CMT Microbiology 4

Question 1

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

Accepted Answer

Clostridium perfringens

Answer

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

Question 2

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?

Accepted Answer

Bacteroides fragilis group

Answer

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

Question 3

Which pair of biochemical results is typical of Mycobacterium tuberculosis?

Accepted Answer

Niacin positive, nitrate positive

Answer

M. tuberculosis accumulates niacin and reduces nitrate.

Question 4

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

Accepted Answer

4 mm

Answer

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

Question 5

A 0.5 McFarland standard corresponds to approximately what bacterial concentration?

Accepted Answer

1.5 × 10^8 CFU/mL

Answer

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

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CMT - Certified Medical Technologist Practice Test

CMT Microbiology 4