Question 1

Which of the following best describes the primary difference between EMB agar and MacConkey agar in the isolation of enteric bacteria?

Accepted Answer

EMB agar uses eosin and methylene blue to inhibit Gram positive bacteria and differentiate lactose fermenters, while MacConkey agar uses bile salts and crystal violet for inhibition and neutral red for lactose fermentation detection.

Question 2

A patient presents with severe diarrhea described as 'rice water stools.' Which organism is most likely responsible, and what is a key laboratory feature for its identification?

Accepted Answer

Vibrio cholerae; Gram negative pleomorphic rods with severe watery diarrhea.

Question 3

Which of the following results from a Triple Sugar Iron (TSI) agar test indicates fermentation of only glucose?

Accepted Answer

Yellow butt and red slant

Question 4

A microbiologist observes a metallic green sheen on EMB agar. Which organism is most likely present, and what does this indicate about its metabolic properties?

Accepted Answer

E. coli; strong lactose fermentation

Question 5

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

Accepted Answer

It is oxidase positive and does not ferment glucose.

Question 6

A TSI slant shows a black butt and yellow slant after incubation. What does this result indicate?

Accepted Answer

Lactose and/or sucrose fermentation, with hydrogen sulfide production

Question 7

Which test in the IMViC series determines the ability of an organism to use citrate as its sole carbon source, and what is a positive result?

Accepted Answer

Citrate test; dark blue color on Simmons citrate agar

Question 8

A laboratory is attempting to distinguish between E. coli and Enterobacter aerogenes using IMViC tests. Which pattern of results would you expect for E. coli?

Accepted Answer

+ Indole, + Methyl Red, - Voges-Proskauer, - Citrate

Question 9

Which of the following best explains why EMB and MacConkey agars are considered both selective and differential?

Accepted Answer

They inhibit Gram positive bacteria and differentiate Gram negative rods based on lactose fermentation.

Question 10

A patient develops septicemia after eating undercooked eggs. Which organism is most likely responsible, and what is a key feature of its identification?

Accepted Answer

Salmonella; non-lactose fermenter and produces black colonies on TSI due to H$$_2S $$production

Question 11

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

Accepted Answer

A positive result is indicated by a red color after addition of reagents, showing butanediol fermentation.

Question 12

Which of the following best describes the appearance of lactose fermenters on MacConkey agar?

Accepted Answer

They form red or purple colonies due to acid production.

Question 13

A microbiologist is using the Enterotube system to identify an unknown Gram negative rod. What is the main advantage of this system?

Accepted Answer

It allows for the performance of 15 standard biochemical tests from a single colony.

Question 14

Which of the following is a unique characteristic of Pseudomonas aeruginosa colonies on Pseudosel agar?

Accepted Answer

They appear green to blue and fluoresce due to pyocyanin and fluorescein production.

Question 15

A Gram negative rod isolated from a burn wound produces a fruity, grape-like odor and grows at 42°C. Which organism is most likely present?

Accepted Answer

Pseudomonas aeruginosa

Question 16

Which of the following best explains the principle behind the indole test?

Accepted Answer

It detects the ability of bacteria to produce indole from tryptophan using Kovac’s reagent.

Question 17

A TSI slant shows a red slant and red butt after incubation. What does this result indicate?

Accepted Answer

No sugar fermentation

Question 18

Which of the following is a common source of Salmonella infection in humans?

Accepted Answer

Consumption of raw or undercooked eggs

Question 19

Which of the following best describes the role of phenol red in the TSI test?

Accepted Answer

It serves as a pH indicator to detect acid production from sugar fermentation.

Enterobacteriaceae and Pseudomonas: Identification and Differentiation in Clinical Microbiology

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