Bacterial Identification: Gram-Positive Cocci, Gram-Negative Cocci, and Gram-Positive Bacilli

Comprehensive question-and-answer practice flashcards reviewing bacterial identification, epidemiology, virulence mechanisms, culture morphology, and differential testing for Gram-positive cocci, Gram-negative cocci, and Gram-positive bacilli.

What is the function of Protein A found on the cell wall of Staphylococcus aureus?

It binds to the Fc portion of IgG.

Which enterotoxins are primarily responsible for Staphylococcus aureus food poisoning, and what is the typical onset time of symptoms?

Enterotoxins A, D, and B; symptoms appear rapidly, approximately $2\text{--}8\,\text{hours}$ after ingestion of contaminated food.

What toxin causes Staphylococcal scalded skin syndrome (Ritter's disease), and which patient population is primarily affected?

Exfoliative toxins; it occurs primarily in newborns and young children.

What is the key virulence component in the pathogenesis of Staphylococcus epidermidis, and what clinical infections are characteristically associated with it?

Biofilm formation; it is a common cause of healthcare-acquired UTI and prosthetic valve endocarditis (frequently involving shunts and prosthetic devices).

What is the primary clinical significance of Staphylococcus saprophyticus?

It is associated with UTIs in young sexually active females and is the second most common cause of uncomplicated cystitis.

What antimicrobial disk does the CLSI M100 document recommend for the detection of methicillin resistance, and what is the gold standard method for MRSA detection?

Cefoxitin (which replaced oxacillin); the gold standard for MRSA detection is nucleic acid probes or PCR.

How are Streptococci classified according to Bergey's/Academic classification based on temperature growth requirements?

1. Pyogenic: does not grow at any temperature other than $37^\circ\text{C}$ (e.g., S. pyogenes, S. agalactiae)
2. Viridans: grows at $45^\circ\text{C}$ and $37^\circ\text{C}$ (e.g., S. mutans, S. mitis)
3. Lactic: grows at $10^\circ\text{C}$ and $37^\circ\text{C}$ (e.g., S. lactis)
4. Enterococcus: grows at $10^\circ\text{C}$, $45^\circ\text{C}$, and $37^\circ\text{C}$ (e.g., E. faecium, E. faecalis)

What are the distinguishing characteristics between Streptolysin O and Streptolysin S produced by Streptococcus pyogenes?

Streptolysin O (SLO) is oxygen labile and immunogenic; Streptolysin S is oxygen stable and nonimmunogenic.

What is the primary virulence factor of Streptococcus pneumoniae, and what characteristic sputum presentation is associated with its pneumonia?

Polysaccharide capsule; it causes pneumonia associated with "rusty" sputum.

Based on the Checkpoint Quiz shown, which group of tests would be most helpful in identifying a beta-hemolytic, catalase-negative, gram-positive coccus isolated from a neonate's CSF?

a. Bacitracin, CAMP, Hippurate hydrolysis

According to the Checkpoint Quiz shown, which characteristic should also be expected of a nonhemolytic, catalase-negative, gram-positive coccus that is PYR positive?

a. Bile esculin positive

According to the Checkpoint Quiz shown, which test can accurately differentiate Neisseria lactamica from Neisseria meningitidis?

a. ONPG

Why must gelatin be added to blood collection bottles when recovering pathogenic Neisseria species?

Pathogenic Neisseria is inhibited by sodium polyanethol sulfonate (SPS); gelatin is added to neutralize the inhibitory effects of SPS.

What plating pattern and temperature restriction apply to clinical specimens collected for the recovery of Neisseria gonorrhoeae?

Swabs should be inoculated directly onto transport systems (such as JEMBEC) rolled in a Z pattern; specimens must NOT be refrigerated.

How do colony morphology and presence of pili differ between Neisseria gonorrhoeae colony types T1–T2 and T3–T5?

Types T1 and T2 possess pili and form smaller, raised colonies; types T3 to T5 lack pili and form larger, flatter colonies.

What are the carbohydrate utilization patterns (glucose, maltose, lactose, sucrose) of Neisseria gonorrhoeae, Neisseria meningitidis, and Neisseria lactamica?

• N. gonorrhoeae: Glucose (+), Maltose (-), Lactose (-), Sucrose (-)
• N. meningitidis: Glucose (+), Maltose (+), Lactose (-), Sucrose (-)
• N. lactamica: Glucose (+), Maltose (+), Lactose (+), Sucrose (-)

What are the distinctive biochemical reactions and physical colony characteristics of Moraxella catarrhalis?

It is asaccharolytic (glucose, maltose, lactose, sucrose negative); DNAse (+), Butyrate esterase (+), and Nitrate reduction (+); colonies have a friable "hockey puck" consistency (can be pushed intact across agar) and a "wagon wheel" appearance.

What are the roles of Fragment A and Fragment B of the diphtheria toxin produced by Corynebacterium diphtheriae?

Fragment A is cytotoxic and disrupts protein synthesis; Fragment B binds to receptors on human cells and mediates the entry of Fragment A.

Which in vitro immunodiffusion test is used to confirm the presence of toxigenic Corynebacterium diphtheriae?

Elek test (toxigenic strains produce precipitin lines).

What colonial and microscopic features are produced by Corynebacterium diphtheriae on Cystine-Tellurite Blood Agar (CTBA) and Loeffler's medium?

On CTBA, it produces black or brownish colonies with a brown halo; on Loeffler's medium, it produces metachromatic granules.

Which Corynebacterium species is lipophilic, rapidly urease-positive, and most commonly associated with UTIs?

Corynebacterium urealyticum.

What are the motility characteristics of Listeria monocytogenes at room temperature versus $37^\circ\text{C}$?

It displays "umbrella-shaped" motility in semi-solid media and tumbling motility in wet preparations at room temperature, but is nonmotile at $37^\circ\text{C}$.

What type of CAMP reaction is produced by Listeria monocytogenes, and what color do its colonies appear on McBride Agar?

A "block type" CAMP reaction; colonies appear blue-green on McBride Agar.

Which nonspore-forming, catalase-negative, gram-positive bacillus produces $H_2S$ and shows "test-tube brush" growth in gelatin stab cultures?

Erysipelothrix rhusiopathiae.

What clinical syndrome and diagnostic microscopic finding are characteristic of Gardnerella vaginalis?

Bacterial vaginosis; diagnosed by the presence of "clue cells" on microscopic examination and a positive "Whiff Test".

Based on the top Checkpoint Quiz in the image, what is the most likely presumptive identification of a gram-positive branching filamentous organism recovered from sputum that is positive with a modified acid-fast stain?

B. Nocardia spp.

What colonial pigment on Sheep Blood Agar (SBA) and exposure route are characteristic of Rhodococcus?

Salmon pink pigment on SBA; acquired through contact with farm animals and feces.

What is Waterhouse-Friderichsen syndrome, and which organism is the classic causative agent?

A fulminant manifestation characterized by DIC, septic shock, and bilateral adrenal hemorrhage; caused by Neisseria meningitidis (meningococcemia).