Final answer:
Salmonella does not ferment lactose and will therefore appear as colorless colonies on Nutrient Agar, MacConkey Agar, and EMB Agar. On XLD Agar, it will form red colonies. The coloration on agar plates helps distinguish between different types of bacteria.
Step-by-step explanation:
Salmonella is a gram-negative bacterium that does not ferment lactose. On various agar plates used to differentiate and identify bacteria, its appearance will vary:
- On Nutrient Agar, Salmonella will appear as colorless or pale colonies as this medium is not differential.
- On MacConkey Agar, Salmonella colonies will also be colorless because it cannot ferment lactose, unlike some other bacteria that would appear pink if they could.
- EMB Agar contains indicator dyes that distinguish between lactose fermenters and non-lactose fermenters. Salmonella, being a non-lactose fermenter, will form colorless colonies.
- XLD Agar is designed for the isolation of Salmonella and Shigella; on this medium, Salmonella typically produces red colonies with or without black centers.
EMB Agar, being a selective medium with eosin and methylene blue, inhibits the growth of gram-positive bacteria. Likewise, MacConkey Agar selectively grows gram-negative bacteria and differentiates lactose fermenters from non-fermenters. When working with these mediums, it's important to understand the expected outcomes for the specific bacteria being tested.