Microbiology Flashcards

An anaerobic, large boxcar-shaped gram-positive rod shows a double zone of beta-hemolysis on blood agar. What is the most likely organism?

Answer: Clostridium perfringens

C. perfringens shows double-zone hemolysis from theta and alpha toxins.

An anaerobic gram-negative rod grows in 20% bile and is resistant to kanamycin, vancomycin, and colistin special-potency disks. What is the likely identification?

Answer: Bacteroides fragilis group

The B. fragilis group is bile tolerant and resistant to all three special-potency disks.

Which pair of biochemical results is typical of Mycobacterium tuberculosis?

Answer: Niacin positive, nitrate positive

M. tuberculosis accumulates niacin and reduces nitrate.

In Kirby-Bauer disk diffusion testing, what is the required depth of Mueller-Hinton agar?

Answer: 4 mm

CLSI specifies 4 mm agar depth, since thicker agar gives falsely small zones and thinner agar falsely large zones.

A 0.5 McFarland standard corresponds to approximately what bacterial concentration?

Answer: 1.5 × 10^8 CFU/mL

A 0.5 McFarland turbidity equals about 1.5 × 10^8 CFU/mL.

A Staphylococcus aureus isolate is erythromycin resistant and clindamycin susceptible. A flattened clindamycin zone near the erythromycin disk indicates what?

Answer: Inducible clindamycin resistance

A positive D-test shows erm-mediated inducible MLSB resistance, so clindamycin should be reported as resistant.

For phenotypic ESBL confirmation, which result is positive?

Answer: Zone increase of ≥5 mm with clavulanate added to ceftazidime or cefotaxime

CLSI defines ESBL confirmation as a ≥5 mm zone increase when clavulanic acid is added.