BIOLOGY Volume 2 - A Guide to General Biology - 2004

12. MICROBIOLOGY AND BIOTECHNOLOGY

12.2. Culture Media

A medium is a solid or liquid substance containing nutrients for the cultivation (growth) of microorganisms, as well as animal Cells or plant Tissues. A culture refers to a population of microbial cells growing on or in such a medium.

12.2.1. Solid and Liquid Media

Microorganisms can be grown on a solid medium or in a liquid medium (broth).

Solid Media

Solid media are extremely convenient for growing Bacteria and Fungi; they are prepared by mixing a liquid nutrient solution with a gelling agent (usually Agar) at a concentration of 1–2%, resulting in nutrient agar. Agar is an extract derived from red Algae. At a 1–2% concentration, agar melts at 90–100 °C and solidifies at approximately 44 °C. Agar can be pre-sterilized by heating (section 12.2.4) and then cooled. Microorganisms are streaked or spread across the agar surface after it has solidified, or, if they can withstand temperatures around 44 °C, added to the agar just before it sets, which ensures an even distribution throughout the medium. Agar is transparent and, being a complex polysaccharide, resistant to microbial degradation; these are among its key advantages. Examples of The Use of solid and liquid media will be discussed in subsequent sections. Solid media are also occasionally used for plant tissue cultures (ch. 22).

Liquid Media

Liquid media are frequently used to study population growth (section 12.5). The cells are placed in a test tube plugged with cotton wool or a metal cap, or in a Glass bottle with a screw cap, such as a universal McCartney bottle (Fig. 12.2), which holds about 25 cm³ of medium—just enough to pour a single Petri dish. Before the medium is used for Cell culture, it must be sterilized. The Introduction of a small number of cells into the medium is called inoculation. Following inoculation, the medium is incubated in a thermostat at the optimal Temperature for the growth of the given microorganism. The growing cells become randomly distributed throughout the medium.

When using large volumes of medium, the culture is agitated to prevent cells from settling. Mechanical shakers or magnetic stirrers are employed for this purpose. In addition, sterile air is bubbled through the medium to ensure the maintenance of an optimal oxygen concentration throughout. To filter and sterilize the air, commercially available filters or filters made of glass or non-absorbent cotton wool are used. The air enters via a sparger—a device with numerous tiny holes designed to produce fine bubbles. It is attached to the end of a tube leading to the bottom of the culture vessel. Liquid cultures can be grown either as batch cultures or continuous cultures (section 12.10).