Lysis of the cell walls of yeast ( Saccharomyces cerevisiae) by soil fungi

Abstract

Two fungi, Fusarium sp. and an unidentified sterile fungus (no. 6), have been isolated from soil and their lytic activity towards cell walls of yeast (Saccharo-myces cerevisiae) examined. Growth of the two fungi on solid and liquid media containing autoclaved whole yeast or yeast cell walls resulted in lysis of the cell walls in both instances. Filtrates from cultures of both fungi grown on autoclaved walls, or whole yeast, showed glucanase activity when this was estimated with laminarin as substrate and this activity was greatest at pH 5·0. The cell walls of preheated whole yeast lost much of their opacity (phase-contrast microscopy) when incubated, in the presence of 2-mercaptoethanol at pH 5·0 and 7·5, with the culture filtrate from the sterile fungus no. 6. Isolated walls (unheated) were also incubated in a similar manner and chemical analysis showed that the walls had lost 77% (at pH 5·0) and 65% (at pH 7·5) of their anthrone-positive constituents after 6 days at 30 °C; the walls lost much of their opacity when incubated at pH 7·5 only. In the absence of 2-mercaptoethanol the losses amounted to 37% at pH 5·0 and 25% at pH 7·5. Ultra-thin sections of whole yeast and shadowed preparations of isolated cell walls examined in the electron microscope provided important information on the manner in which these materials were degraded.