Inactivation of Escherichia coli in droplets at different ambient relative humidities : Effects of phase transition, solute and cell concentrations

Previous studies have indicated that ambient relative humidity (RH) plays an important role in the inactivation of bacteria and viruses in droplets. However, characteristics of the RH-dependent inactivation between bacteria and viruses have been reported to differ. Furthermore, how the droplet medium and cell concentration influence inactivation remains unclear. In this study, inactivation of the model bacterium Escherichia coli in droplets of Luria-Bertani (LB) broth and artificial saliva (AS) as a function of RH and initial cell concentration was investigated in a flow cell under a well-controlled environment. Phase transition of aqueous droplets of the two media was observed to occur at ∼50% RH, and their hygroscopicity at different RH were similar. At an initial cell concentration of 10¹⁰ CFU/mL, relative viability (RV) in AS droplets showed a V-shape dependence on RH with a minimum at 60%, while RV in LB droplets showed a gradual decrease with decreasing RH. As the initial cell concentration decreased from 10¹⁰ to 10⁸ CFU/mL in LB droplets, RV prominently decreased, especially at moderate RH (∼60%), with a V-shape dependence on RH at the lower cell concentration. A drop in RV was also found in AS droplets at a lower initial cell concentration, to below the detection limit in some cases. By considering the bacteria-to-water volume ratio, a high cell concentration in droplets was found to reduce inactivation due to the osmotic stresses. We conclude that for freshly emitted saliva droplets containing a low concentration of bacteria, an ambient RH of ∼60% is optimal for natural inactivation.