Question

E. coli colonies grown in Eosine methylene blue agar appear as

• A. Greenish metallic sheen
• B. Blackish
• C. Pinkish
• D. Transparent

Hint:

Differential Media Colony Appearances:
E. coli on EMB Agar $\rightarrow$ GREENISH METALLIC SHEEN.
Salmonella on SS Agar $\rightarrow$ Colorless colonies with black $\text{H}_2\text{S}$ centers.
Vibrio cholerae on TCBS $\rightarrow$ Yellow sucrose colonies.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Diagnostic differential bacteriological media: on Eosin Methylene Blue (EMB) agar, rapid vigorous lactose fermentation by Escherichia coli precipitates eosin-methylene blue dye complexes, producing a pathognomonic bright greenish metallic sheen.
Key Formula or Approach:
\[ \textit{E. coli} \xrightarrow{\text{Vigorous Lactose Fermentation (Low pH)}} \text{Precipitates Eosin Y + Methylene Blue} \implies \mathbf{Bright \text{ } Greenish \text{ } Metallic \text{ } Sheen} \]

Step 2: Detailed Explanation:
Differential bacteriological characteristics on Eosin Methylene Blue (EMB Levine) Agar:
- EMB agar is a selective and differential culture medium used for detecting and confirming coliform bacteria and fecal contamination in water and seafood:
1. {Escherichia coli is a vigorous, rapid lactose-fermenting bacterium that produces high concentrations of organic mixed acids, dropping the medium pH below $4.5$.
2. This acidic environment causes the two indicator dyes (Eosin Y and Methylene Blue) to precipitate together within the colonies, producing characteristic, pathognomonic dark nucleated colonies with an intense Greenish Metallic Sheen under reflected light.
3. (Slow/weak fermenters like Enterobacter appear pink/mucoid; non-fermenters appear colorless).

Step 3: Final Answer:
Thus, E. coli colonies on EMB agar appear with Greenish metallic sheen, matching option (A).