Bacterial Survival in Systems of Low Moisture Content

Abstract

Summary The technique of performing viable counts using graduated pipettes with roll-tubes has been examined and the accuracy of the method has been assessed by statistical analysis. It has been shown that satisfactory viable counts can be obtained of spores of B. subtilis in suspension or in powders. The spreading surface growth of the organism has been shown to have no significant effect upon the count and roll-tube counts may be performed satisfactorily in its presence. It has been shown that if even suspensions of B. subtilis spores are spray-dried the viable organisms are evenly distributed in the resultant powder. It has been shown that if even suspensions of Bact. lactis aerogenes are spray-dried the viable organisms are not evenly distributed in the powder as taken from the spray-drier, but that an even distribution can be obtained if the powder is thoroughly mixed by mechanical means. The effect of storage on the viability of the organisms in peptone powders has been examined. The viable count of B. subtilis was found to undergo no significant diminution after 6 months’ storage, whereas the viable count of Bact. lactis aerogenes fell rapidly, most of the organisms dying within the first few days. It is concluded that spray-dried powders containing spores of B. subtilis are very suitable for examining the effects of environmental conditions on the spores. On the other hand, the use of powders containing organisms, such as Bact. lactis aerogenes, which show a high mortality on spray-drying, involves greater difficulties, which may be inherent in the problem. It is a pleasure to express our thanks to Professor M. S. Bartlett for suggestions and advice concerning the statistical treatment of the results reported in this paper.