Question 1

Which of the following is an example of a normal microbiota species found on the skin, written in correct scientific format?

Accepted Answer

Staphylococcus epidermidis

Question 2

Which of the following best describes the difference between selective and differential media?

Accepted Answer

Selective media inhibit the growth of some organisms while allowing others to grow; differential media distinguish between organisms based on metabolic reactions.

Question 3

A black colony surrounded by a zone of white precipitation on Baird-Parker Agar indicates which of the following?

Accepted Answer

Tellurite reduction and lecithinase activity by Staphylococcus aureus

Question 4

Which compound in Mannitol Salt Agar makes it selective?

Accepted Answer

Sodium chloride (NaCl)

Question 5

The catalase test involves the breakdown of hydrogen peroxide. Which equation correctly represents this reaction?

Accepted Answer

$$2\ \mathrm{H}_2\mathrm{O}_2 \rightarrow 2\ \mathrm{H}_2\mathrm{O} + \mathrm{O}_2$$

Question 6

Why is the catalase test useful in differentiating between bacterial groups?

Accepted Answer

It distinguishes between Staphylococci (catalase-positive) and Streptococci (catalase-negative).

Question 7

Which type of epidemic is characterized by a rapid rise and fall in the number of cases, often associated with a single source of exposure?

Accepted Answer

Point-source epidemic

Question 8

Foodborne illnesses can be caused by two general mechanisms. Which of the following correctly identifies them?

Accepted Answer

Infection by live microbes and intoxication by microbial toxins

Question 9

Lipopolysaccharide (LPS) is a component of Gram-negative bacteria. Which statement about LPS in food safety is correct?

Accepted Answer

LPS is heat-stable and cannot be destroyed by normal cooking temperatures.

Question 10

Which of the following is a correct example of a foodborne pathogen commonly found in chicken, written in proper scientific format?

Accepted Answer

Campylobacter jejuni

Question 11

Coliforms are important indicators in food quality control. Which statement best defines coliforms?

Accepted Answer

Gram-negative, lactose-fermenting, rod-shaped bacteria that produce acid and gas at 35-37$$^\circC$$

Question 12

Which medium would you use to perform a total viable cell count in a food sample?

Accepted Answer

Standard Methods Agar (SMA)

Question 13

A food sample is diluted by adding 25 g of food to 225 mL of diluent. What is the dilution factor for this initial step?

Accepted Answer

$$\frac{25}{25+225} = \frac{25}{250} = 1:10$$

Question 14

Which of the following best describes the microscopic appearance and oxygen requirements of Campylobacter after Gram staining?

Accepted Answer

Gram-negative, curved rods; microaerophilic

Question 15

How does VRB-MUG agar allow for the detection of E. coli in food samples?

Accepted Answer

E. coli hydrolyzes MUG, producing a fluorescent product under UV light

Question 16

What is the primary purpose of using a stomacher machine in food microbiology sample preparation?

Accepted Answer

To homogenize the food sample and release microbes into suspension

Question 17

Which of the following is NOT a step in the Gram stain procedure?

Accepted Answer

Addition of mannitol salt

Question 18

A student observes a petri plate with three different colony types. Which of the following is the best way to describe the number of kinds of microorganisms present?

Accepted Answer

There are three kinds of microorganisms, each represented by a distinct colony morphology.

Question 19

Which of the following best describes a typical colony of Staphylococcus aureus on Mannitol Salt Agar?

Accepted Answer

Yellow colony with yellow medium due to mannitol fermentation

Question 20

Which of the following is NOT a correct observation to record when describing a bacterial colony?

Accepted Answer

Gram reaction

Question 21

A student is asked to perform a serial dilution and plate 0.1 mL of a $$10^{-4}$$ dilution onto an agar plate. After incubation, 50 colonies are counted. What is the concentration of bacteria in the original sample (CFU/mL)?

Accepted Answer

$$\frac{50}{0.1\ \mathrm{mL}} 	imes 10$$^{4}$$ = 5.0 	imes 10$$^{6}$$\ \mathrm{CFU/mL}$$

micro bio lab exam 1

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