Variations in the aerial viability associated with variation in morphology of color variants of Serratia marcescens.

Serratia marcescens, a nonpathogen, greatly facilitates the general study of airborne bacteria. However, the value of many studies has been lessened by the omission of strain designation which makes comparison of results from different investigators impossible. The presence of variants in S. marcescens strain ATCC 274 used in these studies has been reported previously (Bunting, 1940a, b, 1942; Bunting and Ingraham, 1942). In our laboratory the test variant of this strain exhibited stability of response to atmospheric exposure over a considerable period of time but then began showing aberrant responses to the standard atmosphere test condition. A study of the variants isolated from S. marcescens strain ATCC 274 and their stability and response to atmospheric exposure as airborne particles was made. The essential purpose was to characterize the stable variant in this strain so that constancy of response of the test culture could be assured. The need for standard organisms to be used in the study of airborne bacteria under various conditions of temperature, relative humidity, and aerial disinfection should be evident. Such practice is of long standing in the evaluation of bactericides and antibiotics. The particular organism reported here is not presented as a suggested standard test bacterium but to indicate the variations encountered and their significance within a specifically designated culture when used in studies of airborne bacteria.