Understanding the survival of airborne microbes during aerosolization and desiccation

Airborne microbes critically impact our everyday lives, e.g., rainfall, disease spread, land fertilization, and food production. Yet the survival of microbes during aerosolization and atmospheric transport is not well understood. An essential factor for microbial survival is the availability of water during and after aerosolization. Specifically, the ability to tackle the stress induced by desiccation, such as adjustment to osmolarity changes and biofilm formation, crucially determines their chances of survival. In our experiments, we investigated the effects of desiccation on cell survival for two model organisms: Pseudomonas syringae, a common plant pathogen, and Escherichia coli. Bacteria laden drops were dried under a variety of conditions, from smooth surfaces to porous substrates to acoustic levitation. We find that in pure water, desiccated bacteria die almost immediately regardless of the conditions. However, the presence of a small amount of salt (1-5% of NaCl) dramatically increases the survival of desiccated bacteria by several orders of magnitudes. Furthermore, divalent salts such as MgCl2 and CaCl2 can specially increase the survival of P. syringae. We hypothesize that the presence of salt crystals can help retains water, while divalent ions can stabilize cell membrane and facilitate biofilm formation. Although previous studies have suggested that a nonmonotonic dependence on relative humidity (RH) for cell survival, we find that high RH always leads to better cell survival compared to low RH. We also show that dense bacterial suspensions dried under acoustic levitation lead to high survival rates by trapping residual water.