Q: During a viable plate count, a student plates 0.1 ml of a $$10^{-4}$$ dilution and counts 120 colonies. What is the concentration of bacteria in the original sample (CFU/ml)?

$$120 \times 10$$^{4}$$ \times 10 = 1.2 \times 10$$^{7}$$ CFU/ml

Question 1

Which of the following best describes the purpose of a Gram stain in bacterial identification?

Accepted Answer

To distinguish between bacteria with thick and thin peptidoglycan layers in their cell walls

Question 2

A student observes purple, spherical cells arranged in tetrads under a microscope after performing a Gram stain. Which organism is most likely being observed?

Accepted Answer

Micrococcus luteus

Question 3

Which of the following stains would be most appropriate to identify Mycobacterium tuberculosis in a clinical sample?

Accepted Answer

Acid-fast stain

Question 4

What is the total magnification when using a 10x ocular lens and a 40x objective lens?

Accepted Answer

$$10 	imes 40 = 400$$

Question 5

Which of the following best describes a selective medium?

Accepted Answer

A medium that inhibits the growth of some organisms while allowing others to grow

Question 6

During a viable plate count, a student plates 0.1 ml of a $$10^{-4}$$ dilution and counts 120 colonies. What is the concentration of bacteria in the original sample (CFU/ml)?

Accepted Answer

$$120 	imes 10$$^{4}$$ 	imes 10 = 1.2 	imes 10$$^{7}$$ CFU/ml

Question 7

Which of the following is a goal of aseptic technique in the microbiology lab?

Accepted Answer

To prevent contamination of both the environment and the culture

Question 8

Which of the following best describes the function of the condenser on a compound light microscope?

Accepted Answer

It focuses light onto the specimen

Question 9

A student is preparing a smear from a solid medium. Which step is unique to this process compared to preparing a smear from a broth culture?

Accepted Answer

Adding a drop of water to the slide before transferring bacteria

Question 10

Which of the following best explains why agar plates are incubated upside down?

Accepted Answer

To prevent condensation from dripping onto the agar surface and spreading colonies

Question 11

Which of the following best describes a differential medium?

Accepted Answer

A medium that distinguishes between organisms based on their biochemical properties

Question 12

Which of the following is a correct statement about the use of the acid-fast stain?

Accepted Answer

It is used to identify bacteria with waxy mycolic acids in their cell walls

Question 13

Which of the following best describes the term 'parfocal' as it relates to microscope objectives?

Accepted Answer

Objectives that remain in focus when switching between magnifications

Question 14

Which of the following is a correct pairing of a bacterium and its typical laboratory identification?

Accepted Answer

Escherichia coli: Gram-negative coccobacilli, metallic green colonies on EMB

Question 15

Which of the following best describes the purpose of heat-fixing a bacterial smear before staining?

Accepted Answer

To adhere the bacteria to the slide and kill them

Question 16

A student is asked to calculate the total dilution after performing three serial 1:10 dilutions. What is the total dilution factor?

Accepted Answer

$$10^{-3}$$

Question 17

Which of the following best describes the difference between a pour plate and a spread plate in a viable plate count?

Accepted Answer

A pour plate uses 1 ml of inoculum, while a spread plate uses 0.1 ml

Question 18

Which of the following best describes the function of the mordant in the Gram stain procedure?

Accepted Answer

It forms a complex with the primary stain to help it remain in Gram-positive cells

Question 19

Which of the following is an example of a defined (synthetic) medium?

Accepted Answer

Glucose mineral salts agar

Question 20

Which of the following best describes the information gained from a simple stain?

Accepted Answer

Cell shape, size, and arrangement

Microbiology Lab Exam 1 Study Guide: Microscopy, Staining, Media, and Lab Techniques

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