CLT CLT Microbiology & Urinalysis

Question 1

Which culture medium is selective for gram-negative enteric bacteria and inhibits gram-positive organisms?

Accepted Answer

MacConkey agar

Answer

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?

Accepted Answer

Significant bacteriuria consistent with urinary tract infection

Answer

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?

Accepted Answer

To remove the crystal violet-iodine complex from gram-negative cells

Answer

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?

Accepted Answer

Escherichia coli

Answer

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?

Accepted Answer

It is a Staphylococcus species

Answer

Staphylococci are catalase-positive, while Streptococci are catalase-negative; this is a key initial test to differentiate these two genera of gram-positive cocci.

(CLT) Clinical Laboratory Technologist Certification Practice Test

(CLT) Clinical Laboratory Technologist Certification Practice Test

CLT CLT Microbiology & Urinalysis