Rough and Smooth Variants of Cl. Welchii

Rough and smooth variants of Cl. welchii have been described by several investigators. 1 All found similar colony types but report with no uniformity on the characteristics of the broth cultures of the variants. In the course of some work on the effect of varying gaseous mixtures and pressures on the growth of anaerobic organisms, the variants of 2∗ strains of Cl. welchii were studied.

Five distinct colony types were observed consistently on freshly poured blood agar plates containing about 5% horse or rabbit blood. The plates were incubated for 48 hours, either in desiccators in which a negative pressure of at least 28 inches of mercury was obtained with a cenco hyvac pump and maintained, or in desiccators which were alternately evacuated and refilled with nitrogen 3 times before sealing for the period of incubation. Little difference was noted in the characteristic growth of the variants when the cultures were incubated at 20°C. or 37°C.

The colonies of the 2 smoother variants differed in opacity and in their ability to spread but both gave glistening, homogeneous rounded colonies with entire edges. The thin transparent haloes of the spreading type were suggestive of those of the smooth motile colonies of some of the Salmonella strains. Rapid transfer in veal infusion broth (pH 7.2) of cultures, made from the smoother colonies when the strains were originally plated, resulted in the isolation of these 2 characteristically smooth variants. The broth cultures were incubated in an atmosphere of nitrogen in small anaerobic jars as suggested by Leifson. 2 The supernatant broth at first showed only slight clouding but gradually this increased until the broth was almost uniformly turbid; the slight sediment was of a very stringy, mucilaginous consistency.