In normal bacteriology, the most important requirement of a culture medium is the ability to allow detectable growth from a minute inoculum, possibly a single organism, within the shortest period of incubation. The medium which forms the basis of the majority of culture media is referred to as nutrient broth. It is designed to support the growth of a wide range of bacteria and consists in the main of meat extracts, peptone and mineral salts in clear solution at a pH of approximately 7.4.
Meat extracts supply a wide range of growth factors including mineral salts and amino acids. Peptone is a source of nitrogen obtained by the peptic digestion of protein to give a heat-stable mixture of proteoses, peptones, polypeptides and amino acids. Several varieties of peptone are commercially available, two of the most widely used types being bacteriological and proteose peptones. The latter is especially rich in amino acids such as tryptophan, which is necessary for satisfactory indole production. The mineral salts essential to growth consist of sulphates, chlorides and phosphates of the acid radicals and calcium, phosphate and sodium among the bases.
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