CLS CLS Clinical Microbiology and Immunology 1 — Questions and Answers
Question 1: Which organism is identified by oxidase-positive results, blue-green pigment (pyocyanin), and a grape-like odor?
- Escherichia coli
- Pseudomonas aeruginosa (Correct answer)
- Klebsiella pneumoniae
- Proteus mirabilis
Correct answer: Pseudomonas aeruginosa
Pseudomonas aeruginosa produces pyocyanin (blue-green pigment), has a characteristic grape-like odor, is oxidase-positive, and does not ferment lactose.
Question 2: The catalase test is used to differentiate which two clinically important genera?
- Gram-positive from Gram-negative bacteria
- Staphylococcus (positive) from Streptococcus (negative) (Correct answer)
- Aerobic from anaerobic organisms
- Lactose fermenters from non-fermenters
Correct answer: Staphylococcus (positive) from Streptococcus (negative)
The catalase test differentiates Staphylococcus (catalase-positive, bubbles with H₂O₂) from Streptococcus (catalase-negative, no bubbles).
Question 3: Which agar medium is both selective (for gram-negatives) and differential (for lactose fermentation)?
- Blood agar
- Chocolate agar
- MacConkey agar (Correct answer)
- Mannitol salt agar
Correct answer: MacConkey agar
MacConkey agar selects for gram-negative bacteria using bile salts and crystal violet, and differentiates lactose fermenters (pink colonies) from non-fermenters (colorless).
Question 4: The VDRL (Venereal Disease Research Laboratory) test is used to screen for which sexually transmitted infection?
- HIV
- Hepatitis B
- Syphilis (Correct answer)
- Gonorrhea
Correct answer: Syphilis
The VDRL is a non-treponemal flocculation test that screens for syphilis by detecting reagin antibodies produced in response to Treponema pallidum infection.
Question 5: Which gram-negative rod produces urease, causes struvite kidney stones, and displays 'swarming' motility on agar?
- Escherichia coli
- Klebsiella pneumoniae
- Proteus mirabilis (Correct answer)
- Enterococcus faecalis
Correct answer: Proteus mirabilis
Proteus mirabilis hydrolyzes urea with urease, raising urine pH and promoting struvite (magnesium ammonium phosphate) stone formation, and characteristically swarms on non-inhibitory agar.
Question 6: What is the gold standard method for definitive diagnosis of Mycobacterium tuberculosis infection?
- Tuberculin skin test (TST)
- Chest X-ray
- Mycobacterial culture (Correct answer)
- QuantiFERON-TB Gold test
Correct answer: Mycobacterial culture
Mycobacterial culture remains the gold standard because it confirms viable organisms and allows antibiotic susceptibility testing for drug-resistant TB.
CLS CLS Clinical Microbiology and Immunology 1
Which organism is identified by oxidase-positive results, blue-green pigment (pyocyanin), and a grape-like odor?