COLONY PHENOTYPE SWITCHING IN CLINICAL AND NONCLINICAL ISOLATES OF SACCHAROMYCES-CEREVISIAE

During studies on the virulence and genetics of isolates of Saccharomy ces cerevisiae we noted colony phenotype switching. Virulent clinical (YJM128, YJM436) and avirulent non-clinical (Y55, YJM237) isolates of S. cerevisiae were scored for colony phenotypes and reversion rates of the switched second-generation colonies on YEPD with phloxine B. YJM1 28 produced six colony phenotypes; 99.7% were large (8-9 mm diameter), smooth, pink with a white edge (type A) and the other five types aros e at frequencies ranging from 2 x 10(-3) to 5 x 10(-5). On replating o f type A, 98.8% of colonies scored arose as type A, 1.1% type J and 0. 1% larger, but similar to type A indicating both stability and reversi bility. YJM436 also varied, with 91.3% arising as a single phenotype; five other phenotypes were observed at frequencies ranging from 7.6 x 10(-2) to 1.5 x 10(-4). For Y55, 0.2% of the colonies were 5-7 mm in s ize with a ridged pink edge (type D); 36.4% of YJM237 were a 5-7 mm, s mooth pink colony (type H). The remaining 99.8% of Y55 and 63.4% of YJ M237 colonies arose as sectored or non-sectored non-reverting petites. These results indicate that virulent isolates of S. cerevisiae tend t o produce multiple colony phenotypes, which are reversible. In contras t, avirulent isolates of S. cerevisiae showed single colony phenotypes and a high frequency of non-reverting petites. Whether selected colon y phenotypes and switching are associated with the virulence of S. cer evisiae remains to be determined.