Name three common differential/selective culture media and what they indicate.

• A. MacConkey agar differentiates Gram-positive lactose fermenters.
• B. MacConkey agar differentiates Gram-negative lactose fermenters (pink colonies); Mannitol Salt Agar selects for Staphylococcus and differentiates S. aureus (mannitol fermentation turning yellow); Blood agar shows hemolysis patterns.
• C. EMB agar differentiates Gram-positive cocci.
• D. Nutrient agar differentiates all bacteria by pigment production.

Answer: (B)

Differential/selective media are used to grow only certain microbes and to reveal specific traits by color changes or visible effects on the plate.

MacConkey agar is selective for Gram-negative bacteria because bile salts and crystal violet inhibit most Gram-positives. It’s also differential for lactose fermentation: lactose-fermenting organisms produce acid that turns the colonies pink (and the surrounding background can change color), while nonfermenters don’t. That combination is why you can quickly spot many enteric bacteria by colony color.

Mannitol Salt Agar is highly selective for staphylococci due to its very high salt concentration. It’s also differential for Staphylococcus aureus because mannitol fermentation acidifies the medium, turning it yellow around colonies. Nonfermenting staphylococci keep the medium from turning yellow.

Blood agar isn’t selective, but it’s differential for hemolysis. Different bacteria lyse red blood cells in characteristic ways: beta-hemolysis creates clear zones, alpha-hemolysis gives a greenish discoloration, and gamma-hemolysis shows no change. This helps identify organisms based on their hemolytic patterns.

The other statements don’t fit because EMB agar is designed to differentiate Gram-negative lactose fermenters (and it inhibits Gram-positives, not cocci) rather than differentiating Gram-positive cocci. Nutrient agar is a general-purpose medium, not used for differentiating bacteria by pigment production or other traits.