Final answer:
MacConkey Agar is used to isolate and differentiate enteric bacteria by inhibiting Gram-positive bacteria and distinguishing those that can or cannot ferment lactose.
Step-by-step explanation:
MacConkey Agar (MAC) is a selective and differential medium designed to isolate and differentiate enteric bacteria based on their ability to ferment lactose. The agar contains bile salts and crystal violet, which inhibit the growth of Gram-positive bacteria, thus selecting for Gram-negative bacteria. It also contains lactose and neutral red, a pH indicator that turns pink if acid byproducts from lactose fermentation are present, identifying lactose-fermenting bacteria such as E. coli with bright pink colonies. Non-lactose fermenters like Serratia marcescens show colorless or transparent colonies. Lactose fermentation is a key test in differentiating enteric bacteria.
Other related media used for similar purposes include Sorbitol-MacConkey Agar (SMAC), which is used to detect E. coli O157:H7 by the absence of sorbitol fermentation, and Mannitol Salt Agar (MSA) which is selective for Staphylococcus species through mannitol fermentation and salt tolerance.