Bacteriology Comprehensive

SAINT MOLBOG, SAINT MAHON, SAINT ALVIN PRAY FOR ME

A test for the hydrolysis of esculin in the presence of bile is especially useful in identifying species of the genus

A. Abiotrophia
B. Corynebacterium
C. Enterococcus
D. Staphylococcus
D. Staphylococcus

C. Enterococcus

C. Enterococcus and other group D streptococci can be presumptively identified based on their ability to hydrolyze esculin in the presence of 1-4% bile salts. The medium is made selective for enterococci by the addition of either sodium azide or 4% bile salts. Organisms able to grow on this medium and hydrolyze esculin produce esculetin, which reacts with an iron salt to form a black color in the agar.

The organism associated with a disease characterized by the presence of a pseudomembrane in the throat and the production of an exotoxin that is absorbed into the bloodstream with a lethal effect is

A. Arcanobacterium haemolyticum
B. Staphylococcus aureus
C. Streptococcus pyogenes
D. Corynebacterium diphtheriae

D. Corynebacterium diphtheriae

D. Strains of Corynebacterium diphtheriae
infected by a lysogenic bacteriophage produce an extremely potent exotoxin. Absorption of the toxin may cause a rapidly fatal hypertoxic disease characterized by myocarditis and neuritis. This disease most commonly affects children aged 1 to 10 years. Transmission is by contact with a human carrier or with contaminated fomites.

Enterotoxin produced by Staphylococcus
aureus is responsible for causing

A. Carbuncles
B. Enterocolitis
C. Impetigo
D. Scalded skin syndrome

B. Enterocolitis

B. Staphylococcal enterocolitis food poisoning cases result from the ingestion of contaminated foods containing preformed thermostable enterotoxin. This form of intoxication causes a perfuse and watery diarrhea due to the loss of electrolytes and m fluids into the lumen. In many cases, the causative agent may never be recovered from patient specimens

Abiotrophia, formerly known as nutritionally variant streptococci, will not grow on routine blood or chocolate agars because they are deficient in

A. Hemin
B. Pyridoxal
C. Vitamin B12
D. Thiophene-2-carboxylic hydrazide

B. Pyridoxal

B. Nutritionally variant streptococci (NVS) are now termed Abiotrophia. These clinically significant microorganisms, which account for 5-6% of the cases of endocarditis, are frequently not able to be recovered because of insufficient quantities of vitamin B6 in the culture medium. The routine use of a m pyridoxal disk, a streak of Staphylococcus, or vitamin B6-supplemented culture media is required for isolation.

Exfoliatin produced by Staphylococcus aureus is responsible for causing

A. Enterocolitis
B. Toxic shock syndrome
C. Scalded skin syndrome
D. Staphylococcal pneumonia

C. Scalded skin syndrome

C. Scalded skin syndrome is a form of dermatitis produced by strains of Staphylococcus aureus that elaborate exfoliative toxin. Two types of this toxin have been identified: exfoliation A and exfoliation B. This potent toxin acts by disturbing the adhesive forces between cells of the stratum granulosum, which causes the appearance of the clear, large, flaccid bullae and the skin to peel off. Infants and children are most commonly affected with this form of dermatitis, beginning about the face and trunk and subsequently spreading to the extremities.

Streptococcus pyogenes can be presumptively identified using a(an)

A. PYR disk
B. ONPG disk
C. SPS disk
D. Optochin disk

A. PYR disk

A. Presumptive identification of group A streptococci can be achieved through the PYR (L-pyrrolidonyl-(3-naphthlylamide) disk test. The use of a 0.04-unit bacitracin disk is no longer recommended because groups C and G streptococci are also susceptible to this agent. A positive test result is interpreted as a bright red color change within 5 minutes.

A gram-positive coccus that is catalase positive, nonmotile, lysostaphin resistant, and modified oxidase positive is best identified as a member of the genus

A. Micrococcus
B. Lactococcus
C. Pediococcus
D. Staphylococcus

A. Micrococcus

A. Staphylococci and micrococci are both catalase positive gram-positive cocci. Staphylococci are more clinically significant, so it is important to differentiate Micrococcus from Staphylococcus. Micrococci are modified oxidase positive, whereas Staphylococci are negative

Nocardia asteroides infections in humans
characteristically produce

A. Carbuncles
B. Draining cutaneous sinuses
C. Septic shock
D. Serous effusions

B. Draining cutaneous sinuses

B. Nocardiosis is characterized by mycetoma
or chronic suppurative infection. Draining sinus tracts in the subcutaneous tissue are a common manifestation of the disease. Nocardia spp. are soil saprophytes that may produce disease in humans either by the inhalation of contaminated material or through skin abrasions. Microscopic examination of pus from suspected cases will demonstrate partially acid-fast, gram-positive, branching filamentous or coccoid organism.

Erysipelothrix infections in humans characteristically produce

A. Pathology at the point of entrance of the organism
B. Central nervous system pathology
C. Pathology in the lower respiratory tract
D. The formation of abscesses in visceral organs

A. Pathology at the point of entrance of the organism

A. Infection caused by Erysipelothrix rhusiopathiae in humans is primarily erysipeloid. Erysipeloid is usually the result of contact with an infected animal or contaminated animal product. The characteristic presentation is cutaneous spreading lesions of the fingers or hand that are raised and erythematous. Although generally confined to the skin, E. rhusiopathiae has been implicated in rare cases of endocarditis.

In the CAMP test, a single streak of a beta-hemolytic Streptococcus is placed perpendicular to a streak of beta-lysinproducing Staphylococcus aureus. After incubation, a zone of increased lysis in the shape of an arrowhead is noted; this indicates the presumptive identification of

A. S. agalactiae
B. S. bovis
C. S. equinus
D. S. pyogenes

A. S. agalactiae

A. Streptococcus agalactiae isolates can be presumptively identified by the demonstration of a positive CAMP reaction. CAMP is an acronym for the scientists (Christie, Atkins, MunchPetersen) who discovered this phenomenon. Group B streptococci elaborate the CAMP factor, which acts to enhance the zone of hemolysis produced by beta-lysin-producing strains of Staphylococcus aureus. Incubation of test plates should be earned out in ambient air, because increased CO, and anaerobic incubation increase the rate of false-positive CAMP reactions by group A streptococci.

Staphylococcus saprophyticus, a recognized pathogen, is a cause of

A. Furuncles
B. Impetigo
C. Otitis media
D. Urinary tract infections

D. Urinary tract infections

D. The recovery rate of coagulase-negative
Staphylococcus saprophyticus from urinary tract infections in young females is second only to that of Escherichia coli. The organism has a predilection for the epithelial cells of the urogenital tract and is often seen in large numbers adhering to these cells on Gram stain. Key to the identification of this coagulase-negative Staphylococcus is its resistance to novobiocin.

Color Plate 26 • shows the Gram stain of a blood culture on a 23-year-old pregnant woman who presented with fever and flulike symptoms in her ninth month. The isolate on blood agar produced small, translucent beta-hemolytic colonies. Which of the following is the most likely etiologic agent in this case?

