4 - Culture and preservation

Summary

Introduction

It is essential that the most appropriate growth conditions and preservation techniques are used to ensure the viability, purity and stability of maintained microorganisms. Ideally, the methods used should retain all characteristics throughout storage.

Many filamentous fungi grow on culture media and can be kept viable by periodic transfer. However, the properties of fungi in culture may be unstable through loss of plasmids, spontaneous mutations or genetic recombination (due to the presence of heterokaryons, the parasexual cycle or normal sexual events). These phenomena can result in modification of a strain's characteristics, and conditions of preservation and storage should be selected to minimise the risk of such changes.

Growth of cultures

The major factors affecting growth are medium, temperature, light, aeration, pH and water activity.

Media

The growth requirements for fungi may vary from strain to strain, although cultures of the same species and genera usually grow best on similar media. The source of isolates can give an indication of suitable growth conditions, thus isolates from jam can be expected to grow well on high-sugar media, species from leaves may sporulate best in light, those from marine situations may require salt and those from deserts and the tropics, high growth temperatures.

Cultures are usually best grown on agar slopes in test-tubes or culture bottles. A list of recommended media and growth temperatures for common species is given in Smith & Onions (1983). Details of the most frequently used media are given in the Appendix at the end of this book.