Effect of Water Vapor on Lyophilized Serratia marcescens and Escherichia coli.

Dried Serratia marcescens ATTC 14014 and Escherichia coli ATTC 4157 cells were exposed to various partial pressures of purified water vapor. The colony-forming ability of the S. marcescens was unimpaired when the dried organisms were stored in water-vapor atmosphere such that P/P(0) < 0.55 or P/P(0) = 1.0 (where P is the pressure of the water vapor in contact with the organisms, and P(0) is vapor pressure of pure water at 25 C). During storage under water-vapor atmospheres with P/P(0) between 0.6 and 1.0, the colony-forming ability of the dried S. marcescens was destroyed. The inactivation by water vapor followed the expression - ln N/N(0) = Kt, where N(0) and N are the number of viable organisms before and after exposure, respectively, t is time, and K is a pseudo constant which is dependent upon the partial pressure of the water vapor at 25 C. Similar results were obtained with dried E. coli. The addition of solutes to the suspending media before freeze-drying was found to influence the stability of the organisms during exposure to water vapor.