Q: If a bacterium produces a positive bile esculin test, what is the chemical reaction responsible for the black coloration?

$$\text{Esculin} \xrightarrow{\text{hydrolysis}} \text{Esculetin} + \text{Glucose}$$ $$\text{Esculetin} + \text{Fe}^{3+} \rightarrow \text{Black complex}$$

Question 1

The image shows two bile esculin agar tubes after inoculation. The tube on the right is blackened, while the tube on the left remains yellow. What does the black coloration in the right tube indicate about the bacterial species inoculated?

Accepted Answer

The bacteria can hydrolyze esculin in the presence of bile, producing esculetin which reacts with ferric ions to form a black complex.

Question 2

Which group of bacteria is most likely to produce the black coloration seen in the right tube of the bile esculin test?

Accepted Answer

Enterococci

Question 3

What is the role of ferric citrate in the bile esculin test, as demonstrated by the black coloration in the right tube?

Accepted Answer

It acts as an indicator by reacting with esculetin to form a black complex.

Question 4

A student observes no color change in a bile esculin tube after incubation. Which of the following is the most likely explanation?

Accepted Answer

The bacteria cannot hydrolyze esculin in the presence of bile.

Question 5

Which component of the bile esculin agar is responsible for selecting against Gram negative bacteria?

Accepted Answer

Sodium azide

Question 6

If a bacterium produces a positive bile esculin test, what is the chemical reaction responsible for the black coloration?

Accepted Answer

$$	ext{Esculin} \xrightarrow{	ext{hydrolysis}} 	ext{Esculetin} + 	ext{Glucose}$$

$$	ext{Esculetin} + 	ext{Fe}^{3+} ightarrow 	ext{Black complex}$$

Question 7

Why is bile esculin agar considered both selective and differential?

Accepted Answer

It selects for enteric bacteria with bile salts and differentiates based on esculin hydrolysis.

Question 8

A clinical isolate produces a black precipitate in bile esculin agar. Which of the following is the most appropriate next step for further identification?

Accepted Answer

Perform a catalase test to differentiate Staphylococci from Streptococci.

Question 9

Which of the following best explains why a black color develops only in the presence of both bile and esculin?

Accepted Answer

Bile allows only certain bacteria to grow, and esculin hydrolysis produces esculetin, which reacts with ferric ions.

Question 10

A laboratory technician accidentally omits ferric citrate from the bile esculin agar. What is the most likely result?

Accepted Answer

No black coloration will develop, even if esculin is hydrolyzed.

Biochemical Tests for Bacterial Identification

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