Q: Which of the following equations correctly calculates colony-forming units per milliliter ($$CFU/mL) $$when 0.1 mL of a $$10^{-4}$$ dilution yields 50 colonies on a plate?

$$CFU/mL = \frac{\text{Number of colonies}}{\text{Volume plated (mL)}} \times \text{Dilution factor}$$

Question 1

A bacterial cell is placed in a solution and begins to swell and eventually bursts. Which type of solution was the cell most likely placed in, and what is the direction of water movement?

Accepted Answer

Hypotonic solution; water moves into the cell

Question 2

Which of the following best describes the function of catalase in a bacterial cell?

Accepted Answer

It catalyzes the conversion of hydrogen peroxide into water and oxygen

Question 3

A microbiologist is culturing a bacterium that grows best at 55°C. Which term best describes this organism?

Accepted Answer

Thermophile

Question 4

During Gram staining, what is the primary function of the mordant (iodine)?

Accepted Answer

To form a complex with crystal violet, making it less soluble

Question 5

Which of the following statements about endospores is correct?

Accepted Answer

Endospores are resistant to heat due to the presence of dipicolinic acid and calcium

Question 6

A culture medium contains 7.5% NaCl and phenol red. Only Staphylococcus species grow, and colonies that ferment mannitol turn yellow. What type of medium is this?

Accepted Answer

Selective and differential medium

Question 7

Which of the following best explains why acid-fast bacteria, such as Mycobacterium, retain carbolfuchsin after acid-alcohol decolorization?

Accepted Answer

Their cell walls contain high concentrations of mycolic acids

Question 8

A student observes a bacterial sample under a microscope using oil immersion at 1000x total magnification. What is the main purpose of using oil immersion in this context?

Accepted Answer

To increase the refractive index and improve resolution

Question 9

Which of the following equations correctly calculates colony-forming units per milliliter ($$CFU/mL) $$when 0.1 mL of a $$10^{-4}$$ dilution yields 50 colonies on a plate?

Accepted Answer

$$CFU/mL = \frac{	ext{Number of colonies}}{	ext{Volume plated (mL)}} 	imes 	ext{Dilution factor}$$

Question 10

A bacterial culture is described as 'microaerophilic.' Which growth condition would best support its growth?

Accepted Answer

Atmosphere with low oxygen concentration

Question 11

Which of the following best describes the difference between basic and acidic dyes in bacterial staining?

Accepted Answer

Basic dyes have positively charged chromophores and stain the cell

Question 12

A patient is diagnosed with a bacterial infection. The physician prescribes penicillin. What is the primary mechanism by which penicillin acts on bacteria?

Accepted Answer

Inhibits cell wall synthesis by interfering with peptidoglycan cross-linking

Question 13

Which of the following best explains why Gram-negative bacteria appear pink after Gram staining?

Accepted Answer

Their thin peptidoglycan layer allows the crystal violet-iodine complex to be washed out, so they take up the counterstain

Question 14

Which of the following is a correct statement about Louis Pasteur's contributions to microbiology?

Accepted Answer

He disproved spontaneous generation using swan-neck flask experiments

Question 15

A microbiologist wants to isolate a pure culture from a mixed sample. Which plating technique is most appropriate for this purpose?

Accepted Answer

Streak plate

Question 16

Which of the following best describes the function of a capsule in bacterial cells?

Accepted Answer

It protects the cell from phagocytosis and desiccation

Question 17

A laboratory technician uses filtration to sterilize a solution. Which type of material is most likely being removed by this method?

Accepted Answer

Bacterial cells

Question 18

A bacterial sample is described as 'TNTC' after plating. What does this indicate about the sample?

Accepted Answer

There are too many colonies to count

Question 19

Which of the following best describes the role of thioglycolate in culture media?

Accepted Answer

It reduces oxygen to create an anaerobic environment

Question 20

A scientist observes Brownian movement in a wet mount preparation. What is the most accurate interpretation of this observation?

Accepted Answer

The bacteria are moving due to random collisions with water molecules

Lab Exam #1 Review: Core Concepts in Microbiology

Study Guide - Practice Questions

Study Guide - Flashcards