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

A microbiologist streaks a sample on Mannitol Salt Agar (MSA). After incubation, yellow colonies surrounded by yellow medium are observed. What does this indicate?

Accepted Answer

The organism ferments mannitol, producing acid that turns the medium yellow.

Question 3

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

Accepted Answer

Selective media inhibit certain organisms, while differential media distinguish between organisms based on biochemical reactions.

Question 4

A catalase test is performed by adding hydrogen peroxide to a bacterial colony. Bubbles form immediately. What is the chemical reaction occurring?

Accepted Answer

$$2H_2O_2 \rightarrow 2H_2O + O_2$

Question 5

Which of the following best explains why the oxidase test is useful in microbiology?

Accepted Answer

It identifies bacteria that use oxygen as a terminal electron acceptor in the electron transport chain.

Question 6

During a foodborne illness investigation, a sample of chicken is tested and found to contain Campylobacter jejuni. What is the Gram stain result and oxygen requirement for this organism?

Accepted Answer

Gram-negative spiral, microaerophile

Question 7

Which of the following best defines a coliform?

Accepted Answer

A Gram-negative, lactose-fermenting, rod-shaped bacterium that produces acid and gas at 35-37°C.

Question 8

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

Accepted Answer

$$\frac{1}{10}$$

Question 9

Which medium would you use to detect fecal coliforms in a water sample, and what is the key differential reaction?

Accepted Answer

VRB-MUG Agar; fluorescence under UV due to $$\beta-$$glucuronidase activity

Question 10

Why is a stomacher machine used in food microbiology sample preparation?

Accepted Answer

To homogenize the food sample, ensuring even distribution of microorganisms.

Question 11

Which of the following best describes the appearance of Campylobacter under the microscope after Gram staining?

Accepted Answer

Pink, curved or spiral rods ("gull-wing" shape)

Question 12

Which of the following is a correct interpretation of a black colony with a white precipitate on Baird-Parker Agar?

Accepted Answer

The organism reduces tellurite and hydrolyzes lecithin.

Question 13

Which of the following best describes the two general ways microbes cause foodborne illness?

Accepted Answer

By direct infection (growth in host) and by intoxication (toxin production in food).

Question 14

Lipopolysaccharide (LPS) is a component of which type of bacteria, and what is its significance in food safety?

Accepted Answer

Gram-negative bacteria; it is heat-stable and can cause illness even after cooking.

Question 15

Which of the following is the most common foodborne pathogen found in chicken, written in correct scientific format?

Accepted Answer

Salmonella enterica

Question 16

A student is asked to describe a bacterial colony. Which of the following is NOT a required characteristic?

Accepted Answer

Gram reaction

Question 17

Which of the following best explains why total viable cell counts are important in food quality control?

Accepted Answer

They estimate the number of living microorganisms capable of growth under specific conditions.

Question 18

A Gram stain is performed on a throat swab. Which of the following observations would most likely indicate the presence of Streptococcus pyogenes?

Accepted Answer

Gram-positive cocci in chains

Question 19

Which of the following best describes the purpose of streaking for isolation on an agar plate?

Accepted Answer

To obtain single, isolated colonies from a mixed culture.

Question 20

A student is asked to calculate the final dilution after making two serial dilutions: first, 1 mL of sample into 9 mL of diluent, then 1 mL of that dilution into 99 mL of diluent. What is the final dilution factor?

Accepted Answer

$$\frac{1}{10} 	imes \frac{1}{100} = \frac{1}{1000}$$

Microbiology Lab Exam Study Guide: Skin, Throat, Food Quality, and Lab Techniques

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