A. Listeria monocytogenes
B. Propionibacterium acnes
C. Streptococcus agalactiae
D. Streptococcus pyogenes

A. Listeria monocytogenes

A. The organism seen in Color Plate 26 is Listeria monocytogenes. Listeria is an important animal and human pathogen that is known to cause abortion, meningitis, and septicemia in humans. This gram-positive rod is actively motile at room temperature (but not at 35°C), hydrolyzes esculin, produces catalase, and is oxidase negative. When recovered on sheep blood agar plates from clinical samples, it is often initially confused with group A or group B streptococci because of its beta-hemolysis.

The etiologic agent most commonly associated with septicemia and meningitis of newborns is

A. Streptococcus agalactiae
B. Streptococcus bovis group
C. Streptococcus pneumoniae
D. Streptococcus pyogenes

A. Streptococcus agalactiae

A. Streptococcus agalactiae (group B Streptococcus) is a principal cause of bacterial meningitis and septicemia in neonates. The organism, which is a part of the indigenous microbial flora of the vagina, is transmitted by the mother before birth, usually as the baby passes through the birth canal. Neonatal infection with group B streptococci may occur either as an early-onset disease (at birth) or as a delayed-onset syndrome that manifests itself weeks after birth.

Which of the following is the most commonly isolated species of Bacillus in opportunistic infections such as bacteremia, post-traumatic infections of the eye, and endocarditis?

A. B. circulans
B. B. cereus
C. B. licheniformis
D. B. subtilis

B. B. cereus

B. The vegetative cells and spores of Bacillus
cereus are widely distributed in the environment. The virulence mechanisms of B. cereus are an enterotoxin and a pyogenic toxin. Accidents in nature resulting in cuts or abrasions contaminated with soil or vegetation, intravenous drug abuse, ingestion of contaminated foods, and traumatic introduction into a normally sterile site through the use of contaminated medical equipment are associated with infection.

Loeffler's serum medium is recommended for the cultivation of

A. Abiotrophia sp.
B. Corynebacterium diphtheriae
C. Leuconostoc sp.
D. Streptococcus agalactiae

B. Corynebacterium diphtheriae

B. The formation of the characteristic
Corynebacterium diphtheriae granules and cellular morphology seen in methylene blue stains is enhanced when the organism is grown on Loeffler's serum medium. Although this medium is primarily designed for the recovery of C. diphtheriae from clinical samples, it is not a differential medium. The agar slant, when inoculated, may demonstrate growth of corynebacteria within 8 to 24 hours

On Tinsdale agar, colonies of Corynebacterium diphtheriae are characterized by the observance of

A. Liquefaction of the agar surrounding
the colonies on the medium
B. Opalescent colonies with a white precipitate in the surrounding agar
C. Black colonies on the culture medium surrounded by brown halos
D. Pitting of the agar medium surrounding the colonies

C. Black colonies on the culture medium surrounded by brown halos

C. Tinsdale medium, for the primary isolation of Corynebacterium diphtheriae, not only inhibits indigenous respiratory flora but differentiates colonies of C. diphtheriae. The potassium tellurite in the medium is taken up by colonies of Corynebacterium, causing them to appear black. Colonies of C. diphtheriae are presumptively identified when black colonies surrounded by a brown halo are seen on this agar medium. However, other corynebacteria and some staphylococci will produce a similar reaction.

Precipitates of diphtheria toxin and antitoxin formed in agar gels are an in vitro means for detecting toxigenic strains of Corynebacterium diphtheriae. The name of this test procedure is the

A. D-test
B. Elektest
C. Hodge test
D. Naglertest

B. Elektest

B. The Elek immunodiffusion test is recommended for detecting toxigenic strains of Corynebacterium diphtheriae. In the test, diphtheria antitoxin is impregnated on a sterile filter paper strip, which is pressed onto the surface of an Elek agar plate. Test and control strains are then inoculated perpendicular to the strip on both sides and without touching the strip. A positive reaction by toxigenic strains produces a precipitin line at a 45-degree angle to the inoculum streak.

The etiologic agent of the disease erysipelas is

A. Staphyloccus aureus
B. Streptobacillus moniliformis
C. Streptococcus agalactiae
D. Streptococcus pyogenes

D. Streptococcus pyogenes

D. Erysipelas results from person-to-person transmission of group A streptococci. Symptoms occur when nasopharyngeal infection spreads to the face. The rare complication of an upper respiratory infection with Streptococcus pyogenes is characterized by sensations of burning and tightness at the site of invasion. Erythema associated with this superficial cellulitis rapidly spreads with an advancing elevated margin. Erysipelothrix rhusiopathiae causes a similar disease referred to as erysipeloid.

Staphylococcus aureus, when present, could most likely be recovered from a stool sample if the primary plating medium included

A. Bismuth sulfite
B. Phenylethyl alcohol
C. Thiosulfate citrate bile salts sucrose
D. Xylose-lysine-desoxycholate

B. Phenylethyl alcohol

B. Phenylethyl alcohol agar (PEA) is a selective medium for the isolation of gram-positive cocci. Blood agar medium is supplemented with 0.15% phenylethyl alcohol, which is inhibitory to most gram-negative aerobic bacilli. This medium is particularly helpful when a specimen containing gram-positive cocci is contaminated with a Proteus spp. due to the inhibition of swarming
by PEA.

A common member of the normal flora of
the upper respiratory tract is

A. Corynebacterium jeikeium
B. Lactobacillus
C. Staphylococcus epidermidis
D. Viridans streptococcus

D. Viridans streptococcus

D. Viridans streptococci are the most common normal flora in upper respiratory cultures. They are opportunistic pathogens with low virulence. Subacute endocarditis is seen in patients with previously damaged heart valves.

Streptococci obtain all their energy from
the fermentation of sugars to

A. Formic acid
B. Lactic acid
C. Succinic acid
D. Valeric acid

B. Lactic acid

B. Whether growing aerobically or anaerobically, streptococci obtain all their energy from the fermentation of sugars to lactic acid. Streptococci are all catalase negative and grow on coventional media such as sheep blood agar. Most are part of the normal flora of human skin, throat, and intestine but produce a wide variety of infections when introduced in tissues or blood.

Streptococci are unable to synthesize the
enzyme

A. Catalase
B. Kinase
C. Hyaluronidase
D. Lipase

A. Catalase

A. Organisms that synthesize the enzyme catalase are able to protect themselves from the killing effects of H2O2 by converting it to H2O and O2. Streptococci are unable to synthesize the heme prosthetic group for this enzyme and are catalase negative. Therefore, they grow better on blood-containing media because of the catalaselike activity of hemoglobin

The beta-hemolysis produced by group A Streptococcus seen on the surface of a sheep blood agar plate is primarily the result of streptolysin

A. H
B. M
C. O
D. S

D. S

D. Streptolysin S is primarily responsible for the beta-hemolysis seen on the surface of a sheep blood agar plate inoculated with a group A streptococcus. Of the two hemolysins secreted by beta-hemolytic group A streptococcus, Streptolysin S is stable in the presence of atmospheric oxygen. Streptolysin O is inactivated in the presence of oxygen, and it is best demonstrated when the agar has been stabbed and subsurface hemolysis is revealed.

When an infection due to Streptomyces is suspected, isolates can be separated from
most other bacteria by

A. Heat shocking the culture
B. Incubating the culture at 25°C
C. Incubating the culture at greater than 37°C
D. Drying the specimen before inoculating the culture media

B. Incubating the culture at 25°C

B. Streptomyces are weak pathogens rarely associated with disease. The bacteria normally inhabit the soil. The most common human infection is myectoma, which is most frequently caused by S. somaliensis. Some strains of Streptomyces grow better at 25°C than at 35°C.

