Question 1

Which of the following best describes the principle behind negative staining in microbiology?

Accepted Answer

The dye is negatively charged and repelled by the negatively charged cell surface, resulting in a stained background and clear cells.

Question 2

A student is using a microscope with a 10x eyepiece and a 40x objective lens. What is the total magnification being used?

Accepted Answer

$$10 	imes 40 = 400$$

Question 3

During Gram staining, what color will a Gram-negative cell appear after the decolorization step but before the counterstain is applied?

Accepted Answer

Colorless

Question 4

Which of the following is a correct safety precaution when working in a microbiology lab?

Accepted Answer

Always flame your loop before and after use to prevent contamination.

Question 5

A microbiologist streaks a plate for isolation but finds that no isolated colonies are present after incubation. Which of the following is the most likely explanation?

Accepted Answer

The streaking technique did not sufficiently dilute the bacteria across the plate.

Question 6

Which of the following media is both selective and differential, and is commonly used to isolate Gram-negative enteric bacteria?

Accepted Answer

Eosin methylene blue (EMB) agar

Question 7

A catalase test is performed by adding hydrogen peroxide to a bacterial colony. Which result indicates a positive catalase test, and what is the underlying reaction?

Accepted Answer

Bubbles form; the enzyme catalase converts $$2H_2O_2$$ into $$2H_2O + O_2$.

Question 8

In an ELISA, which of the following best explains how the presence of a specific antigen is detected?

Accepted Answer

A color change occurs when an enzyme-linked antibody binds to the antigen, indicating a positive result.

Question 9

A patient’s blood sample is tested using an agglutination assay for West Nile Virus (WNV). The sample agglutinates only in the presence of anti-WNV antibodies. What does this indicate?

Accepted Answer

The patient has antibodies against WNV, indicating prior exposure or infection.

Question 10

Which of the following best describes the morphology of yeast compared to molds and algae?

Accepted Answer

Yeast are unicellular, molds are multicellular with hyphae, and algae are photosynthetic eukaryotes.

Question 11

A microbiologist is testing the effectiveness of an antimicrobial drug using the disk diffusion method. The zone of inhibition around the disk is measured to be 18 mm. According to the standard chart, a zone of 18 mm is classified as 'intermediate.' What does this mean?

Accepted Answer

The organism may respond to the drug at higher doses, but the drug may not be reliably effective.

Question 12

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

Accepted Answer

It focuses light onto the specimen to improve illumination.

Question 13

Which of the following tests is used to detect the ability of bacteria to deaminate phenylalanine, and what is the positive result?

Accepted Answer

Phenylalanine deaminase test; a green color develops after adding ferric chloride.

Question 14

Which of the following statements about mannitol salt agar is correct?

Accepted Answer

It is selective for halotolerant (salt-tolerant) bacteria and differential for mannitol fermentation.

Question 15

A student performs a starch hydrolysis test and observes a clear zone around the bacterial growth after adding iodine. What does this indicate about the organism?

Accepted Answer

The organism produces amylase and hydrolyzes starch.

Question 16

Which of the following best describes the purpose of aseptic technique in microbiology?

Accepted Answer

To prevent contamination of cultures and the environment by unwanted microorganisms.

Question 17

In the OF glucose test, what result would indicate that an organism is a fermenter of glucose?

Accepted Answer

Both the open and closed tubes turn yellow.

Question 18

Which of the following best explains how an agglutination assay can distinguish between different viruses in a patient sample?

Accepted Answer

Antibodies specific to each virus will only cause agglutination if their target virus is present.

Question 19

Which of the following is a key difference between simple staining and Gram staining?

Accepted Answer

Simple staining reveals cell morphology, while Gram staining differentiates bacteria based on cell wall structure.

Question 20

A microbiologist observes a slide under high power and sees spiral-shaped bacteria. What is the correct term for this cell morphology?

Accepted Answer

Spirillum

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

Study Guide - Practice Questions

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