Question 1

Which of the following best describes the primary selective and differential properties of MacConkey agar?

Accepted Answer

It selects for Gram negative bacteria and differentiates based on lactose fermentation.

Question 2

A water sample is tested for coliforms using the Most Probable Number (MPN) method. After incubation, several lactose broth tubes show acid and gas production. What does this result indicate?

Accepted Answer

The water sample likely contains coliform bacteria, indicating possible fecal contamination.

Question 3

Which of the following is a distinguishing biochemical feature of Pseudomonas aeruginosa compared to Enterobacteriaceae?

Accepted Answer

Oxidase positive reaction

Question 4

A TSI slant shows a yellow butt and red slant after incubation. What does this result indicate about the organism's metabolic capabilities?

Accepted Answer

The organism ferments only glucose.

Question 5

Which of the following is the correct sequence of tests in the IMViC series for differentiating Enterobacteriaceae?

Accepted Answer

Indole, Methyl Red, Voges-Proskauer, Citrate

Question 6

A patient presents with severe dehydration and 'rice water stool.' Which organism is most likely responsible, and what is its key pathogenic mechanism?

Accepted Answer

Vibrio cholerae; exotoxin production in the small intestine

Question 7

On EMB agar, a colony displays a metallic green sheen. What does this indicate about the organism?

Accepted Answer

It is a lactose fermenter, likely E. coli.

Question 8

Which of the following best explains why coliforms are used as indicators of water contamination?

Accepted Answer

They are normal gut flora, ferment lactose with acid and gas production, and survive outside the body.

Question 9

A TSI slant shows a black precipitate in the butt. What does this result indicate?

Accepted Answer

The organism produces hydrogen sulfide ($$H_2S).$$

Question 10

Which of the following is NOT a characteristic of Pseudomonas aeruginosa?

Accepted Answer

Fermentation of glucose

Question 11

A water sample is filtered using the membrane filter technique and the filter is placed on differential media. What does this method allow you to determine?

Accepted Answer

The direct count of live bacteria in the water sample

Question 12

Which test would you use to confirm the ability of an organism to use citrate as its sole carbon source, and what is a positive result?

Accepted Answer

Simmons citrate agar; development of a dark blue color

Question 13

Which of the following is a correct interpretation of a positive methyl red test?

Accepted Answer

The organism carries out mixed acid fermentation, lowering the pH.

Question 14

A patient is diagnosed with typhoid fever. Which organism is responsible, and how is it primarily transmitted?

Accepted Answer

Salmonella typhi; fecal contamination of water

Question 15

Which of the following best describes the purpose of the Voges-Proskauer test in the IMViC series?

Accepted Answer

To detect the production of acetylmethylcarbinol (acetoin) from glucose fermentation

Question 16

Which of the following is a key reason why EMB agar is used in the confirmed test for coliforms?

Accepted Answer

It differentiates lactose fermenters by color change due to acid production.

Question 17

A Gram stain of a bacterial isolate from a water sample shows pink rods. Which of the following is the most likely identity of the organism?

Accepted Answer

Gram negative bacilli

Question 18

Which of the following is a plausible explanation for why a lactose fermenting colony appears pink on MacConkey agar?

Accepted Answer

Acid production from lactose fermentation causes the pH indicator to turn pink.

Question 19

Which of the following is NOT a reason why coliforms are used as indicators of fecal contamination in water?

Accepted Answer

They are always pathogenic.

Question 20

A laboratory technician observes a green, fluorescent colony on Pseudosel agar from a wound swab. Which organism is most likely present, and what is a key identifying test?

Accepted Answer

Pseudomonas aeruginosa; oxidase test

Isolation and Identification of Enterobacteriaceae and Pseudomonas: Laboratory Methods and Clinical Relevance

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