Question

Why is the chemical makeup of a complex media unknown? Differential media agar plates often change color. What is the color change actually telling us about the microbe? What would happen if the color indicator was excluded when making a differential media agar plate? Would the microbes behave differently on plates without the color indicators?

Answer 1

Complex media is made up of complex materials such as blood, milk, yeast extract or beef extract etc. The chemical composition of these materials is hard to determine because different components such as proteins, amino acids, carbohydrates etc are present at different amounts. And each of the amount of the chemical components are not constant. Thus, the chemical makeup of any complex media is unknown.

Differential media often contains certain coloured chemical compounds which give different colour to the media. And these coloured chemical compounds help to differentiate between different microorganisms as not all microorganisms are capable of degradation of the coloured chemical compounds. Thus, the microorganisms which are not capable of degradation these materials can't grow on differential media. Thus, we can differentially grow different microorganisms such as gram negative or gram positive bacteria on differential media. The type of bacteria which are capable of growing in differential media must contain certain specific enzymes to degrade the coloured chemical compounds.

e.g. Eosin methylene blue allows growth of gram negative bacteria only not gram positive bacteria.