Final answer:
MacConkey agar is a selective and differential medium that inhibits Gram-positive bacteria and differentiates between lactose-fermenting and non-lactose fermenting enteric bacteria, with the former producing hot pink colonies.
Step-by-step explanation:
MacConkey agar serves as a selective and differential medium, primarily used for the isolation and differentiation of enteric bacteria. It contains bile salts and crystal violet, substances that inhibit the growth of many Gram-positive bacteria, hence supporting the growth of Gram-negative bacteria. Specifically, it favors Enterobacteriaceae, which are adapted to survive in the presence of bile salts due to their natural habitat in the intestine. Additionally, MacConkey agar allows the differentiation between lactose-fermenting and non-lactose fermenting bacteria. This is accomplished through the incorporation of lactose and a pH indicator in the medium, which causes lactose-fermenting bacteria to produce acid and turn the colonies and surrounding medium hot pink, indicating their presence.
The correct responses to the question are that MacConkey agar is a selective and differential medium for the isolation and differentiation of enteric bacteria (1) and it inhibits the growth of Gram-positive bacteria (2). Also, it allows the growth of lactose-fermenting bacteria (3). The incorrect statement is that it contains bile salts and crystal violet to inhibit the growth of Gram-negative bacteria (4), as it actually promotes their growth.