The production of H2S is one characteristic
used to differentiate which of the aerobic gram-positive bacilli?

A. Corynebacterium
B. Erysipelothrix
C. Lactobacillus
D. Nocardia

B. Erysipelothrix

B. Erysipelothrix is a nonmotile, catalasenegative, gram-positive bacillus that often appears as long filaments. Unlike other aerobic grampositive bacilli, this organism produces H2S, which can be demonstrated in triple sugar iron agar. Erysipeloid, a skin disease of the hands usually associated with the handling of infected animals, is the human infection produced most commonly by this agent.

Growth in a 48-hour semisolid agar stab culture at room temperature reveals lateral
filamentous growth away from the stab near the top of the medium. This observation is most characteristic of which organism?

A. Rhodococcus sp.
B. Corynebacterium urealyticum
C. Enterococcusfaecalis
D. Listeria monocytogenes

D. Listeria monocytogenes

D. Listeria monocytogenes is motile at room
temperature. When inoculated into a semisolid medium, growth away from the stab is characteristic of motility. Motility is generally enhanced just below the agar surface, giving the growth pattern an "umbrella" appearance. L. monocytogenes is nonmotile at 35°C.

A former species of Corynebacterium pathogenic for swine, horses, and cattle is also known to cause disease in compromised hosts. This organism when grown on culture media produces pale pink colonies that help to presumptively identify it as

A. Arcanobacterium hemolyticum
B. Actinomyces naeslundii
C. Gardnerella vaginalis
D. Rhodococcus equi

D. Rhodococcus equi

D. Rhodococcus equi is found in soil and commonly produces disease among livestock. These gram-positive bacilli can demonstrate primary mycelia and were formerly in the genus Nocardia. This species is characterized by its pink pigmentation on culture media and its inability to ferment carbohydrates.

Which one of the following is not appropriate when describing Streptococcus pneumoniae ?

A. Bile-resistant
B. Alpha-hemolytic
C. Lancet-shaped, gram-positive diplococcus
D. Virulent strains are encapsulated.

A. Bile-resistant

A. Streptococcus pneumoniae, a primary etiologic agent of lobar pneumonia, is an encapsulated, gram-positive, lanceolate diplococcus. Fastidious in its growth requirements, the organism on sheep blood agar produces characteristic alpha-hemolytic colonies, which are convex and often mucoid in appearance and bile soluble. Upon aging, colonies of S. pneumoniae undergo autolytic changes. There are approximately 80 types of pneumococci based on specific capsular antigens.

Nocardia can be differentiated from Actinomyces based on

A. Nocardia being an obligate anaerobe
B. The partial-acid fast staining reaction
of Actinomyces
C. The production of sulfur granules in
cases of nocardiosis
D. Nocardia being catalase positive

D. Nocardia being catalase positive

D. Species of the genus Nocardia are ubiquitous in the soil and thus characteristically produce exogenous forms of infection as a result of inhalation of contaminated fomites or a traumatic incident with soil contamination. A diagnostic
characteristic, depending on the species, is the acid fastness of the filamentous bacilli or
coccoid forms. Unlike Actinomyces spp., which are catalase-negative, gram-positive, non-sporeforming anaerobic bacilli, Nocardia spp. are catalase-positive aerobic organisms. "Sulfur granules" are characteristic of actinomycotic pus and upon examination would reveal nonacid-fast branching filaments.

Enterococcus faecium is characteristically

A. Inhibited by the presence of bile in
culture media
B. Able to grow in the presence of high
concentrations of salt
C. PYR negative
D. Beta-hemolytic

B. Able to grow in the presence of high
concentrations of salt

B. Enterococcus faecium is an important agent of human infection. Their differentiation from other enterococcal strains is of importance because of their resistance to most clinically useful antimicrobial agents, including vancomycin. The ability to tolerate a high concentration of salt is characteristic of the clinically significant species of Enterococcus. E. faecium is PYR positive
and is usually nonhemolytic.

A negative PYR (L-pyrolidonyl anaphthylamide) test is demonstrated by

A. Enterococcus faecalis
B. Enterococcus faecium
C. Streptococcus pyogenes
D. Viridans streptococci

D. Viridans streptococci

D. Viridans streptococci do not produce the
enzyme pyroglutamyl aminopeptidase and, therefore, in the PYR test do not produce a positive or red color. The PYR test is used predominantly for the presumptive identification of group A streptococci and Enterococcus. Micrococcus and Lactococcus are known to produce a positive reaction as well, although the reaction may be delayed.

A Gram stain of a sputum specimen from a patient with a suspected case of lobar pneumonia reveals many white blood cells and many gram-positive cocci, which are primarily diplococci. Which of the following statements would be appropriate, given these findings?

A. A PYR test should be performed on the culture isolate.
B. An Elek test should be performed on the culture isolate.
C. An optochin test should be performed on the culture isolate.
D. A hippurate hydrolysis test should be performed on the culture isolate.

C. An optochin test should be performed on the culture isolate.

C. Streptococcus pneumoniae is a leading cause of lobar pneumonia as well as other serious bacterial infections. The Gram stain smear of clinical specimens can provide a rapid presumptive diagnosis when the characteristic morphology and Gram reaction is observed. The optochin disk test can be performed to presumptively identify this
organism. Optochin lyses pneumococci, producing a zone of inhibition around the disk.

A child presented in August at the pediatric clinic with a superficial skin infection of the neck. The large, itchy lesions were cultured, and the diagnosis of impetigo was made. One of the etiologic agents of this clinical condition is

A. Erysipelothrix rhusiopathiae
B. Corynebacterium diphtheriae
C. Staphylococcus saprophyticus
D. Streptococcus pyogenes

D. Streptococcus pyogenes

D. Bacteriologic cultures of a typical impetigo lesion may yield either a pure culture of Streptococcus pyogenes or a mixed culture of S. pyogenes and Staphylococcus aureus. The thick crust form of impetigo, which is most commonly seen, is primarily caused by S. pyogenes. It is the bullous form of impetigo for which S. aureus is the etiologic agent. The route of infection is direct inoculation of the causative agents into abraded or otherwise compromised areas of the skin.

An identifying characteristic of
Staphylococcus aureus is
A. DNase negative
B. Coagulase negative
C. Mannitol fermentation postive
D. Growth inhibition in presence of increased salt

C. Identifying characteristics of Staphylococcus
aureus include the production of the extracellular
enzymes coagulase and DNase and its ability to
grow in the presence of high salt concentrations.
Differential and selective media, such as mannitol
salt agar, have been developed for the recovery of
this organism. Selective media and rapid identification
tests are important for this widely recognized
opportunistic pathogen.

Which of the following organisms is able
to hydrolyze sodium hippurate to benzoic
acid and glycine?
A. Streptococcus agalactiae
B. Streptococcuspneumoniae
C. Listeria monocytogenes
D. Enterococcusfaecalis

A. Group B streptococci (Streptococcus agalactiae),
unlike other streptococci, can hydrolyze
sodium hippurate to benzoic acid and glycine. If
glycine is produced, the addition of ninhydrin to
the medium will reduce the glycine to produce a
purple color. The use of ninhydrin to detect
glycine is a sensitive and rapid test of hippurate
hydrolysis.

