selective and differential media

In you next practical you are going to be plating out bacteria onto agar plates, and there are a couple of terms you are going to come across, selective media and differential media...so, what are these?

Selective media is exactly what it sounds like - 'media' that 'selects'!

(When someone says 'media' in the context of growing bacteria, they really mean food.) 

So, selective media is basically food that you can give to a mixture of bacteria on which only some of the types of bacteria in that mixture will be able to grow.

For example, you might have an antibiotic in the media. In this case, only bacteria that are resistant to that antibiotic will be able to grow on that media - any other bacteria won't survive.

Differential media is more fancy. It is selective as it will only allow certain bacteria to grow, but it also makes the colonies of the strains that it does allow to grow have different morphologies so they can be discriminated between.

An example of differential media is MacConkey agar - MacConkey agar is often used to help identify gut bacteria. You will be using this in the lab! 

What does MacConkey agar contain?

Crystal Violet

Bile Salts 

Methyl red - a pH indicator

Lactose

How does MacConkey agar work?

• The Crystal Violet inhibits gram positive bacteria = only gram negative bacteria will grow
• The Bile Salts inhibit many non-intestinal bacteria = mainly only intestinal bacteria will grow
• If Lactose is fermented the methyl red pH indicator will turn the colony and agar around it red due to acid production
• If Lactose is not fermented, colonies will not turn red as there will be no acid production.

So take a look at the plate below. This shows a plate containing MacConkey agar on which lactose fermenting bacteria have been streaked on the left, and bacteria that don't ferment lactose have been streaked on the right. You can really see the colour change to red/pink when the bacteria ferment lactose.

Bacterial colonies growing on MacConkey agar. Right side - bacteria that don't ferment lactose. Left side - bacteria that do ferment lactose.