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Microbes require nutrients to grow. These are supplied by either solid or liquid culture media. The standard solid medium is nutrient agar, a gelatinous substance derived from seaweed. The basic liquid medium is nutrient broth, typically a mix of water, meat extract peptone, and sodium chloride.
Some microorganisms are more finicky than others and require media enriched with growth-promoting ingredients such as animal blood, glucose or egg. Examples of commonly used enriched media are blood agar, chocolate agar, and Loeffler medium.
Microbiologists also sometimes use special media called differential media that contain various chemicals designed to distinguish microbes by the appearances their colonies take on as they grow. For example, MacConkey agar has a stain called neutral red added and contains lactose (the sugar in milk). Lactic acid turns on the neutral red. Therefore, colonies formed by bacteria that ferment lactose into lactic acid will turn red and stand out from the pale colonies of non-lactose-fermenting bacteria.
When microbiologists want to inhibit the growth of certain microbes while encouraging the growth of others, they use special media called selective media. These media contain elements that inhibit the growth of some kinds of microbes and not others.