Q.80. An example of a differential and selective medium in which colonies of Gram-negative bacteria
produce large amounts of acidic products and appear green with a metallic sheen is
(A) Blood agar
(B) EMB agar
(C) MacConkey agar
(D) Mannitol salt agar

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Introduction to Differential and Selective Media

In microbiology, differential and selective media play a crucial role in identifying specific bacteria based on their growth patterns and metabolic activities. Selective media inhibit unwanted microbes, while differential media distinguish bacteria through visible changes like color or sheen. A classic example involves Gram-negative bacteria producing acidic products from lactose fermentation, leading to unique colony appearances.

This article breaks down a key MCQ: An example of a differential and selective medium in which colonies of Gram-negative bacteria produce large amounts of acidic products and appear green with a metallic sheen is (A) Blood agar (B) EMB agar (C) MacConkey agar (D) Mannitol salt agar.

Correct Answer: (B) EMB agar
EMB (Eosin Methylene Blue) agar perfectly matches this description, making it essential for isolating enteric Gram-negative pathogens like Escherichia coli.

Why EMB Agar Shows Green Metallic Sheen on Gram-Negative Bacteria

EMB agar is both selective (targets Gram-negative enteric bacteria using bile salts and crystal violet, which inhibit Gram-positive growth) and differential (detects lactose fermentation).

• Strong lactose fermenters like E. coli produce large amounts of acidic products.

• These acids lower the pH, causing eosin dye to form a green metallic sheen on colonies— a hallmark sign.

• Non-fermenters appear colorless or pink.

This makes EMB agar ideal for stool sample analysis in clinical labs, helping spot coliforms quickly.

Detailed Explanation of All Options

Let’s compare each option to clarify why only EMB agar fits the “green metallic sheen” criterion.

• (A) Blood Agar
  Blood agar is primarily differential, not selective for Gram-negatives. It uses 5% sheep blood to detect hemolysis (alpha, beta, gamma) in a wide range of bacteria. Gram-negatives may grow but show no acidic products or green sheen—instead, you see clearing or greening from hemolysins. Not the answer.

• (B) EMB Agar (Correct)
  As explained, EMB agar selects Gram-negatives and differentiates via lactose acid production. The green metallic sheen from eosin dye uptake is specific to heavy fermenters. Key for identifying E. coli in water or food testing.

• (C) MacConkey Agar
  Like EMB, MacConkey is selective for Gram-negatives (bile salts/crystal violet) and differential for lactose fermentation. Fermenters form pink-red colonies due to neutral red dye and acid; no green metallic sheen. Great for Salmonella/Shigella but doesn’t match the description.

• (D) Mannitol Salt Agar (MSA)
  MSA is selective for Gram-positive staphylococci (7.5% NaCl inhibits others) and differential for mannitol fermentation. Fermenters turn the medium yellow (phenol red indicator); Gram-negatives barely grow. No green sheen or acid focus on Gram-negatives.

Applications and Exam Tips

EMB agar’s green metallic sheen is a high-yield fact for exams like NEET PG, CSIR NET, or biotech undergrad tests. Use it to differentiate E. coli (sheen) from Salmonella (colorless). Always pair with MacConkey for confirmation in labs.