Media may be dispensed in bottles with rubber-lined metal screw caps, polypropylene caps or tubes plugged with non-absorbent cotton wool. Small bottles may be sterilized with their caps screwed down firmly, but not packed tightly in the baskets. The larger bottles should have their caps loosened before heating and subsequently tightened for storage. A useful method of distinguishing between types of media is the use of colored non-absorbent cotton wool plugs in tubes or, in the case of bottles, colored caps or beads may be used.
Cotton wool plugs must not be so tight that air is excluded. On the other hand, they should be firm enough to allow one to raise each tube by its plug. Media for current use should always be stored in a dust-free cupboard or in a cool, moist atmosphere. For longer periods store at 4-6 degree Celsius. Each batch should be tested before use and labeled with a batch number. The shelf-life of media varies considerably. Poured plates will be used within a few days of preparation but agar slopes should be checked to ensure that moisture is still present. Supplies of the basal media should be such that each batch is renewed within 3 months of manufacture.
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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