CHAPTER 6: BACTERIOLOGY - Flashcards

A curated set of practice Q&As covering key Gram-positive bacteria, their identifying features, virulence factors, and common laboratory tests drawn from the lecture notes.

What percentage of the population carries Staphylococcus aureus as resident flora and where is it most commonly carried?

Approximately 30% of people, primarily in the anterior nares.

What are the key identifying features of Staphylococcus aureus?

Gram-positive cocci in clusters; catalase positive; coagulase positive; beta-hemolysis on sheep blood agar; grows on most media; often mannitol fermenter on MSA (yellow colonies); detected surface proteins by latex agglutination (clumping factor and protein A).

What mechanism underlies MRSA resistance?

Altered penicillin-binding proteins that reduce binding of beta-lactam antibiotics.

What do VISA and VRSA stand for and what is their significance?

VISA = vancomycin-intermediate S. aureus; VRSA = vancomycin-resistant S. aureus; rare but concerning resistance patterns.

What are the major characteristics of coagulase-negative Staphylococci (CoNS)?

Very common skin flora; mostly nonpathogenic but can cause disease in immunocompromised individuals; gram-positive cocci in clusters; catalase positive; coagulase negative.

Which Staphylococcus species is most commonly pathogenic in device-related infections and what is a key distinguishing test?

Staphylococcus epidermidis; novobiocin sensitive; commonly associated with catheter-related infections.

How can Micrococcus be distinguished from Staphylococcus in the lab?

Micrococcus typically is (i) catalase positive and (ii) oxidase positive with yellow colonies and a positive modified oxidase test; Staphylococcus is catalase positive but oxidase negative.

What are the lab features of Group A Streptococcus (Streptococcus pyogenes)?

Beta-hemolytic on SBA; bacitracin sensitive; PYR positive; virulence factors include M protein and PYogenic exotoxins.

What toxins are associated with Streptococcus pyogenes and scarlet fever?

Streptococcal pyrogenic exotoxins (Spe A, Spe B, Spe C, Spe F) that act as superantigens.

How is Group A Streptococcus presumptively identified in the lab?

Bacitracin (A disk) sensitivity and PYR positivity; latex agglutination tests for group A antigen.

What are the key features of Group B Streptococcus (Streptococcus agalactiae)?

Normal flora of GI tract; important in OB/GYN infections; early onset neonatal sepsis; CAMP test and hippurate hydrolysis positive; PYR negative; bacitracin resistant.

What distinguishes Group B Strep in the lab?

CAMP test positive; hippurate hydrolysis positive; PYR negative; bacitracin resistant.

What characterizes Group D streptococci and viridans streptococci?

Group D: normal fecal/oral flora; bile-esculin positive; alpha- or nonhemolytic; associated with wound/UTI; viridans streptococci: normal oral flora, alpha-hemolytic, optochin resistant, bile insoluble; endocarditis on damaged valves.

What are the hallmark features of Streptococcus pneumoniae?

Alpha-hemolytic diplococci; optochin sensitive; bile soluble; rust-colored sputum; capsule; grows in 5-10% CO2; autolysis after 48 hours.

What are the defining tests for Enterococcus species?

Bile-esculin positive; growth in 6.5% NaCl; PYR positive; Lancefield group D antigen; often vancomycin-resistant enterococci (VRE) due to altered cell wall targets.

What is the mechanism of vancomycin resistance in Enterococcus (VRE)?

Altered peptidoglycan cross-link target (D-Ala-D-Ala to D-Ala-D-Lac or D-Ala-D-Ser). Most VRE are E. faecium.

What are Leuconostoc spp. known for in antibiotic resistance and key tests?

Vancomycin resistant; PYR negative; LAP negative; catalase negative.

What are Gemella spp. identified by lab tests?

PYR and LAP positive; bile-esculin negative; associated with various infections.

How does Listeria monocytogenes typically appear in motility tests?

Umbrella motility in semisolid media at room temperature and tumbling motility in wet mount; CAMP positive; beta-hemolysis on SBA.

What are the hallmark features of Corynebacterium diphtheriae?

Diphtheroid morphology; metachromatic granules; Elek test for toxin production; toxin encoded by phage; pseudomembrane formation in throat.

What media are used to grow Corynebacterium diphtheriae and how are toxigenic strains detected?

Cystine-tellurite and Tinsdale media show characteristic black colonies; Elek test detects toxin production; Loeffler broth supports pleomorphism.

What is notable about Corynebacterium jeikeium?

Nosocomial infections; multidrug-resistant; pyrazidamidase positive.

What is notable about Corynebacterium urealyticum?

Causes UTIs; rapid urease positive; grows slowly.

