What distinguishes selective and differential media?

Question

For example what makes MacConkey agar or Mannitol Salt Agar both selective and differential??

Answer 1

A selective medium has a tendancy to kill many microorganisms which might otherwise grow there. This is great if your goal is to only have cultures of one type or of a specific type.

MacConkey agar is selective because only Gram-negative bacteria will survive. Mannitol Salt Agar is selective because only microorganisms capable of surviving its high salt content will survive (generally Staphylococcus and Micrococcaceae).

A differential medium doesn't necessarily kill anything, but has some feature that lets you pick out cultures that have specfic characteristics. Usually this is some kind of dye, colour change, or such.

MacConkey agar is differential because it contains 'neutral red'... bacteria which can digest the lactose on the plate produce acid which turns that indicator red; those that can't will produce ammonia which causes no colour change. Mannitol Salt Agar is differential because it contains 'phenol red'. Those bacteria which digest the mannitol produce acid and change the colour from red to yellow.

Answer 2

For example Maconkey agar or MAC will grow many forms of Gram negative Bacteria.
But Enteric Hemmaorgenic O157:H7 E.coli will have RED colonies in color this is distinctive to MacConkey agar.

Answer 3

A selective medium only allows certain types of bacteria to grow on it, for example, just gram negatives can grow on some agars, and just Staph can grown on others.

A differential medium is one which allows you to distinguish between bacteria which can grow on it.

You should be able to find what those two agars are useful for...

Answer 4

Perfect for my assignment though i dont need those compounds