3 - Isolation, classification and identification of microbes

Summary

Isolation of bacteria

The earliest attempts to produce solid cultures included solidifying meat extracts with gelatin. Such media had two principal disadvantages: firstly, gelatin liquefies at about 37 °C, the optimum temperature for the growth of many human pathogens; secondly, many bacteria possess the ability to digest gelatin. Consequently, gelatin-based media tend to become liquefied under conditions where it is desirable to use a solid growth medium.

Agar is an inexpensive polysaccharide, obtained from certain seaweeds. In solution it can form a gel, and it provides an excellent substitute for gelatin as a solid support for microbiological media. Agar is generally resistant to microbial degradation and once gelled it remains solid at temperatures just below 100 °C. Once molten, agar suspensions remain liquid at temperatures of about 45 °C. This permits heat-labile supplements to be added to agar-based media without loss through heat degradation. Because of these properties, agar is the gelling agent used most widely in bacteriology.

During the earliest days of bacteriology, the most successful growth media were those derived from boiled extracts of meats of various types. Even now, brain–heart infusion broth is a rich growth medium often used to culture fastidious bacteria. Early culture media were very variable in their content. This caused problems with the standardisation of growth and also of bacterial characteristics. Today, many bacteriological growth media are still based upon peptones. These consist of a complex mixture of water-soluble products obtained from the hydrolysis of proteins derived from lean meats and other sources including heart muscle, casein and soya flour.