What Arcanobacterium species are clinically significant and how can A. haemolyticum be identified?

A. haemolyticum, A. pyogenes; CAMP inhibition test helps identify A. haemolyticum.

What is a key infection associated with Erysipelothrix rhusiopathiae?

Erysipeloid (cellulitis-like infection of the skin) after animal exposure; nonmotile; catalase negative; H2S positive.

What are the general features of Nocardia spp.?

Partially acid-fast, aerobic, branching Gram-positive filaments; slow growth; common in immunocompromised hosts.

What distinguishes Bacillus anthracis morphologically and clinically?

Large, nonhemolytic colonies with “Medusa-head” projections; nonmotile; spore-forming; zoonotic anthrax.

What distinguishes Bacillus cereus clinically and lab wise?

Food poisoning and wound infections; beta-hemolytic; motile; often contaminant; grows on SBA.

What is characteristic of Clostridium perfringens in culture and testing?

Gas gangrene; double zone beta-hemolysis on anaerobic SBA; lecithinase positive; negative for reverse CAMP test.

What are the major Clostridium species to know clinically?

C. tetani (tetanus); C. botulinum (botulism); C. perfringens (gas gangrene/food poisoning); C. difficile (pseudomembranous colitis).

What is the hallmark disease caused by Clostridium difficile and how is it diagnosed?

Antibiotic-associated pseudomembranous colitis; toxins A and B detected in stool; CCFA selective medium used for isolation.

What are Actinomyces israelii and its hallmark presentation?

Actinomycosis with sulfur granules; molar-tooth-like colonies; anaerobic; normal oral flora.

What is the clinical significance of Propionibacterium acnes?

Normal skin flora; common contaminant in blood cultures; anaerobic gram-positive bacillus.

What is Mobiluncus associated with clinically?

Bacterial vaginosis; curved anaerobic bacilli; clue cells on microscopy.

What are the general features of Lactobacillus in clinical relevance?

Normal flora of GI and female genital tract; acidifying vaginal environment; rarely pathogenic.

What are Eubacterium and Bifidobacterium described as clinically?

Normal, mostly nonpathogenic flora of oral and intestinal tracts.

What are anaerobic cocci types and their typical habitats?

Peptococcus (P. niger) and Peptostreptococcus (P. anaerobius, P. magnus = Finegoldia magna) among others; part of normal gut/oral flora; can cause polymicrobial infections.

What is the general characteristic of anaerobic Gram-negative cocci like Veillonella?

Small cocci; reduce nitrate to nitrite; nonfermenters; inhibited by kanamycin/colistin; resistant to vancomycin.

What is the importance of Chlamydia trachomatis clinically?

Causes lymphogranuloma venereum, trachoma, urethritis, conjunctivitis; diagnosed by cell cultures, DFA, NAATs, and serology.

What distinguishes Chlamydophila pneumoniae in disease?

Mild respiratory infections; human-to-human transmission; diagnosed by fluorescent antibodies; serology.

What disease is caused by Chlamydophila psittaci and how is it acquired?

Psittacosis (ornithosis); acquired by inhalation of dried bird droppings; occupational hazard for those in contact with birds.

What is the common diagnostic approach to Rickettsia/Ehrlichia infections?

Obligate intracellular; vector-borne (ticks, mites, lice); diagnosed by serology, PCR; immunohistology; Weil-Felix test is outdated.

What is unique about Mycoplasma and Ureaplasma in terms of cell walls and antibiotics?

Lack a cell wall; pleomorphic; resistant to beta-lactams; require special media (SP4, A8, Shepard’s 10B); M. pneumoniae causes walking pneumonia.

What are the characteristic growth requirements and diagnosis for Mycobacteria?

Acid-fast, high lipid content; require digestion/decontamination of samples; Niacin test, nitrate reductase, niacin accumulation; Runyon groups; thin, slow-growing; NAATs increasingly used.

What is the significance of the Runyon groups for Mycobacteria?

Group by growth rate and photoreactivity: slow growers (groups 1-3) including M. tuberculosis; rapid growers (group 4). (Note: Runyon groupings are older classification no longer universally used.)

Which toxin is associated with Corynebacterium diphtheriae and how is toxigenicity detected?

Diphtheria toxin; detected via Elek test for toxin production; toxigenic strains linked to phage carrying the toxin gene.

What is the role of the Hib vaccine in Haemophilus influenzae disease?

Vaccination against H. influenzae type b has greatly reduced invasive Hib disease, including meningitis in children.

What growth requirements distinguish Haemophilus spp. from other bacteria?

Requires X factor (hemin) and/or V factor (NAD); grows best on chocolate agar; satellite growth around S. aureus colonies on SBA.

