Q: A urine culture is performed using a 10 µL calibrated loop. After incubation, 45 colonies are counted. Calculate the bacterial concentration in cfu/mL.

$$\frac{45}{0.01} = 4500$$ cfu/mL$$\rightarrow$$ $$45 \times 100 = 4500$$ cfu/mL

Q: What is the equation for calculating viral load (PFU/mL) in a plaque assay?

$$\text{Viral Load (PFU/mL)} = \frac{\text{Number of Plaques}}{\text{Dilution Factor} \times \text{Volume of Diluted Virus Added}}$$

Q: A plaque assay is performed using 0.1 mL of a $$10^{-2}$$ dilution, and 38 plaques are counted. What is the viral load (PFU/mL)?

$$\frac{38}{0.01 \times 0.1} = 38,000$$ PFU/mL

Question 1

Refer to the image showing six tubes with varying turbidity and surface growth (Figure 2-31). Which tube best demonstrates the growth pattern of an obligate aerobe, and why?

Accepted Answer

The tube with heavy growth only at the top, because obligate aerobes require oxygen for growth.

Question 2

In Figure 1-5, three tubes are shown: one with a slant, one with a butt, and one with a liquid medium. Which tube is most appropriate for testing motility, and what is the principle behind this test?

Accepted Answer

The liquid medium tube, because motile bacteria will cause turbidity throughout the medium.

Question 3

A MacConkey agar plate (see image) shows both pink and colorless colonies. What does this indicate about the bacteria present?

Accepted Answer

Both lactose fermenters (pink) and non-lactose fermenters (colorless) are present.

Question 4

In the citrate utilization test (see image with green and blue tubes labeled A and B), which tube is positive, and what is the pH indicator used?

Accepted Answer

Tube B is positive; the pH indicator is bromothymol blue.

Question 5

A black precipitate forms in a SIM tube (see image with tubes A and B, one black, one tan). What does this indicate about the microorganism?

Accepted Answer

The microorganism reduces sulfur to hydrogen sulfide.

Question 6

A blood agar plate shows complete clearing around colonies (see image). What type of hemolysis is this, and what does it indicate?

Accepted Answer

Beta hemolysis; complete lysis of red blood cells.

Question 7

You perform an antimicrobial susceptibility test using Mueller-Hinton agar. The zone of inhibition for gentamicin (10 µg) is 14 mm. According to the provided chart, how would you interpret this result for Staphylococcus species?

Accepted Answer

Intermediate

Question 8

A urine culture is performed using a 10 µL calibrated loop. After incubation, 45 colonies are counted. Calculate the bacterial concentration in cfu/mL.

Accepted Answer

$$\frac{45}{0.01} = 4500$$ cfu/mL$$ightarrow$$ $$45 	imes 100 = 4500$$ cfu/mL

Question 9

What is the equation for calculating viral load (PFU/mL) in a plaque assay?

Accepted Answer

$$	ext{Viral Load (PFU/mL)} = \frac{	ext{Number of Plaques}}{	ext{Dilution Factor} 	imes 	ext{Volume of Diluted Virus Added}}$$

Question 10

A plaque assay is performed using 0.1 mL of a $$10^{-2}$$ dilution, and 38 plaques are counted. What is the viral load (PFU/mL)?

Accepted Answer

$$\frac{38}{0.01 	imes 0.1} = 38,000$$ PFU/mL

Question 11

Which patient is responding better to antiviral medication based on the viral load data provided (see table with Patient 1 and Patient 2 viral loads over time)?

Accepted Answer

Patient 1, because their viral load drops to zero by 3 months.

Question 12

What is the minimum bacterial count (cfu/mL) in a urine culture that is interpreted as a positive UTI?

Accepted Answer

$$\geq 10^5$ cfu/mL

Question 13

Which of the following is a selective agent in MacConkey agar, and what does it select for?

Accepted Answer

Crystal violet; selects for Gram-negative bacteria

Question 14

In a SIM test, after adding Kovac's reagent, a red ring forms at the top of the tube. What does this indicate?

Accepted Answer

Positive for indole production

Question 15

Which of the following is NOT a standardized component of the Kirby-Bauer antimicrobial susceptibility test?

Accepted Answer

Type of bacterial species

Question 16

What is the purpose of diluting a patient sample before performing a plaque assay?

Accepted Answer

To obtain a countable number of plaques on the plate

Question 17

Which of the following best describes a provirus?

Accepted Answer

Viral DNA integrated into the host genome

Microbiology Lab Exam 2 Review – Biochemical and Differential Tests, Antimicrobial Susceptibility, Plaque Assay, and Urine Cultures

Study Guide - Practice Questions

Study Guide - Flashcards