What is the main purpose of MacConkey Agar?

MacConkey Agar is primarily used to isolate and differentiate Gram-negative enteric bacteria, particularly those that can ferment lactose. This medium contains bile salts and crystal violet, which inhibit the growth of Gram-positive bacteria, allowing for the selective isolation of Gram-negative organisms.

The presence of lactose and a pH indicator (usually neutral red) allows for differentiation based on lactose fermentation. Lactose fermenters produce acid, which lowers the pH and results in a color change, typically appearing pink on the agar. Non-lactose fermenters, on the other hand, do not produce acid, and thus maintain their original color or appear colorless. This feature makes MacConkey Agar not only a tool for isolation but also a differential medium that allows microbiologists to identify lactose fermenters and assess their fermentation capability, which is an important characteristic in microbiological diagnostics.

The other options involve different purposes that do not accurately represent the main function of MacConkey Agar. For instance, it does not isolate or differentiate Gram-positive bacteria, nor is it specifically designed for fastidious organisms or for facilitating the growth of anaerobes.