CLT CLT Microbiology & Urinalysis 1 — Questions and Answers
Question 1: Which culture medium is selective for gram-negative enteric bacteria and inhibits gram-positive organisms?
- Blood agar
- MacConkey agar (Correct answer)
- Chocolate agar
- Sabouraud dextrose agar
Correct answer: MacConkey agar
MacConkey agar contains bile salts and crystal violet that inhibit gram-positive bacteria, making it selective for gram-negative enteric organisms.
Question 2: A urine culture grows >100,000 CFU/mL of a single organism. How should this result be interpreted?
- Contamination
- Significant bacteriuria consistent with urinary tract infection (Correct answer)
- Normal flora
- Requires repeat culture only
Correct answer: Significant bacteriuria consistent with urinary tract infection
A colony count of ≥100,000 CFU/mL (10^5) of a single organism in a clean-catch midstream urine specimen is the classic threshold for significant bacteriuria indicating a UTI.
Question 3: In the Gram stain procedure, what is the purpose of the decolorization step using acetone-alcohol?
- To fix bacteria to the slide
- To remove the crystal violet-iodine complex from gram-negative cells (Correct answer)
- To counterstain gram-positive cells
- To lyse the bacterial cell wall
Correct answer: To remove the crystal violet-iodine complex from gram-negative cells
Acetone-alcohol decolorizes gram-negative bacteria by dissolving their thin peptidoglycan layer and outer lipid membrane, releasing the crystal violet-iodine complex.
Question 4: Which organism is the most common causative agent of urinary tract infections in otherwise healthy women?
- Staphylococcus aureus
- Klebsiella pneumoniae
- Escherichia coli (Correct answer)
- Pseudomonas aeruginosa
Correct answer: Escherichia coli
Escherichia coli accounts for approximately 80–85% of uncomplicated UTIs in women due to its ability to adhere to uroepithelial cells.
Question 5: What does a positive catalase test result (bubbling with hydrogen peroxide) indicate about a gram-positive coccus?
- It is a Streptococcus species
- It is a Staphylococcus species (Correct answer)
- It is anaerobic
- It ferments glucose
Correct answer: It is a Staphylococcus species
Staphylococci are catalase-positive, while Streptococci are catalase-negative; this is a key initial test to differentiate these two genera of gram-positive cocci.
Question 6: Which component of a routine urinalysis evaluates the kidney's ability to concentrate urine?
- pH
- Specific gravity (Correct answer)
- Protein
- Glucose
Correct answer: Specific gravity
Specific gravity (normal 1.003–1.030) measures urine concentration and reflects the kidney's ability to dilute or concentrate urine based on hydration status.
CLT CLT Microbiology & Urinalysis 1
Which culture medium is selective for gram-negative enteric bacteria and inhibits gram-positive organisms?