Survival of vole bacilli in fresh and freeze-dried suspensions

Abstract

Using either colony counts or direct microscopic counts to estimate viability, investigations have been made of the survival of both fresh and freeze-dried suspensions under different conditions of preparation and storage. Both types of product were preserved best at 4°C.; incubation at 37°C. was particularly destructive, especially for dried bacilli. Exposure to window light was rapidly lethal, the fresh preparations in this case being particularly susceptible. Aqueous 0·1 per cent albumin or human serum was a satisfactory suspending fluid for fresh preparations, but was completely inadequate for drying purposes, for which various fluids containing moderate concentrations (5–7·5 per cent) of glucose were most satisfactory. No influence of culture age on survival of freeze-dried suspensions could be demonstrated. Under optimal conditions of preparation and storage, viability of vole bacillus suspensions appeared to be at least equal to that of BCG. Fresh preparations were able to remain fully viable for more than a week and to retain 50 per cent viability after five weeks. Frozen-dried suspensions showed an initial fall to not less than 10 per cent generally 20–40 per cent, at which level most preparations remained for test periods of from six to nine months at 4° C.