MLPAO Microbiology and Parasitology Fundamentals 2

Question 1

What does a positive oxidase test indicate, and which clinically significant organism is oxidase-positive?

Accepted Answer

Presence of cytochrome c oxidase; Pseudomonas aeruginosa

Answer

The oxidase test detects the presence of cytochrome c oxidase, an enzyme in the electron transport chain. Pseudomonas aeruginosa is a clinically important oxidase-positive, gram-negative rod. The test uses a reagent (tetramethyl-p-phenylenediamine) that turns dark purple when oxidized by the enzyme.

Question 2

Which type of hemolysis on blood agar is produced by Group A Streptococcus (Streptococcus pyogenes)?

Accepted Answer

Beta hemolysis (complete clearing)

Answer

Group A Streptococcus (S. pyogenes) produces beta hemolysis, which is complete lysis of red blood cells creating a clear zone around colonies on blood agar. This is due to the production of streptolysins O and S. Beta-hemolytic streptococci are further classified by Lancefield grouping into groups A, B, C, F, and G.

Question 3

What is the purpose of the acid-fast stain (Ziehl-Neelsen or Kinyoun)?

Accepted Answer

Detecting Mycobacterium species that resist decolorization due to mycolic acid in their cell wall

Answer

The acid-fast stain detects mycobacteria (including M. tuberculosis) that retain carbol-fuchsin despite acid-alcohol decolorization due to their high mycolic acid content. Acid-fast bacilli (AFB) appear red against a blue (methylene blue) background. This stain is the first-line test for suspected tuberculosis.

Question 4

Which medium is specifically used for isolating Neisseria gonorrhoeae?

Accepted Answer

Thayer-Martin agar (modified)

Answer

Modified Thayer-Martin agar is a chocolate agar-based selective medium containing antibiotics (vancomycin, colistin, nystatin, and trimethoprim) that suppress normal flora while allowing N. gonorrhoeae to grow. The organism requires CO2 enrichment (5-10%) and grows best at 35-37°C.

Question 5

What is the minimum inhibitory concentration (MIC)?

Accepted Answer

The lowest concentration of antibiotic that inhibits visible bacterial growth

Answer

The MIC is the lowest concentration of an antimicrobial agent that prevents visible growth of a bacterium after overnight incubation. It is the reference standard for antimicrobial susceptibility testing and is used to guide antibiotic selection and dosing. MIC is determined by broth microdilution, Etest, or automated systems.

MLPAO - Medical Laboratory Professionals Association of Ontario Practice Test

MLPAO - Medical Laboratory Professionals Association of Ontario Practice Test

MLPAO Microbiology and Parasitology Fundamentals 2