Which of the following is not characteristic
of Listeria monocytogenes ?
A. CAMP test postive
B. Catalase negative
C. Esculin hydrolysis positive
D. Motile

B. Listeria monocy to genes and Streptococcus
agalactiae produce an extracellular factor
known as the CAMP factor. The test is performed
by making a streak of the test isolate perpendicular
to a streak of Staphylococcus aureus.
A positive CAMP reaction is indicated by a zone
of enhanced beta-hemolysis (arrowhead shape)
at the point where the zone of hemolysis produced
by S. aureus joins with that produced by
the beta-hemolytic test isolate. Unlike 5. agalactiae,
L. monocytogenes is catalase positive.

Which of the following is not associated with Staphylococcus aureus?
A. Endotoxin production
B. Clumping factor production
C. Deoxyribonuclease production
D. Hemolysin production

A. The production of hemolysins and the
enzymes coagulase and DNase is associated with
the virulence of staphylococci. The coagulaseproducing
staphylococci are most commonly
producers of staphylolysins, which produce betahemolysis
when the isolate is grown on sheep
blood agar. Many factors contribute to staphylococcal
virulence by overcoming the host's natural
defenses. Endotoxin is found in the cell wall
of gram-negative bacteria.

Which of the following is a characteristic of staphylococci that would help in their isolation from clinical specimens?
A. Bile resistance
B. Growth at 55°C
C. High salt tolerance
D. Resistance to novobiocin

C. The physiology of staphylococci enables
them to remain infectious in the environment
longer than many other pathogenic bacteria.
Staphylococci are somewhat heat resistant and
can survive dry conditions. Their high salt tolerance
enables strains to grow in salt-preserved foods and causes cases of food poisoning. Staphylococci,
however, cannot resist temperatures as
high as 55 °C for long periods, and they are
not bile resistant. Most species are sensitive to
novobiocin.

Which of the following species of Bacillus is nonmotile?
A. B. cereus
B. B. subtilis
C. B. anthracis
D. B. thuringiensis

C. Motility is a key test for the differentiation of
Bacillus anthracis from other species of Bacillus.
Suspect Bacillus colonies are inoculated in a
broth medium and allowed to grow to a visible
turbidity. A sample of this actively growing culture
should be examined using the hanging-drop
technique for motility. B. anthracis is nonmotile
and can therefore be easily differentiated from
commonly encountered motile species.

Which one of the following diseases involves erythrogenic toxin?
A. Cutaneous anthrax
B. Diphtheria
C. Impetigo
D. Scarlet fever

D. The rash of scarlet fever is a result of the
action of an erythrogenic toxin produced by group A streptococci. Because of the rapid diagnosis and treatment of group A streptococci
infections, scarlet fever is rare in most developed countries. The other diseases listed do not
involve an erythrogenic toxin.

Cultures of the posterior pharynx are most commonly submitted to the clinical laboratory for the detection of
A. Coiynebacterium diphtheriae
B. Staphylococcus aureus
C. Streptococcus pneumoniae
D. Streptococcus pyogenes

D. Cultures of the tonsillar fossae and posterior
pharynx are most commonly obtained in suspected
cases of streptococcal pharyngitis. Streptococcus
pyogenes is most often associated with
cases of pharyngitis but is also the agent of scarlet
fever and erysipelas in addition to wound infections
(e.g., necrotizing fasciitis). Rapid identification
of this organism and prompt antimicrobial
therapy are required to prevent sequelae (i.e.,
rheumatic fever and acute glomerulonephritis).

Streptococcus sanguis, a viridans streptococcus, is most commonly associated with which of the following clinical conditions?
A. Otitis media
B. Pharyngitis
C. Relapsing fever
D. Subacute bacterial endocarditis

D. Subacute bacterial endocarditis is an inflammation
of the lining membrane of the heart,
which most often is caused by a member of the
viridans group of streptococci. Streptococcus
sanguis is one of several species that may lodge
in an abnormal heart or on valves damaged by
previous infection. Viridans streptococci are
normal inhabitants of the human upper respiratory
tract.

Rust-colored sputum in cases of lobar pneumonia is characteristic of which of
the following possible etiologic agents?
A. Corynebacteriumjeikeium
B. Staphylococcus aureus
C. Streptococcus pneumoniae
D. Streptococcus pyogenes

C. Streptococcus pneumoniae is most commonly
associated with cases of lobar pneumonia.
Patients characteristically produce blood-tinged,
rust-colored sputum in which the characteristic
gram-positive lanceolate diplococci can be found.
S. pneumoniae forms alpha-hemolytic colonies
when grown on sheep blood agar.

A urine culture from a 23-year-old female grew a catalase-positive gram-positive coccus (> 100,000 cfu/mL), which would most likely be
A. Staphylococcus saprophyticus
B. Enterococcus faecalis
C. Streptococcus bovis group
D. Streptococcus viridans

A. Staphylococcus saprophyticus is recognized
as an etiologic agent of uncomplicated cystitis
cases in young females. These nonhemolytic,
coagulase-negative staphylococci closely resemble
S. epidermidis on sheep blood agar. Identification
of 5. saprophyticus is facilitated by
demonstrating its resistance to novobiocin.

Cystine-tellurite blood agar plates are recommended for the isolation of
A. Corynebacterium diphtheriae
B. Streptococcus agalaciae
C. Streptococcus pyogenes
D. Group D streptococci

A. Clinical material sent to the laboratory for
the recovery of Corynebacterium diphtheriae
should be inoculated on cystine-tellurite agar
plates or Tinsdale medium. On tellurite-containing
media, colonies of this pathogen will appear
dark-brown to black, which aids in their differentiation.
Suspicious colonies should be further
tested for their biochemical activity and toxin
production.

The pulmonary form of anthrax is known as
A. Valley fever
B. Walking pneumonia
C. Farmers' lung
D. Woolsorters disease

D. Bacillus anthracis is the causative agent of
woolsorters disease or the pulmonary form of
anthrax. The mode of infection is the inhalation
of spores by the patient, usually during the performance
of his/her occupation (sheep shearing
or processing of animal hair). Prompt diagnosis
and treatment of this disease is needed because it
is known to progress rapidly to a fatal form of
septicemia.

Pleomorphic gram-positive bacilli in a Gram stain best describes

A. Bacillus anthracis
B. Bacillus subtilis
C. Listeria monocytogenes
D. Corynebacterium pseudodiphtheriticum

D. Corynebacterium pseudodiphtheriticum is morphologically similar to all other members of the genus Corynebacterium. They are all grampositive, non-spore-forming bacilli that characteristically resemble Chinese characters or palisades. These bacteria often stain irregularly and have a pleomorphic club-shaped appearance.

An aerobic gram-positive rod known to cause bacteremia in hospitalized immunocompromised patients is

A. Bacillus anthracis
B. Corynebacterium jeikeium
C. Corynebacterium ulcerans
D. Corynebacterium urealyticum

B. Corynebacterium jeikeium is a low virulence organism resistant to multiple antimicrobials. Its multiple drug resistance allows it to remain in hospital environments, and it is often cultured from the skin of hospitalized patients. In compromised patients it has been implicated in cases of septicemia, wound infections, and endocarditis in association with intravenous catheter use.

