Cultures of aerobic organisms require oxygen Cultures of anaerobic organisms must be free of oxygen Cultures of facultative anaerobes can be grown with or without oxygen. The exact chemical composition is not known. It is media that can show the difference between the growth of different kinds of organisms on it. For example, nutrient agar media with sheep blood in it can be used to tell the difference between different species within the genus Streptococcus . The different species all grow on this media, but their colonies look different.
Selective media allows the growth of some microorganisms while inhibiting the growth of others.
For example, many gram negative bacteria cannot grow in high salt concentrations, whereas most gram positive bacteria can. Bacterial growth media with high salt selects against gram negative bacteria and selects for gram positive bacteria. It can both select for a certain kind of microorganisms and provide way to differentiate between different kinds of bacteria that grow out. The acid produced by fermentation turns a pH indicator purple, so that purple colonies are identified as lactose fermentors. Not all microorganisms can be grown in culture .