Summary: Twenty ‘commensal’ oral or ‘pathogenic’ vaginal isolates of were examined for colony morphology on malt/yeast-extract and serum-based agar media. Diverse and variable colony morphology was seen on serum agar. In 17 strains, selective subculture of morphologically atypical colonies produced progeny which had reverted to the morphology of the majority of parental colonies. However, in one strain, a highly stable colony variant was isolated which did not revert on subculture. In two further strains, variants were isolated which could be maintained with at least 99% homogeneous colony type by selective colony subculture, but reversion to the parental type or switching to other morphologies occurred at rates of 10to 10: a rapid switching phenomenon. The relative proportions of mycelial or yeast forms were the main determinants of colony morphology. The variants were biotyped using a selection of biochemical tests. The stable variant differed from its parent in several characters, including rate of production of a proteinase enzyme. The pathogenicity of variants was compared in mice, and both stable and switching variants differed in virulence from their parental strains. Colony-type variation on suitable media is thus a powerful tool in the isolation of mutants or variants of which differ from ‘isogenic’ parents in significant biological properties. Such variants may aid identification and characterization at the molecular level of determinants of, for example, pathogenicity and morphogenesis.


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