Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): On Eosin Methylene Blue (EMB) agar, some lactose fermenting species of enterobacteria, such as Escherichia coli will grow into pink colonies, but Salmonella does not.
Reason (R): Members of genus Salmonella are non lactose fermenters.
In light of the above statements, choose the most appropriate answer from the options given below.

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A).
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A).
• C. (A) is correct but (R) is not correct.
• D. (A) is not correct but (R) is correct.

Hint:

To avoid confusion, always associate these classic diagnostic appearances:
- E. coli on MacConkey Agar \(\rightarrow\) Pink colonies (due to neutral red indicator).
- E. coli on EMB Agar \(\rightarrow\) Green metallic sheen on dark colonies.
- Non-lactose fermenters (Salmonella, Shigella) \(\rightarrow\) Colorless colonies on both media.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
This question deals with diagnostic microbiology, focusing on the selective and differential properties of Eosin Methylene Blue (EMB) agar.
EMB agar is widely used to isolate and differentiate Gram-negative enteric bacilli (enterobacteria) based on their ability to ferment lactose and sucrose.
The medium contains eosin Y and methylene blue dyes, which serve both as selective agents inhibiting Gram-positive bacteria and as pH indicators.

Step 2: Detailed Explanation:
Let us critically evaluate both the Assertion (A) and the Reason (R):
- Assertion (A): "On Eosin Methylene Blue (EMB) agar, some lactose fermenting species of enterobacteria, such as Escherichia coli will grow into pink colonies, but Salmonella does not."
This statement is incorrect.
On EMB agar, Escherichia coli is a rapid and highly vigorous lactose fermenter.
It produces large quantities of acidic byproducts during fermentation, which dramatically lowers the local pH.
This acidic environment causes the coprecipitation of the eosin and methylene blue dyes within the colony.
As a result, E. coli colonies do not appear pink; instead, they develop a highly distinctive, classic dark blue-black center with a characteristic green metallic sheen.
Slow or weak lactose fermenters (such as Klebsiella or Enterobacter species) produce pink or purple mucoid colonies on EMB.
It is on MacConkey agar (not EMB) that rapid lactose fermenters like E. coli form bright pink colonies due to the neutral red pH indicator.
Thus, the assertion contains a scientifically inaccurate description of E. coli's colony morphology on EMB agar.
- Reason (R): "Members of genus Salmonella are non lactose fermenters."
This statement is correct.
Members of the genus Salmonella are classic non-lactose fermenters (NLFs).
Since they do not ferment lactose, they do not produce acidic end-products, and the indicators remain unaltered.
On EMB agar, Salmonella colonies appear colorless, translucent, or pale amber, taking on the natural color of the medium.
Since Assertion (A) is incorrect and Reason (R) is correct, the correct option is (D).

Step 3: Final Answer:
Assertion (A) is not correct, but Reason (R) is correct.
Therefore, the correct choice is Option (D).