What organism is commonly associated with dog/cat bites and bites from animals leading to cellulitis?

Pasteurella multocida; grows on SBA but not on most enteric media; puncture wounds from animals.

What organism is associated with erysipeloid after occupational exposure to animals?

Erysipelothrix rhusiopathiae; nonmotile, catalase negative; hydrogen sulfide positive.

What are the key features of Pseudomonas aeruginosa in the lab?

Oxidase positive; Gram-negative rod; motile; nonfermenter; grape-like odor; pyocyanin pigment (blue-green); grows on cetrimide agar; CF-related mucoid strains.

What distinguishes Moraxella catarrhalis among nonfermenters?

Gram-negative diplococci; oxidase positive; catalase positive; DNase positive; asaccharolytic.

What medium is used to isolate Legionella pneumophila and what is a common diagnostic test?

BCYE (buffered charcoal yeast extract) agar; urine antigen test and direct fluorescent antibody testing; requires cysteine for growth.

What is the clinical relevance of Gardnerella vaginalis and how is BV diagnosed?

Associated with bacterial vaginosis; small gram-variable coccobacilli; clue cells on microscopy; Nugent scoring; cultures are not routinely used.

What is the role of Campylobacter jejuni in human disease and its lab requirements?

Major cause of bacterial gastroenteritis; curved microaerophilic Gram-negative rod; grows best at 42°C; Campy media; darting motility; hippurate positive.

What are the hallmark features of Vibrio cholerae in lab culture?

Halophilic Gram-negative rod; grows on TCBS agar; V. cholerae O1 causes classical cholera with rice-water stools; enterotoxin (choleragen) stimulates cAMP and secretory diarrhea.

What is the key diagnostic approach for Legionella in clinical specimens?

Urine antigen test; culture on BCYE; special staining (silver); direct fluorescent antibody testing; poor Gram stain visibility.

What is the typical specimen used for Bordetella pertussis culture and why?

Nasopharyngeal swab on Regan-Lowe medium (or Bordet-Gengou) with selective antibiotics; PCR increasingly used for rapid diagnosis.

What is the basic lab approach to diagnosing Chlamydia trachomatis?

Cytology or cell culture for inclusions; NAATs (most common); DFA and serology also used; cannot be cultured on routine media.

Which Mycoplasma species are important human pathogens and what are their growth traits?

Mycoplasma pneumoniae (primary atypical pneumonia); Mycoplasma hominis; Ureaplasma urealyticum; lack cell wall; require special media; fried egg colony morphology for some species.

What stain is primarily used to visualize acid-fast Mycobacteria and what is required for staining?

Ziehl-Neelsen or Kinyoun stain; carbol-fuchsin as primary stain; heating required for Ziehl-Neelsen; Mycobacteria are acid-fast due to lipid-rich cell walls.

What is the difference between nontuberculous Mycobacteria and M. tuberculosis in Niacin testing?

M. tuberculosis is typically Niacin positive; many NTM are Niacin negative (varies by species). Niacin testing helps differentiate M. tuberculosis from other mycobacteria.

What are Runyon groups, and why are they less commonly used today?

Groups by growth rate and pigment production (photochromogens, scotochromogens, nonchromogens, rapid growers); older nomenclature; modern methods often rely on molecular tests instead.

What lab test is a classic presumptive indicator of Streptococcus pneumoniae in culture?

Optochin susceptibility (zone of inhibition) and bile solubility; S. pneumoniae is optochin sensitive and bile soluble.

What is the CAMP test used for and which organism gives a positive result?

Used to help identify Streptococcus agalactiae (Group B) by showing enhanced beta-hemolysis at the intersection with Staphylococcus aureus beta-lysin; positive CAMP indicates S. agalactiae.

What is a characteristic virulence factor of Streptococcus pyogenes?

M protein that inhibits phagocytosis; spectrum of toxins including Spe exotoxins; superantigen activity.

Which organism is the principal cause of neonatal meningitis and septicemia?

Streptococcus agalactiae (Group B Streptococcus).

What is a major virulence mechanism of Staphylococcus aureus in wound infections?

Exotoxins such as toxins (e.g., TSST-1 in toxic shock) and hemolysins; coagulase and DNase as virulence factors.

What organism is associated with a chalky white colony appearance and can cause endocarditis in older adults (viridans group)?

Viridans streptococci; alpha-hemolytic; optochin resistant; bile insoluble; normal oral flora; endocarditis on damaged valves.

Which organism is often implicated in catheter-related UTIs and is coagulase-negative?

Staphylococcus epidermidis; part of normal skin flora; causes device-associated infections.

