The Survival of Airborne Microorganisms II. Experiments With Escherichia Coll Near O°c

Abstract

The humidity/death rate relationships of 2 types of airborne Escherichia coli were studied at approx. 0.5[degree]C over the range 45-100% relative humidity (R. H.). Support was obtained for previous findings with psychrophilic bacteria of min. death rates at about 70% R. H. Evidence suggests that after spraying into the air from an aqueous suspension, bacterial cells come rapidly into equilibrium with the atmosphere,and that measured death rates are associated with these equilibrium conditions. At least 2 separate lethal mechanisms operating near 50% and 90% R. H.,respectively, seem to be potentially available for destruction of airborne bacteria. For E. coli, at least, the lower-humidity mechanism probably is a direct water activity effect. The high-humidity mechanisms seems to be neither a water activity effect nor a "cold shock" reaction. Death of E. coli at low humidities is not affected by the age of the cells, but at high humidities death is most rapid with young cultures. Differences in behavior at high humidities are observed between rough and smooth, and aerated anaerobic cultures.