Microbiology 5 Flashcards

A clinical laboratory receives a urine culture specimen. After 24 hours of incubation on blood agar, small, non-hemolytic colonies are observed. Gram stain reveals gram-positive cocci in clusters. The isolate tests catalase-positive and coagulase-negative. Which organism is most likely responsible?

Answer: Staphylococcus epidermidis

Staphylococcus epidermidis is a coagulase-negative, catalase-positive gram-positive coccus that commonly appears in clusters. It is a frequent cause of urinary tract infections, especially in catheterized patients. S. aureus is coagulase-positive, S. pyogenes is catalase-negative and grows in chains, and Enterococcus faecalis is catalase-negative.

Which of the following bacterial structures is primarily responsible for resistance to phagocytosis by host immune cells?

Answer: Capsule

The bacterial capsule is a polysaccharide layer surrounding the cell wall that inhibits phagocytosis by preventing opsonization and engulfment by macrophages and neutrophils. Flagella provide motility, pili mediate attachment and conjugation, and endospores are survival structures — none primarily function to resist phagocytosis.

A 35-year-old patient presents with profuse, watery 'rice-water' stools after traveling to Southeast Asia. The causative organism produces a toxin that permanently activates adenylyl cyclase by ADP-ribosylating the Gs alpha subunit. Which pathogen is responsible?

Answer: Vibrio cholerae

Vibrio cholerae produces cholera toxin, which ADP-ribosylates the Gs alpha subunit of adenylyl cyclase, locking it in an active state. This causes massive cAMP accumulation in intestinal epithelial cells, leading to secretion of water and electrolytes and the characteristic profuse, watery 'rice-water' diarrhea. The other organisms cause diarrhea through different mechanisms.

In anaerobic bacteriology, which medium additive is used to lower the oxidation-reduction potential and ensure an anaerobic environment suitable for fastidious anaerobes?

Answer: Sodium thioglycolate

Sodium thioglycolate is a reducing agent commonly added to anaerobic media. It reacts with dissolved oxygen and lowers the oxidation-reduction (redox) potential, creating the low-oxygen environment necessary for the growth of fastidious anaerobes. Bile salts are selective agents, crystal violet inhibits gram-positive organisms, and potassium tellurite is used selectively for Corynebacterium diphtheriae.

A technologist is performing a Kirk-Bauer disk diffusion susceptibility test. The Mueller-Hinton agar plate is incubated overnight and zones of inhibition are measured. Which organism requires supplemented Mueller-Hinton agar for accurate susceptibility testing?

Answer: Streptococcus pneumoniae

Streptococcus pneumoniae is a fastidious organism that cannot grow adequately on standard Mueller-Hinton agar. It requires blood-supplemented Mueller-Hinton agar (5% sheep blood) for reliable disk diffusion susceptibility testing. Gram-negative organisms like E. coli, Pseudomonas, and Klebsiella grow well on standard Mueller-Hinton agar without supplementation.

A fungal culture from a patient with chronic pulmonary disease demonstrates dimorphism — growing as a mold at 25°C with tuberculate macroconidia and as a yeast at 37°C. This organism is most likely endemic to which geographic region?

Answer: Mississippi and Ohio River Valleys

Histoplasma capsulatum is a thermally dimorphic fungus endemic to the Mississippi and Ohio River Valleys. It forms mold with characteristic tuberculate (thick-walled, spiny) macroconidia at room temperature and converts to a yeast form at body temperature. Coccidioides is endemic to the desert Southwest, and Blastomyces overlaps in the Mississippi Valley and Great Lakes but has different morphology.