Question 1

Which of the following best describes the function of Eosin Methylene Blue (EMB) agar?

Accepted Answer

It is a selective medium for Gram-negative bacteria and differentiates lactose fermenters.

Question 2

A microbiologist streaks a sample on Mannitol Salt Agar (MSA) and observes yellow colonies surrounded by yellow media. What is the most likely identity of the organism?

Accepted Answer

Staphylococcus aureus

Question 3

Which of the following equations correctly represents the reaction catalyzed by catalase in the catalase test?

Accepted Answer

$$2H_2O_2 \rightarrow 2H_2O + O_2$

Question 4

A blood agar plate shows complete clearing around colonies. What type of hemolysis is this, and which organism is most likely responsible?

Accepted Answer

Beta-hemolysis; Streptococcus pyogenes

Question 5

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

Accepted Answer

Their thick lipid layer dissolves during decolorization, allowing safranin to stain the cells.

Question 6

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

Accepted Answer

The organism produces amylase and hydrolyzes starch.

Question 7

Which of the following is true regarding the Voges-Proskauer (VP) test?

Accepted Answer

A red color after adding Barritt’s reagents indicates the presence of acetoin.

Question 8

Which of the following best describes the result and interpretation of a Simmon’s Citrate Agar slant that turns prussian blue?

Accepted Answer

Positive for citrase production; organism can use citrate as sole carbon source.

Question 9

A student performs a gelatinase test and finds that the medium remains liquid after refrigeration. What does this indicate?

Accepted Answer

The organism is gelatinase positive and hydrolyzes gelatin.

Question 10

Which of the following organisms is urease positive and would turn the urea agar bright pink within 24 hours?

Accepted Answer

Proteus vulgaris

Question 11

Which of the following statements about the Gram stain procedure is correct?

Accepted Answer

Gram’s iodine acts as a mordant, forming a complex with crystal violet.

Question 12

A microbiologist is testing an unknown bacterium. The catalase test is positive, and the oxidase test is negative. Which genus is most likely?

Accepted Answer

Staphylococcus

Question 13

Which of the following best explains why basic dyes are commonly used to stain bacterial cells?

Accepted Answer

Bacterial cells are slightly negatively charged and attract the colored cation of basic dyes.

Question 14

Which of the following is a correct interpretation of a Durham tube sugar fermentation test that is yellow with a gas bubble in the tube?

Accepted Answer

Positive for fermentation; acid and gas produced.

Question 15

Which of the following is a distinguishing feature of endospore-forming bacteria?

Accepted Answer

They can survive extreme conditions such as heat, drought, and radiation.

Question 16

A microbiologist observes a blue to dark purple color after adding oxidase reagent to a bacterial colony. What does this indicate?

Accepted Answer

The organism is oxidase positive and contains cytochrome C.

Question 17

Which of the following best describes the purpose of the Schaeffer-Fulton stain?

Accepted Answer

To visualize endospores and distinguish them from vegetative cells.

Question 18

A patient’s urine sample is streaked on EMB agar. After incubation, green metallic sheen colonies are observed. What is the most likely organism?

Accepted Answer

Escherichia coli

Question 19

Which of the following is the correct sequence of reagents used in the Gram stain procedure?

Accepted Answer

Crystal violet, Gram’s iodine, alcohol, safranin

Question 20

A microbiologist is interpreting an antibiotic sensitivity test. The zone of inhibition around a kanamycin disk is 20 mm. According to the standard, what is the interpretation?

Accepted Answer

Sensitive

Microbiology Lab Techniques: Media, Biochemical Tests, and Staining

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