The food used to grow microbes is called media (medium = singular). The media may be a liquid and called a broth. Nutrient broth (contains 3 grams of beef extract and 5 grams of peptone per liter) and lactose broth (nutrient broth with 5 grams of lactose per liter) are two familiar liquid media used to grow microbes including coliforms in the examination of water and dairy products. The addition of agar, followed by boiling, will cause the medium to solidify after cooling. Walter and Fannie Hesse were friends of Robert Koch and initiated the use of agar. Agar is an extract from the red algae Gelidium; and it remains solid at body temperature. Solid media can be conveniently used in petri dishes as well as culture tubes (deeps or slants); and broth is generally used only in culture tubes.

Media is also defined as synthetic or non-synthetic. Synthetic media are chemicallydefined; and the exact amount of each ingredient has been determined. Other media like blood agar, tomato juice agar, and brain-heart infusion media are non-synthetic media because the exact composition of blood, tomato juice, and calf heart tissue is not known. Even the nutrient broth which contains unknown amounts of specific amino-acids in the beef extract is a non-synthetic medium.

There are hundreds of different types of media; and the functions are very diverse. General purpose media is used to grow a wide variety of microbes. General nutrient agar and broth are examples. Differential media also grow several types of microbes; and the microbiologist can distinguish the microbes by the colony color, size, or other biochemical changes. Changes in the media such as hemolysis (destruction) of blood and yellow discoloration of mannitol salt media are two observations used to help identify Staphylococcus. Gram-negative rods that ferment lactose have colony colors on Eosine-methylene-blue agar (EMB) that can be used to differentiate E.coli and Enterobacter from Salmonella. Selective media include chemicals or antibiotics that prevent the growth of selected microbes. The addition or rose bengal and streptomycin in the preparation of Marten's media prevents the growth of most bacteria; so Marten's media is used primarily to grow and study fungi. The alcohol concentration in phenylethanol agar and sabouraud media as well as the salt concentration of mannitol salt agar prevent the growth of many microbes and can be used to estimate the presence of gram-positive bcteria, yeast, and Staphylococcus respectively. Enriched media contain ingredients like blood, rabbit testes, yeast extract, and/or special amino-acids; and these special ingredients increase the potential growth of fastifious pathogens. Other media may be used to enumerate (count) microbes, transport microbes, and assay or test the concentration of drugs, antiseptics, preservatives, or cosmetics. The media including Stuarts and Amies which are used to transport microbes are low in nutrients and high in buffers to permit maintenance of the microbe with minimal growth during the time of transportation from one location to another.