Chapter 11 - Methods for the isolation, maintenance and identification of yeasts

Abstract

This chapter focuses on the methods used for the isolation, maintenance, and identification of yeasts. Yeasts have been recovered from widely differing aquatic and terrestrial sources, as well as from the atmosphere. Many types of yeast occur widely, whereas some appear to be confined to restricted habitats. Yeasts seldom occur in the absence of either molds or bacteria. Consequently, selective techniques are often used for recovery of yeasts, employing media which permit the yeast to grow while suppressing molds and bacteria. The composition of such media is determined by the fact that yeasts are, as a rule, capable of developing at pH levels and water activities, which reduce or inhibit the growth of bacteria. Antibiotics may also be used to suppress bacteria. When yeasts are present in low numbers, their isolation may require enrichment using media and conditions which favor the growth of yeasts over other microorganisms. Yeast cultures are best maintained on a medium which contains glucose as the only source of carbon as this reduces the risk of changes in growth and fermentative patterns due to the selection of mutants. Many basidiomycetous yeasts do not survive well during prolonged storage on a glucose-peptone medium, although they grow well on it. Potato-dextrose agar is used when cultures of such yeasts are to be kept for a long time. The majority of yeasts may be stored at temperatures between 4 and 12° C and subcultured at intervals of 6 to 8 months. Yeasts such as Arxiozyma and Malassezia, may have to be subcultured every month.