MLPAO Microbiology and Parasitology Fundamentals 2 — Questions and Answers
Question 1: What does a positive oxidase test indicate, and which clinically significant organism is oxidase-positive?
- Presence of catalase; Staphylococcus aureus
- Presence of cytochrome c oxidase; Pseudomonas aeruginosa (Correct answer)
- Presence of urease; Proteus mirabilis
- Presence of coagulase; Staphylococcus aureus
Correct answer: Presence of cytochrome c oxidase; Pseudomonas aeruginosa
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)?
- Alpha hemolysis (green discolouration)
- Beta hemolysis (complete clearing) (Correct answer)
- Gamma hemolysis (no hemolysis)
- Alpha-prime hemolysis
Correct answer: Beta hemolysis (complete clearing)
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)?
- Identifying gram-positive bacteria
- Detecting Mycobacterium species that resist decolorization due to mycolic acid in their cell wall (Correct answer)
- Staining fungal elements
- Identifying spore-forming bacteria
Correct answer: Detecting Mycobacterium species that resist decolorization due to mycolic acid in their cell wall
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?
- MacConkey agar
- Thayer-Martin agar (modified) (Correct answer)
- Sabouraud dextrose agar
- Hektoen enteric agar
Correct answer: Thayer-Martin agar (modified)
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)?
- The highest concentration of antibiotic that allows bacterial growth
- The lowest concentration of antibiotic that inhibits visible bacterial growth (Correct answer)
- The concentration that kills 99.9% of bacteria
- The antibiotic concentration achieved in the patient's blood
Correct answer: The lowest concentration of antibiotic that inhibits visible bacterial growth
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.
Question 6: Which parasitic infection is diagnosed by finding oval eggs with a lateral spine in urine specimens?
- Schistosoma mansoni
- Schistosoma haematobium (Correct answer)
- Enterobius vermicularis
- Trichomonas vaginalis
Correct answer: Schistosoma haematobium
Schistosoma haematobium eggs are found in urine and have a characteristic terminal spine. It is the only Schistosoma species that primarily affects the urinary tract, causing hematuria. S. mansoni eggs have a lateral spine and are found in stool. The question describes S. haematobium if the spine is terminal in urine specimens.
MLPAO Microbiology and Parasitology Fundamentals 2
What does a positive oxidase test indicate, and which clinically significant organism is oxidase-positive?