Abstract
Small colony variants of bacteria are regarded by some investigators as a filterable stage in a life cycle of the organisms. Other workers do not feel that there is sufficient evidence in support of this theory. Hadley 1 has been one of the chief exponents of the idea that bacterial variation represents a cyclical development of the organisms and in this connection he described a small colony filterable form of the Shiga dysentery bacillus and other bacteria.
In the present investigation small colony variants of Staphylococcus aureus were studied with respect to their occurrence, morphology, physiological and antigenic properties and filterability. The variants were obtained by plating ageing broth or agar cultures and from cultures in lithium chloride broth. Their occurrence was irregular and rare and no means were found by which this type of variation could be induced. Tiny colonies were not observed at any time in very young cultures except occasionally in broth containing lithium chloride. They appeared only after the cultures had aged for a week or two and were more numerous after 3 or 4 weeks.
Well-isolated colonies of Staph, aureus are usually from 1 to 2 mm. in diameter when grown on nutrient agar. Those of the small colony variants could scarcely be discerned without the aid of a microscope. In the various strains they ranged from .04 mm., the smallest ever observed, to 0.5 mm., but most frequently they were about 0.1 mm. in diameter. The small colony variants had lost the power of pigment production, the growth on agar slants being white and translucent.
Morphologically some of the variants were not strikingly different from the original strains. In other cultures, however, the cells were very irregular in size. Some were in tetrads, others in pairs or single. Within each cluster of cells there were marked contrasts in size, some being much larger and others smaller than those of normal cultures.
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