Isolation and Bioassay of a Fungal Morphogen

Abstract

THE staling of fungal cultures is a topic which has not received much attention recently. Pratt¹ described two types of staling agent, organic acids and potassium bicarbonate. She suggested that staling due to the formation of bicarbonate by the carbon dioxide of respiration is of primary importance and occurs whenever the medium is such that a basic radical is set free. She determined that the organic acid radicals were not effective as staling agents in the alkaline-staled medium. Pratt investigated the staling of a species of Fusarium in Richards's solution and her criterion of staleness was based on the average germ-tube length of conidia of Botrytis cinerea sown in drops of the filtered medium in which the Fusarium had grown. More recently Park² has investigated staling in F. oxysporum using agar cultures of Aspergillus niger as the test organism. Drops of culture medium were pipetted on to the margin of colonies of A. niger and the criterion of staleness was the cessation of extension-growth of the marginal hyphæ with accompanying vacuolation. Vacuolation has since been adopted as the best indication of ageing, since, whereas the stale culture fluid would always induce vacuolation, extension-growth was not permanently inhibited under all conditions of test. It is also important to note that a wide range of substances will cause hyphæ to branch or to cease extension-growth, but no substance among several tried has yet been found to reproduce the vacuolation effect caused by the stale medium from which the bulk of the mycelium has been removed by filtration through nylon gauze. It is of additional interest that both extracts of soil and of pond-water can also induce vacuolation of our test organism. The substance or substances responsible may therefore be associated with microbial habitats of different sorts.