A total of 17 presumptive killer yeast strains were tested in vitro for growth inhibitory and killing activity against a range of fungal pathogens of agronomic, environmental and clinical significance. Several yeasts were identified which displayed significant activity against important pathogenic fungi. For example, isolates of the opportunistic human pathogen, Candida albicans, were generally very sensitive to Williopsis mrakii killer yeast activity, whilst killer strains of Saccharomyces cerevisiae and Pichia anomala markedly inhibited the growth of certain wood decay basidiomycetes and plant pathogenic fungi. Results indicate that such yeasts, together with their killer toxins, may have potential as novel antimycotic biocontrol agents.

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 Candid0 spp. and Sacchammyces cerewisiae. Williopsis mrakii (NCYC 500) showed extensive antiCandida activity against strains isolated from clinical specimens. W. mrakii killer factor was produced in minimal media as a function of growth and its activity reached constant levels as cells entered stationary phase. The proteinaceous killer toxin was found to be unstable outwith a specific range of temperature and pH (above 30 "C and pH 49), and further analysis showed that the active toxin molecule was an acidic polypeptide with a relative molecular mass between 1+59 kDa. A t critical concentrations the killer factor exerted a greater effect on stationary phase cells of Candida than cells from an exponential phase of growth. A t low concentrations, the killer toxin produced a fungistatic effect on sensitive yeasts but at higher concentrations there was evidence to suggest that membrane damage accounted for the zymocidal effects of the killer factor. The cidal nature of the 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 Candide.

The pale yellow redox indicator 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) is reduced to a dark blue end-product, MTT-Formazan, by the mitochondrial dehydrogenases of living cells. MTT reduction can be measured spectrophotometrically at a wavelength of 570 nm and a method is described to assay the cidal activity of Williopsis mrakii killer toxin against sensitive cells of Candida glabrata. The MTT assay is rapid, quantitative and compares favourably with traditional plating techniques for the assessment of sensitive viability.

The pale yellow redox indicator 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) is reduced to a dark blue end-product, MTT-Formazan, by the mitochondrial dehydrogenases of living cells. MTT reduction can be measured spectrophotometrically at a wavelength of 570 nm and a method is described to assay the cidal activity of Williopsis mrakii killer toxin against sensitive cells of Candida glabrata. The MTT assay is rapid, quantitative and compares favourably with traditional plating techniques for the assessment of sensitive viability.