ANTICANDIDA ACTIVITY OF A NOVEL KILLER TOXIN FROM THE YEAST WILLIOPSIS-MRAKII

A screening of putative killer yeast strains showed that spore-forming ascomycetous yeasts of the genera Pichia and Williopsis displayed the broadest range of activity against sensitive strains of Candida spp. and Saccharomyces cerevisiae. Williopsis mrakii (NCYC 500) showed exte nsive anti-Candida activity against strains isolated from clinical spe cimens. W. mrakii killer factor was produced in minimal media as a fun ction of growth and its activity reached constant levels as cells ente red stationary phase. The proteinaceous killer toxin was found to be u nstable outwith a specific range of temperature and ph (above 30 degre es C and ph 4.0), and further analysis showed that the active toxin mo lecule was an acidic polypeptide with a relative molecular mass betwee n 1.8-5.0 kDa. At critical concentrations the killer factor exerted a greater effect on stationary phase cells of Candida than cells from an exponential phase of growth. At low concentrations, the killer toxin produced a fungistatic effect on sensitive yeasts but at higher concen trations there was evidence to suggest that membrane damage accounted for the zymocidal effects of the killer factor. The cidal nature of th e toxin was reflected in a rapid decrease in sensitive cell viability. Findings presented suggest that W. mrakii killer toxin has potential as a novel antimycotic agent in combatting medically important strains of Candida.