Introductory Microbiology

Preparation of media

• Some naturally occurring substances are used for the cultivation of bacteria. Notable among these is milk, usually skimmed rather than whole. Such natural materials are merely dispensed into tubes or flasks and sterilized before use. Media of the nutrient both or nutrient agar types are prepared by compounding the required individual or ingredients or more conveniently by adding water to a dehydrated product which contains all the ingredients. Practically, all media are available commercially in powdered form. The preparation of bacteriological media usually involves the following steps:
1) Each ingredient or the complete dehydrated medium is dissolved in the appropriate volume of distilled water in earlenmeyer flasks (250 ml or 500 ml capacity).
2) The pH of the liquid medium or broth is adjusted by adding 1N HCl or 1N NaOH.
3) If a solid medium is desired, agar is added and the medium is boiled to dissolve the agar. Flasks are then plugged with non-adsorbent cotton plugs. The plugged flasks should be wrapped by brown paper and placed for sterilization by autoclaving at a pressure of 15lbs/inch2 (at temperate 1210c) for 15min. Some media that are heat labile are sterilized by filtration. It is first to be noted that all the glassware in use should be sterilized in an oven at 1700c for 3hr. before using them. Such sterilized glassware (Petri dishes) are needed for pouring the medium used for culturing the microorganisms.
• Each and every biological process requires energy for their vital activities. The basic cell building requirements are supplied by the nutrition, which is manipulated according to its requirements. Nutrition not only provides energy but also acts as precursors for growth of microorganisms. The nutritional requirement of an organism depends upon the biochemical capacity. If an organism is capable of synthesizing its own food using various inorganic components, requires a simple nutritional diet whereas organisms unable to meet such synthesis requires complex organic substances.