What is the difference between a selective medium and a differential medium?
Question
I remember once during lab prep, we were asked to bring examples of media types, and I got confused between selective and differential media. Later, while reading a practical guide, I learned that selective media are designed to suppress unwanted microbes, while differential media help distinguish among those that grow. This explanation really helped clarify the difference for me.
Answer ( 1 )
Selective medium
Selective media contain ingredients that inhibit unwanted groups of microbes while allowing the target group to grow. Classic examples are bile salts and crystal violet in MacConkey agar that suppress Gram‑positives so Gram‑negative enterics flourish.
Differential medium
Differential media include indicators that reveal biochemical differences between colonies that do grow. They do not stop anybody, they just colour code them. In MacConkey lactose and neutral red turn lactose fermenters pink while non‑fermenters stay pale.
Key difference
Selective = who can grow, differential = how the colonies look once they have grown. A plate can be one, the other, or both. MacConkey is both, blood agar is differential only, mannitol salt agar is both but more selective for Staphylococcus.
Why it matters
Using the right medium saves time in diagnostics by reducing background flora and giving visual clues such as colour change or hemolysis pattern.