What is the purpose of selective media like MacConkey agar?

• A. To suppress unwanted organisms and encourage growth of specific groups (e.g., Gram-negative intestinal bacteria) by bile salts and crystal violet.
• B. To differentiate Gram-positive cocci by catalase activity.
• C. To identify spore-forming bacteria.
• D. To promote growth of all bacteria.

Answer: (A)

Selective media aim to suppress unwanted organisms while supporting the growth of a targeted group. MacConkey agar uses bile salts and crystal violet to inhibit most Gram-positive bacteria, letting Gram-negative intestinal bacteria grow. This makes it useful for isolating enteric Gram-negative organisms from mixed samples. It’s also differential for lactose fermentation, so lactose fermenters appear pink, helping distinguish between certain Gram-negative groups. The other options describe tests or media for non-selective growth or different targets (catalase testing for Gram-positive cocci, spore identification, or growing all bacteria), which aren’t the purpose of MacConkey agar.