What is a key lab feature of Enterococcus species that distinguishes them in bile-esculin slants?

Bile-esculin positive with growth; hydrolyze esculin; grow in 6.5% NaCl; PYR positive; Lancefield group D antigen.

What organism is an important cause of nosocomial infections and often demonstrates beta-hemolysis on SBA with “Medusa-head” colonies?

Bacillus anthracis exhibits large, nonhemolytic colonies with Medusa-head projections; Av: B. cereus shows beta-hemolysis.

What organism is the classic cause of gas gangrene and has subterminal spores?

Clostridium perfringens; beta-hemolysis on SBA under anaerobic conditions; lecithinase positive; reverse CAMP test (with S. agalactiae).

What laboratory test confirms toxin production by Corynebacterium diphtheriae?

Elek test detects diphtheria toxin production.

Which organism is the classical cause of diphtheria and forms a pseudomembrane in the throat?

Corynebacterium diphtheriae.

What organism is commonly involved in dog/cat bite infections and is often cultured on SBA but not on MacConkey?

Pasteurella multocida.

Which genus includes species that produce a purple pigment on agar and are associated with environmental and wound infections?

Chromobacterium violaceum.

What organism is a major cause of lobar pneumonia and is bile soluble and optochin sensitive?

Streptococcus pneumoniae.

What organism is commonly found as a contaminant in blood cultures due to skin flora and is associated with prosthetic devices?

Staphylococcus epidermidis.

Which organism is a common cause of BV and forms clue cells on vaginal smears?

Gardnerella vaginalis.

Which organism is associated with “satellitism” around S. aureus on SBA?

Haemophilus influenzae.

Which bacterium is characterized by a blue-pigmented pigment and often requires X and V factors to grow?

Haemophilus spp.; grows on chocolate agar; satellite phenomenon with S. aureus.

What organism is responsible for chancroid and can produce buboes?

Haemophilus ducreyi.

What lab test helps differentiate Neisseria gonorrhoeae from other Neisseria species by carbohydrate utilization?

N. gonorrhoeae ferments only glucose (not maltose, sucrose, or lactose).

What are the four serological characteristics of Enterobacteriaceae?

O (somatic) antigen; K (capsular) antigen; H (flagellar) antigen; Vi antigen (Salmonella Typhi).

What selective medium is used for primary isolation of Yersinia enterocolitica and Aeromonas?

CIN (Cefsulodin-irgasan-novobiocin) agar.

What distinguishes Pseudomonas aeruginosa pigment and its colony describe?

Pyocyanin pigment gives blue-green color; grape-like odor; mucoid strains in CF; oxidase positive; Gram-negative nonfermenter.

What is the difference between Corynebacterium jeikeium and Corynebacterium diphtheriae in clinical relevance?

C. jeikeium is nosocomial and multidrug-resistant; C. diphtheriae is toxigenic via phage and causes diphtheria.

Which organism is associated with swimming pool granuloma and grows best at 25-32°C?

Mycobacterium marinum.

Which media is used to selectively isolate Vibrio spp. and how are V. cholerae and V. parahaemolyticus differentiated on this medium?

TCBS agar; V. cholerae ferments sucrose (yellow colonies); V. parahaemolyticus does not (green/blue-green centers).

What is the hallmark test for obligate intracellular bacteria like Chlamydia and Rickettsia?

NAATs (nucleic acid amplification tests) are common; serology and culture in cell lines are used; DFA and antigen tests also employed.

What organism is associated with “sulfur granules” and a molar-tooth appearance on colonies?

Actinomyces israelii.

Which organism is the etiologic agent of relapsing fever and is transmitted by body lice?

Borrelia recurrentis.

Which organism is a common cause of thread-like Gram-positive rods with beta-hemolysis that can be mistaken for Streptococcus?

Listeria monocytogenes.

What organism is known for its ability to form a biofilm and cause endocarditis on damaged valves, often part of the viridans group?

Viridans streptococci.

What organism is the leading cause of candidial meningitis in neonates?

Neisseria meningitidis (Note: this card is intended for pneumococcal and meningitis context; if confusion arises, replace with a pneumococcal focus).

What is the diagnostic significance of the CAMP test in identifying Group B streptococci?

Positive CAMP test supports Streptococcus agalactiae (Group B) identification.

Which organism is associated with ‘picket fences’ Gram-stain pattern and metachromatic granules on blue stain?

Corynebacterium diphtheriae.

What organism is a common cause of cellulitis after animal bites and is often susceptible to penicillin?

Pasteurella multocida.

Which organism is typically seen as pale, branching Gram-positive bacilli in sulfur granules and is associated with actinomycosis?

Actinomyces israelii.