A bone marrow transplant patient on immunosuppressive therapy developed a pulmonary abscess with symptoms of neurologic involvement. A brain abscess was detected by MRI, and aspirated material grew an aerobic, filamentous, branching gram-positive organism, which stained weakly acid-fast. The most likely etiologic agent in this case would be
A. Actinomyces israelii
B. Nocardia asteroides
C. Mycobacterium tuberculosis
D. Propionibacterium acnes

B. In immunocompromised patients, Nocardia
asteroides can cause invasive pulmonary infection
and can often spread hematogenously throughout
the body. Lesions in the brain are commonly associated
with dissemination and have a poor prognosis.
The organism is ubiquitous in nature, and
infection is acquired by traumatic inoculation or
inhalation.

Which of the following is catalase negative?
A. Bacillus
B. Corynebacterium
C. Leuconostoc
D. Listeria

C. Of the genera listed, only Leuconostoc is
catalase negative. Leuconostoc is vancomycin
resistant and associated with infections in hospitalized
patients. It has also been linked to septicemias
in neonates.

Colonies of Listeria monocytogenes on a
sheep blood agar plate most closely resemble colonies of
A. Corynebacterium diphtheriae
B. Streptococcus agalactiae
C. Streptococcus bovis group
D. Rhodococcus equi

B. Listeria monocytogenes is a small, grampositive
bacillus that is actively motile at
room temperature. When grown on sheep blood
agar, this organism produces small, translucent
beta-hemolytic colonies, which may be visually
mistaken for beta-hemolytic streptococci.
Biochemically L. monocytogenes differs from
streptococci because it possesses the enzyme
catalase.

The most common etiologic agent of infections associated with the surgical insertion of prosthetic devices such as artificial heart valves and cerebrospinal fluid shunts is
A. Corynebacterium urealyticum
B. Staphylococcus capitis
C. Staphylococcus epidermidis
D. Streptococcus mutans

C. Staphylococcus epidermidis is a saprophytic
microorganism found on the skin and mucous
membranes of humans. This coagulase-negative
Staphylococcus is seen frequently as a contaminant
in blood cultures when proper venipuncture
technique has not been used. S epidermidis has
been implicated in serious human infections
associated with the surgical insertion of prosthetic
devices.

The description of "Medusa head" colonies on solid agar is most characteristic
of
A. Bacillus anthracis
B. Enterococcusfaecalis
C. Staphylococcus saprophyticus
D. Streptococcus agalactiae

A. Bacillus anthracis is the etiologic agent of
human anthrax that occurs in any of three forms:
cutaneous, pulmonary, and gastrointestinal. On
Gram stain this organism appears as a large,
spore-forming, gram-positive bacillus that characteristically
grows in long chains. Colonies on
agar plates are large and opaque with fingerlike
projections referred to as "Medusa head" forms.

Which of the following is most likely to be isolated in cultures from the anterior nares of healthcare workers?
A. Bacillus cereus
B. Streptococcuspneumoniae
C. Staphylococcus aureus
D. Staphylococcus saprophyticus

C. Staphylococci colonize various skin and
mucosal surfaces in humans. S. aureus is carried
as transient flora in the anterior nares. S. saprophyticus
is less likely found as normal flora and
is associated with urinary tract infections. Hospital
personnel may harbor resistant strains of
S. aureus, and person-to-person contact is a substantial
infection control concern. Cultures of the
anterior nares are recommended when screening
for earners in the hospital environment.

Ethylhydrocupreine HC1 susceptibility is a presumptive test for the identification of
A. Viridans streptococci
B. Streptococcus pyogenes
C. Streptococcus agalactiae
D. Streptococcus pneumonias

D. The susceptibility of alpha-hemolytic streptococcal
isolates to optochin, or ethylhydrocupreine
HC1, is a presumptive test for the
differentiation of Streptococcus pneumoniae
from viridans streptococci. Viridans streptococci
are typically resistant to this agent and show no
zone of inhibition or a zone of less than 10 mm
with a 6-mm disk. S. pneumoniae characteristically
is susceptible and produces a zone of inhibition
greater than 14 mm

Solubility in the presence of sodium desoxycholate is characteristic of
A. Enterococcusfaecalis
B. Streptococcus agalactiae
C. Streptococcus mutatis
D. Streptococcus pneumoniae

D. Solubility of Streptococcus pneumoniae
colonies by surface-active agents, such as
sodium desoxycholate, is a widely used presumptive
identification procedure. When a 10%
solution of this reagent is applied to test colonies,
5. pneumoniae will be totally dissolved. Colonies
of viridans streptococci typically remain intact
when bile is applied.

Family members attending a picnic
became ill about 2 hours after eating. The
illness was characterized by rapid onset of
violent vomiting. The most likely bacterial
cause of such symptoms would be food
poisoning caused by
A. Enterococcusfaecium
B. Bacillus subtilis
C. Listeria monocytogenes
D. Staphylococcus aureus

D. The ingestion of food contaminated with
enterotoxin produced by Staphylococcus aureus
is the most likely cause of the disease in the case
described. 5. aureus multiplies rapidly in improperly
stored food. Within a few hours, levels of 105
organisms per gram of food can be found. Enterotoxin
is elaborated when the organism reaches
stationary growth phase. Ingestion of small
amounts of toxin results in a rapid onset (1-6
hours) of vomiting and diarrhea as a result of a
neural response.

The novobiocin susceptibility test is used
for the identification of
A. Corynebacterium diphtheriae
B. Streptococcus pyogenes
C. Streptococcus pneumoniae
D. Staphylococcus saprophyticus

D. Most strains of Staphylococcus saprophyticus
are resistant to novobiocin. This organism is
frequently found in urine culture of young
women and may be misidentified as S. epidermidis.
A 5-ug disk is used in the test, and a zone
of 16 mm or less determines resistance.

Tellurite reduction is used for the presumptive identification of
A. Bacillus anthracis
B. Corynebacterium diphtheriae
C. Erysipelothrix rhusiopathiae
D. Staphylococcus saprophyticus

B. On serum-cystine-sodium thiosulfate-tellurite
medium (Tinsdale medium), Corynebacterium
diphtheriae is differentiated from other cornybacteria
and other bacteria of the respiratory tract by
its ability to produce black colonies surrounded
by a brown-black halo after 48 hours of incubation.
Growth factors needed by C. diphtheriae are
provided by the addition of the serum. Potassium
tellurite is inhibitory to many gram-positive and
gram-negative bacteria, but corynebacteria are
resistant.

The etiologic agent of the majority of adult joint infections is
A. Abiotrophia sp.
B. Leuconostoc sp.
C. Staphylococcus aureus
D. Streptococcus pneumoniae

C. Staphylococcus aureus is the predominant
pathogen involved in joint infections of adults.
Bacterial arthritis can occur following infection
in other parts of the body or bacteremia. Streptococcus
pyogenes and Neisseria gonorrhoeae
each account for a significant number of adult
infections, whereas Streptococcus pneumoniae
and Haemophilus influenzas predominate in
childhood infections.

Which of the following is associated with infections in humans often linked to deli meats and improperly pasteurized dairy
products?
A. Bacillus subtilis
B. Listeria monocytogenes
C. Leuconostoc
D. Streptococcus agalactiae

B. Listeria monocytogenes is a cause of human
and bovine abortion. In humans, the mother's
symptoms are usually mild, resembling the flu
and causing a low-grade fever. The organism can
be isolated from aborted fetuses as well as from
the maternal placenta. When infection with this
etiologic agent is detected early, appropriate
therapy can be initiated, which may prevent the
death of the fetus.

