Describe how each of the following would appear when grown on EMB agar. (4 marks)

Eosin Methylene Blue (EMB) agar is a selective and differential medium primarily used to isolate gram-negative bacteria, particularly enteric bacilli. It contains dyes (eosin Y and methylene blue) that inhibit the growth of most gram-positive organisms and allows for differentiation based on lactose fermentation. Here's how various types of bacteria would typically appear when grown on EMB agar: 1. **Lactose Fermenters (e.g., E. coli)**: - **Appearance**: These colonies typically appear as large, shiny, dark purple colonies with a green metallic sheen. The metallic sheen is a result of strong acid production from lactose fermentation, which lowers the pH and affects the dye interaction. 2. **Non-Lactose Fermenters (e.g., Salmonella, Shigella)**: - **Appearance**: Colonies of these bacteria usually appear as colorless or pale colonies. Since they do not ferment lactose, there is no acid production to react with the dyes, hence the agar remains its original color. 3. **Weak Lactose Fermenters (e.g., Enterobacter, Klebsiella)**: - **Appearance**: Colonies of weak lactose fermenters may appear as light pink to pale purple colonies. These organisms ferment lactose more slowly, producing less acid, so the color change is not as pronounced as in strong fermenters like E. coli. 4. **E. coli Variants with Shiga Toxin (e.g., E. coli O157:H7)**: - **Appearance**: Virulent strains that produce Shiga toxin may appear as colorless or pale colonies similar to non-fermenters, as they do not ferment lactose. This variant may require additional confirmatory testing for proper identification. In summary, when grown on EMB agar, typical observations are: - E. coli: dark purple/green metallic sheen. - Non-lactose fermenters (Salmonella, Shigella): colorless or pale. - Weak fermenters (Enterobacter, Klebsiella): light pink to pale purple. - Pathogenic E. coli (e.g., O157:H7): colorless or pale, similar to non-lactose fermenters.