The survival of bacteria during and after drying

Abstract

An investigation of the survival rate of bacteria after drying in vacuo by the ' spin–freeze' method showed that one of the most important factors was the fluid in which the bacteria were suspended. Broth or nutrient gelatin gave a moderate survival rate, but in various other protein solutions, even though there might be a fair survival immediately after drying, a large fall occurred in the viable count after storage. The addition of glucose to the suspending fluid in concentrations of between 5 and 10% greatly increased the survival rate both immediately and after storage. Some experiments showed that, without added glucose, the longer the drying process was continued the lower was the survival rate, and it is thought that glucose acts by automatically retaining a small amount of water which is necessary for survival. Lactose appears to produce the same effect.It is shown that the highest death-rate generally occurs during the early stages of drying, the further drop after 24 hr. being small if glucose is used in the suspending fluid. Very young cultures (4½–6 hr.) seem to be far more sensitive to drying than older ones (18–24 hr.). The bacteria that survive drying are not more resistant to subsequent drying.Most of these results have been obtained with one strain of paracolon bacillus, but a limited number of other experiments show that the same principles hold good for other organisms such as Salmonella typhi-murium, staphylococci, streptococci, Neisseria gonorrhoeae and Vibrio cholerae. In the case of the last two species the percentage survival is much lower than with the others, but survivals up to 1% have been obtained after storage for from 1 to 3 years.