Bacillus cereus has been implicated as the etiologic agent in cases of
A. Food poisoning
B. Impetigo
C. Pelvic inflammatory disease
D. Toxic shock syndrome

A. Bacillus spp. are gram-positive, spore-forming
bacilli widely found in the environment.
Bacillus cereus is of particular interest as an etiologic
agent of human cases of food poisoning.
This enterotoxin-producing microorganism is
most commonly associated with cases of food
poisoning following ingestion of reheated rice
served at Asian restaurants.

The causative agent of "malignant pustule" is
A. Bacillus anthracis
B. Cojjnebacterium ulcerans
C. Erysipelothrix rhusiopathiae
D. Listeria monocytogenes

A. Bacillus anthracis infects humans by three
routes: respiratory, gastrointestinal, and cutaneous.
Malignant pustule is the name given to
lesions seen in cutaneous anthrax in humans.
The lesion is, however, neither malignant nor a
pustule. The disease produces a localized
abscess on the skin, which forms a characteristic
black eschar surrounded by a red raised ring.

An infant was hospitalized with a severe, tender erythema. The child's epidermis was loose, and large areas of skin could be peeled off. The condition described is most consistent with a clinical syndrome associated with
A. Streptococcus pyogenes
B. Staphylococcus aureus
C. Bacillus anthracis
D. Erysipelothrix rhusiopathiae

B. Scalded skin syndrome is the dermatitis
associated with the effects of the exfoliative
toxin produced by strains of Staphylococcus
aureus. Exfoliatin acts in humans to disrupt the
adhesive forces between cells of the stratum
granulosum, creating large flaccid bullae. This
syndrome occurs primarily in infants and children;
the primary infection is usually unrelated
to the areas where lesions appear.

A catalase-negative gram-positive coccus is isolated from a urine sample of a hospitalized patient. The bacterium produced a black pigment on bile-esculin agar and formed acid from glucose in the presence of 6.5% NaCl. What is the most likely identification of this bacterium?
A. Abiotrophia sp.
B. Enterococcus faecalis
C. Group B streptococci
D. Group D streptococci

B. Enterococcus faecalis and E. faecium grow in
the presence of bile, hydrolyze esculin, and produce
acid from glucose in the presence of high
salt concentration. These bacteria also express
streptococcal group D antigen. The ability to tolerate
high salt concentrations differentiates the
enterococci from the group D streptococci like
the S. bovis group.

A suspension of the test organism for use in broth dilution and disk diffusion testing is adjusted to match the turbidity of a
A. #0.5 McFarland standard
B. #1.0 McFarland standard
C. #2.0 McFarland standard
D. #3.0 McFarland standard

A. Standardization of the susceptibility testing procedure is essential for determining the susceptibility of an organism to antimicrobial agents. A #0.5 McFarland standard is used when adjusting the turbidity of the suspension of test organism. A #0.5 McFarland standard has a turbidity consistent with approximately 1.5 X 108organisms/mL of broth or saline.

When testing the antimicrobial susceptibility of Haemophilus influenzae strains by disk-agar diffusion, the recommended medium is
A. Chocolate agar
B. Charcoal yeast extract agar
C. Mueller-Hinton base supplemented with 5% sheep blood
D. Mueller-Hinton base supplemented with hematin, NAD, and yeast extract

D. Hemophilias Test Medium is recommended for use in the disk-agar diffusion susceptibility testing procedure of Haemophilus. The testing of Haemophilus spp. requires supplemented media to support the growth of these fastidious organisms. In vitro growth of H. influenzae requires the presence of accessory growth factors: X factor (hemin) and V factor (NAD).

The chemotherapeutic agents structurally similar to the vitamin p-aminobenzoic acid that act by inhibiting bacteria via inhibition of folic acid synthesis are
A. Aminoglycosides
B. Penicillins
C. Macrolides
D. Sulfonamides

D. Sulfonamides act to interfere with the ability of bacteria to use p-aminobenzoic acid, which is a part of the folic acid molecule, by competitive inhibition. These chemotherapeutic agents are bacteriostatic and not bactericidal. The drug sulfisoxazole is a member of this group and is used in the treatment of urinary tract infections, especially those caused by Escherichia coli, which must synthesize folic acid for growth

Which of the following organisms would not routinely undergo antimicrobial susceptibiity testing?
A. Escherichia coli
B. Proteus mirabilis
C. Staphylococcus aureus
D. Streptococcus pyogenes

D. Antimicrobial susceptibility testing is not routinely performed on all bacteria. Certain organisms are predictably susceptible to a variety of antimicrobial agents. Therefore, testing is not usually performed even when these organisms are the etiologic agents of infection. Bacteria for which susceptibility tests are usually not performed include Streptococcus pyogenes (group A Streptococcus), Streptococcus agalactiae (group B Streptococcus), and Neisseria meningitidis.

Penicillin is active against bacteria by
A. Inhibition of protein synthesis at the 30S ribosomal subunit
B. Reduction of dihydrofolic acid
C. Inhibition or peptidoglycan synthesis
D. Inhibition of nucleic acid function

C. Inhibitors of peptidoglycan synthesis such as penicillin act to inhibit cell wall development. Bacteria unable to produce peptidoglycan for their cell walls are subject to the effects of varying osmotic pressures. The peptidoglycan component of the cell wall protects the bacterium from lysis.

The minimum bactericidal concentration (MBC) of an antimicrobial agent is defined as the lowest concentration of that antimicrobial agent that kills at least of the original inoculum.
A. 95.5%
B. 97%
C. 99.9%
D. 100%

C. The requirement of 99.9% killing defines the minimum bactericidal concentration (MBC) of an antimicrobial agent. The MBC test is an additional quantitative assessment of the killing effect of a drug on a specific patient isolate. This test, done to evaluate a drug's activity, is sometimes requested in cases of life-threatening infections.

Resistance to clindamycin can be induced in vitro by
A. Ampicillin
B. Erythromycin
C. Gentamicin
D. Penicillin

B. Even though clindamycin and erythromycin are in different classes, the mechanisms of resistance are similar. The presence of erythromycin can induce clindamycin resistance. The D-zone test is used to detect the presence of this inducible resistance.

The term that denotes a situation in which the effect of two antimicrobial agents together is greater than the sum of the effects of either drug alone is
A. Additivism
B. Antagonism
C. Sensitivity
D. Synergism

D. The therapeutic effect of antimicrobial therapy is often increased by the use of a combination of drugs. A combination of antimicrobials is said to be synergistic when the sum of their effects is greater than that derived from either drug when tested independently. A tenfold decrease in the number of viable cells from that obtained by the most effective drug in the combination is the definition of synergism. Synergistic combinations of antimicrobials are used primarily in the treatment of tuberculosis, enterococcal endocarditis, and certain gram-negative bacillus infections.

Beta-lactamase-producing strains of Haemophilus influenzas are resistant to
A. Chloramphenicol
B. Erythromycin
C. Penicillin
D. Trimethoprim sulfamethoxazole

C. Beta-lactamase production by strains of Haemophilus influenzae renders them resistant to the antibacterial effect of penicillin and ampicillin. It is recommended that rapid beta-lactamase testing be performed on isolates in life-threatening clinical infections such as meningitis. The rapid tests all rely on this enzyme's ability to act on a beta-lactamase ring and in turn produce a color change, which denotes a positive result due to the production of penicilloic acid.

