Culture media

Culture media can be classified in several ways.

By what is in them:

• Synthetic (defined) media: the exact chemical make-up is known. M9 is used for E. coli. BG11, for cyanobacteria, needs only CO₂, nitrate and salts, since cyanobacteria make everything else themselves.
• Complex media: some ingredients have an unknown exact composition, such as peptones (partly digested proteins), beef extract and yeast extract. Examples: nutrient broth, tryptic soy broth, MacConkey agar. LB (lysogeny broth), devised by Giuseppe Bertani in 1951, is a common medium for E. coli; potato dextrose agar (PDA) grows fungi, yeasts and moulds.

By physical state:

• Liquid media (broths) contain no agar.
• Solid media contain 1.0-2.0% agar.
• Semi-solid media contain about 0.5% agar.

Agar is a sulphated polymer from red algae. It is ideal for solidifying media because very few microbes can digest it, and it melts and sets at convenient temperatures. It is not a food for them.

By purpose:

• Selective media favour the growth of one group and suppress others. With cellulose as the only carbon source, only cellulose-digesting microbes grow. Bile salts or dyes such as basic fuchsin and crystal violet suppress Gram-positive bacteria.
• Differential media let different microbes grow but make them look different. Blood agar shows which bacteria break down red blood cells (haemolytic, with a clear zone) and which do not. On MacConkey agar, bacteria that ferment lactose make acid and form red/pink colonies; it is also selective, because its bile salts stop Gram-positive bacteria.
• Enrichment media add extracts or blood to grow fastidious microbes that need special nutrients.