Transformative Approach to Investigate the Microphysical Factors Influencing the Airborne Transmission of Pathogens.

Otero Fernandez, Mara; Thomas, Richard J; Oswin, Henry; Haddrell, Allen E; Reid, Jonathan P

Emerging outbreaks of airborne pathogenic infections worldwide, such as the current SARS-CoV-2 pandemic, have raised the need to understand parameters affecting the airborne survival of microbes in order to develop measures for effective infection control. We report a novel experimental strategy, TAMBAS (Tandem Approach for Microphysical and Biological Assessment of Airborne Microorganisms Survival), to explore the synergistic interactions between the physicochemical and biological processes that impact airborne microbe survival in aerosol droplets. This innovative approach provides a unique and detailed understanding of the processes taking place from aerosol droplet generation through to equilibration and viability decay in the local environment, elucidating decay mechanisms not previously described. The impact of evaporation kinetics, solute hygroscopicity and concentration, particle morphology and equilibrium particle size on airborne survival are reported, using Escherichia coli (MRE162) as a benchmark system. For this system, we report that the particle crystallisation does not directly impact microbe longevity, bacteria act as crystallization nuclei during droplet drying and equilibration, and the kinetics of size and compositional change appear to have a larger effect on microbe longevity than equilibrium solute concentration.IMPORTANCE A transformative approach to identify the physicochemical processes that impact the biological decay rates of bacteria in aerosol droplets is described. It is shown that the evaporation process and changes in the phase and morphology of the aerosol particle during evaporation impact microorganism viability. The equilibrium droplet size was found to affect airborne bacterial viability. Furthermore, the presence of Escherichia coli (MRE162) in a droplet does not affect aerosol growth/evaporation, but influences the dynamic behaviour of the aerosol through processing the culture media prior to aerosolization affecting the hygroscopicity of the culture medium; this highlights the importance of the inorganic and organic chemical composition within the aerosolised droplets that impact hygroscopicity. Bacteria act also as a crystallisation nucleus. The novel approach and data has implications for increased mechanistic understanding of aerosol survival and infectivity in bioaerosol studies spanning medical, veterinary, farming, and agricultural fields, including the role of micro-organisms in atmospheric processing and cloud formation.