The agar recommended by the Clinical and Laboratory Standards Institute for routine susceptibility testing of nonfastidious bacteria is
A. MacConkey agar
B. MiddlebrookVHIOagar
C. Mueller-Hinton agar
D. Trypticase soy agar

C. The recommended plating medium for use in both the disk diffusion and tube dilution susceptibility test procedures is Mueller Hinton. Low in tetracycline and sulfonamide inhibitors, this medium has been found to show only slight batch-to-batch variability. For the susceptibility testing of fastidious organisms (e.g., Streptococcus pneumoniae), 5% lysed sheep blood may be added.

The pH of the agar used for the KirbyBauer test should be
A. 7.0-7.2
B. 7.2-7.4
C. 7.4-7.6
D. 7.6-7.8

B. The Kirby-Bauer or disk-agar diffusion susceptibility test requires that the pH of the agar be tested at room temperature to ensure an optimal range of 1.2-1.4 before use in the procedure. A sample of the Mueller-Hinton medium can be tested by macerating it in distilled water and testing with a pH meter electrode; a surface electrode is acceptable for direct testing. Another acceptable method is to allow the agar to solidify around the electrode of a pH meter and then obtain a reading.

Which drug known to be active against parasitic infections has importance as a therapeutic agent in cases of disease caused by anaerobic bacteria?
A. Isoniazid
B. Metronidazole
C. Rifampin
D. Trimethoprim

B. Metronidazole, a drug recommended for the treatment of amebic dysentery and trichomoniasis, is a synthetic compound that acts by inhibiting DNA synthesis. The use of this drug for treating anaerobic infections has gained emphasis in light of resistance patterns of many of the commonly recovered anaerobes. Metronidazole is consistently active against all gram-negative, anaerobic bacilli; is able to cross the blood-brain barrier; and is the only agent consistently bactericidal against susceptible isolates.

An example of a bactericidal antibiotic is
A. Chloramphenicol
B. Erythromycin
C. Tetracycline
D. Tobramycin

D. Tobramycin, an aminoglycoside, is the only antibiotic, of those listed, that is bactericidal. Bactericidal antibiotics actually destroy the bacteria, whereas bacteriostatic drugs only arrest the growth of the microorganism. All aminoglycosides, with the exception of spectinomycin,
are bactericidal in their activity.

The extended-spectrum beta-lactamases confer resistance to
A. Amoxicillin
B. Ceftriaxone
C. Erythromycin
D. Rif ampin

B. The extended spectrum beta-lactamases (ESBLs) confer resistance to the extended spectrum cephalosporins such as ceftriaxone and cefotaxime. ESBLs cleave the antibiotic, inactivating it. So far, ESBLs have only been found ingram-negative bacteria.

Which of the following media should be used for in vitro susceptibility testing of S. pneumoniae?
A. Chocolate agar
B. Charcoal yeast extract agar
C. Mueller-Hinton base supplemented with 5% lysed horse blood
D. Mueller-Hinton base supplemented with 1% hemoglobin and 1% IsoVitaleX

C. Most fastidious bacteria do not grow satisfactorily in standard in vitro susceptibility test systems that use unsupplemented media. For certain species, such as Haemophilus influenzae, Neisseria gonorrheae, Streptococcus pneumoniae,
and other Streptococcus species, modifications have been made to the standard Clinical and Laboratory Standards Institute (CLSI) methods. In the case of S. pneumoniae, current CLSI broth dilution test conditions include cation-supplemented Mueller-Hinton broth with 5% lysed horse blood.

Rapid testing for beta-lactamase production
is recommended, before initiation of antimicrobial therapy, for isolates of
A. Serratia marcescens
B. Haemophilus influenzas
C. Staphylococcus epidermidis
D. Streptococcus pyogenes

B. Haemophilus influenzae should be tested for beta-lactamase production. The test can be performed directly, and the methods are rapid and reliable for the detection of penicillin and ampicillin resistance. Rapid test methods, in general, rely on a color change to detect the presence of this enzyme. A pH indicator may be used to detect the penicilloic acid produced when the beta-lactam ring of penicillin is cleaved, or a color change can be observed when the beta-lactam ring of a chromogenic cephalosporin is hydrolyzed by the enzyme.

The phenomenon of bacterial resistance to the bactericidal activity of penicillins and cephalosporins, with only inhibition of the organism's growth, is known as
A. High-level resistance
B. Intrinsic resistance
C. Inducible resistance
D. Tolerance

D. Tolerance is described as the ability of certain strains of organisms to resist lethal concentrations of antimicrobial agents like penicillin. The growth of these organisms is only inhibited by these cidal drugs. This mechanism of bacterial resistance is attributed to a deficiency of cell wall autolysins.

The supervisor of a microbiology laboratory has been asked to begin performing in vitro antimicrobial susceptibility testing of Mycobacterium tuberculosis because of an increase in the reported resistance in his community. Which of the following methods would be appropriate for this testing?
A. Broth microdilution method using Mueller-Hinton broth
B. Kirby-Bauer method
C. BACTEC method
D. Schlichter method

C. The rise in antimicrobial-resistant isolates of Mycobacteria tuberculosis has been an important public health crisis. The accepted methods for determining the in vitro antimicrobial susceptibility of mycobacteria are based on the growth of the microorganisms on solid or in liquid media containing a specified concentration of a single drug. Two such methods that have been described and are in common use in the U.S. are the agar proportion method and the BACTEC 460TB radiometric method.

Clavulanic acid is classified as a
A. Beta-lactam
B. Beta-lactamase inhibitor
C. Macrolide
D. Aminoglycoside

B. Clavulanic acid is a beta-lactamase inhibitor. It can be administered with amoxicillin or ticarcillin and is effective in treating infections caused by beta-lactamase-producing bacteria such as staphylococci, Klebsiella, and Haemophilus influenzae. Sulbactam and tazobactam are also beta-lactamase inhibitors.

Which of the following antimicrobial agents acts by inhibiting cell wall synthesis?
A. Clindamycin
B. Gentamicin
C. Naladixic acid
D. Vancomycin

D. Vancomycin, which acts to inhibit cell wall synthesis of susceptible bacteria, is produced by an actinomycete. The main activity of this drug is to inhibit peptidoglycan synthesis, but it also has an effect on other aspects of bacterial metabolism. Vancomycin is a bactericidal antibiotic

Metronidazole is most commonly recommended for treatment of infections caused by
A. Aerobic microorganisms
B. Microaerophilic microorganisms
C. Obligate anaerobic microorganisms
D. Obligate intracellular microorganisms

C. Metronidazole, a nitroimidazole derivative, is active against most of the clinically significantanaerobes. Only some of the non-spore forming, gram-positive anaerobic bacilli and gram-positive anaerobic cocci are resistant to this agent. This drug acts to disrupt bacterial DNA through the production of cytotoxic intermediates.

Which of the following antimicrobial agents acts by inhibiting protein synthesis?
A. Gentamicin
B. Methicillin
C. Rifampin
D. Ampicillin

A. Gentamicin is a member of the aminoglycoside group of antibiotics. These drugs act on the 308 ribosomal subunit to inhibit protein synthesis.
Gentamicin is particularly effective against a wide variety of gram-negative bacilli.

