The Aerosol Survival of Escherichia coli jepp Sprayed from Protecting Agents into Nitrogen Atmospheres under Changing Relative Humidity Conditions

The aerosol survival of Escherichia coli jepp, a particularly sensitive strain, has been studied in a nitrogen atmosphere (< 99·9%) when sprayed from suspension in solutions of sodium glutamate or glycerol, and collected into phosphate buffer with or without m-sucrose. The response caused by changes in relative humidity (RH) to 30% and 100% before collection was also examined. These two protecting agents were selected because glycerol readily permeated the cytoplasmic membrane, whereas sodium glutamate did not. E. coli jepp sprayed as a suspension in sodium glutamate and collected in phosphate buffer showed two minima in the survival versus RH curves: (i) a comparatively narrow zone between 90 and 82% RH; (ii) a broad instability zone between 82 and 50% RH. The presence of sucrose in the collecting fluid increased the survival for storage above 50% RH, but decreased it below 50% RH. For storage above 50% RH a change to 100 % RH before collection increased the survival for collection in phosphate buffer but not in phosphate buffer + sucrose; below 50% RH this change decreased the survival. A change to 30% RH before collection had little effect. In similar experiments with glycerol a gradual decline in survival occurred as the RH value was decreased and no critical minima were apparent. Addition of sucrose to the phosphate buffer collecting fluid increased survival. A similar improvement was obtained by changing the RH to 100% before collection into phosphate buffer. The combined effect of having sucrose in the collecting fluid and an RH change were not additive, and were somewhat antagonistic for aerosols stored at low RH values. A change to 30% RH before collection decreased the survival. These results are compared with those for E. coli jepp sprayed from suspension in distilled water and in solutions of raffinose, and show that at high RH values glycerol provided the best protection. At low RH values glycerol was inferior to sodium glutamate and raffinose, which were similar. The importance of the manner in which water re-enters the bacterium on collection is discussed.