Chloramphenicol is an important antimicrobial agent for the treatment of meningitis as well as several other serious infections. Unfortunately, chloramphenicol exhibits significant complications that limit its clinical usefulness. These effects include
A. Allergic reactions and anaphylaxis
B. Bone marrow suppression and aplastic anemia
C. Significant gastrointestinal manifestations
D. Photosensitivity

B. Bone marrow toxicity is the major complication of chloramphenicol. Reversible bone marrow suppression with anemia, leukopenia, and thrombocytopenia occurs as a direct result of the agent on hematopoiesis. The second form of bone marrow toxicity is a rare but usually fatal aplastic anemia. The mechanism of this response is not known.

Which of the following is not one of the standard control organisms used for the weekly testing of antimicrobial disks?
A. Staphylococcus epidermidis (ATCC 25833)
B. Pseudomonas aeruginosa (ATCC 27853)
C. Escherichia coli (ATCC 25922)
D. Enterococcus faecalis (ATCC 29212)

A. Standard quality control strains maintained by the American Type Culture Collection (ATCC) should be tested routinely as recommended by the Clinical and Laboratory Standards Institute. Guidelines developed for the quality assurance of the disk-diffusion antimicrobial susceptibility test procedure recommended that the following organisms be used for this purpose: Pseudomonas aemginosa (ATCC 27853), Staphylococcus aureus (ATCC 25923), Escherichia coli (ATCC 25922), and Enterococcus fecalis (ATCC 29212). Cultures of these organisms should be frozen or lyophilized to maintain their antimicrobial susceptibility pattern. Testing should not be done from stored cultures but rather from freshly grown 18- to 24-hour cultures.

When using the rapid chromogenic cephalosporin method for the detection of beta-lactamase production by an organism, a positive test is indicated by the color
A. Yellow
B. Green
C. Red
D. Blue

C. The chromogenic cephalosporin method is the most sensitive test for detecting the production of beta-lactamase enzymes. This yellow compound will become red if the organism produces the enzyme that breaks the beta-lactam ring. Nitrocefin, the commonly used compound, has a high affinity for most bacterial beta-lactamases.

Enterobius vermicularis infection is usually diagnosed by finding
A. Eggs in perianal specimens
B. Larvae in perianal specimens
C. Larvae in feces
D. Eggs in the feces

A. Because the eggs of E. vermicularis are usually deposited on the perianal area, cellulose tape slides are recommended for collecting the eggs. Recovery is best if specimens are collected late in the evening or before bathing or defecating in the morning. The gravid female worms usually migrate at night to the perianal region to deposit eggs. Because their migration is sporadic, several consecutive collections may be necessary to detect the infection.

The best direct diagnosis of Echinococcus granulosus infection in humans is made by identification of
A. Adult worms in the intestine
B. Adult worms in tissues
C. Eggs in feces
D. Hydatid cysts in tissues

D. When E. granulosus eggs are ingested by an intermediate herbivorous host, including humans, they usually develop into hydatid cysts in which invaginated larval scolices are produced. These cysts are most often in the liver or lung. Although clinical findings can provide a presumptive diagnosis, this is best confirmed by the finding, at surgery, of encysted larval scolices. Each scolex is capable of developing into an adult worm after ingestion by a dog or related animal, the definitive host.

Which statement is correct for specimen collection and processing?
A. Stool samples can contain urine.
B. Stools can be frozen without affecting parasitic structure.
C. Liquid stools are best for detecting ameba and flagellate trophozoites.
D. Unpreserved stools can remain at room temperature for up to 72 hours.

C. Fresh liquid stools are more likely to contain motile protozoan trophozoites that can be detectedin saline wet mounts. Urine in stool specimens can damage parasite morphology, as does freezing. Unpreserved stool specimens should only be left at room temperature up to a couple of hours before examining or placing into a preservative.

Cysts are the infective stage of this intestinal flagellate.
A. Balantidium coli
B. Dientamoebafragilis
C. Entamoeba coli
D. Giardia lamblia

D. Cysts are the infective stage of most intestinal parasites. D. fragilis is currently classified as a flagellate, even though it does not produce a flagellum. However, D. fragilis does not have a cyst stage. G. lamblia is the only flagellate in the list of protozoa with a cyst stage. B. coli is a ciliate and E. coli is an ameba.

Eggs or larvae recovered in the stool are not routinely used to diagnose infections caused by which one of the following helminths?
A. Trichinella spiralis
B. Strongyloides stercomlis
C. Necator americanus
D. Ascaris lumbricoides

A. Although T. spiralis adults live in the intestinal mucosa, they are rarely seen. The female deposits living larvae into the mucosa or lymphatic vessels, from which they normally enter the bloodstream and are disseminated throughout the body. They then burrow into muscle fibers. Although larvae may occasionally be liberated into the intestinal lumen, the definitive diagnostic procedure is the demonstration of larvae in skeletal muscle, not in feces.

Many parasites have different stages of growth within different hosts. The host where the sexual reproductive stage of the parasites exists is called the
A. Commensal
B. Definitive host
C. Intermediate host
D. Vector

B. In parasites with a sexual and asexual stage of development, the definitive host is the host in which the sexual stage of the parasite occurs. The intermediate host is the host in which the asexual stage of the parasite is found. Vectors are arthropods, like mosquitoes and ticks that transmit infectious agents. A commensal is an organism that benefits from an existence with a host but does not damage the host.

Species identification of an immature amebic cyst can be very difficult. The presence of a large glycogen mass is sometimes seen in
A. Dientamoeba fragilis
B. Endolimax nana
C. Entamoeba coli
D. Entamoeba histolytica

C. Young cysts of E. coli can have a large glycogen mass that pushes two nuclei to the outer edge of the cell. No cyst stage is known for D. fragilis. The cysts of E. nana and E. histolytica do not contain glycogen.

Which of the following is typical in cysts of lodamoeba biitschlii?
A. A glycogen mass
B. Blunt chromatoidal bars
C. Four nuclei with large karyosomes
D. Many ingested bacteria

A. Mature cysts of I. biitschlii are usually ovoid, with a single nucleus with a large eccentric karyosome. The cytoplasm contains a compact mass of glycogen, which appears as a clear area in unstained or permanently stained preparations but stains dark brown with iodine. Chromatoid bodies are not present.

Which of the following is the most important feature in differentiating cysts of Entamoeba histolytica from E. dispar?
A. Number of nuclei
B. Size of the cyst
C. Shape of the karyosome
D. Distinguishing surface antigens by immunologic assays

D. E. histolytica and E. dispar cannot be morphologically differentiated. The cyst stage of both organisms has four nuclei with a centrally located karyosome. E. histolytica is a wellrecognized intestinal parasite, whereas E. dispar is considered nonpathogenic. Immunologic assays to detect antigens or molecular biology assays are necessary to differentiate these two species.

Which of the following findings in a peripheral blood smear is especially associated with tissue-invading helminths but may also be found in a variety of allergic conditions and other diseases?
A. Eosinophilia
B. Leukopenia
C. Lymphocytosis
D. Neutropenia

A. Although the condition may vary from patient to patient, eosinophilia is often found in association with infections with tissue invading nematodes. Eosinophilia of 40-80% is not unusual in trichinosis and in visceral larva migrans. It may also be present in strongyloidiasis, early in Ascaris and hookworm infections, and in filariasis, which may also cause pulmonary eosinophilia.