Transformative Approach To Investigate the Microphysical Factors Influencing Airborne Transmission of Pathogens

Abstract

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.

Accurate Representations of the Microphysical Processes Occurring During the Transport of Exhaled Aerosols and Droplets

“…It is important to note that an accurate understanding of the phase and time-dependent moisture content may play an important role in understanding the survival of viruses and bacteria in evaporating droplets. 36…”

The Dynamics of SARS-CoV-2 Infectivity with Changes in Aerosol Microenvironment

“…To provide insight into the underlying mechanisms that drive the observed airborne loss of SARS-CoV-2 infectivity, the microphysical changes (depicted in Figure S1A) taking place in the droplets hosting the virus were explored in real time and in situ using the CK-EDB with a time-resolution of <100 ms (36,(39)(40)(41)(42)(43). For context, the phase changes that occur during the evaporation of aqueous sodium chloride at an RH below the efflorescence threshold are shown in Figure S2.…”

“…We have previously reported a unique approach to the study of infectious aerosol and the interplay between aerosol microphysics and pathogen survival, using complementary aerosol analysis techniques to assess the underlying mechanisms that govern the airborne longevity of pathogens (36,37). The aerosol stability of viruses and bacteria is investigated using the CELEBS technique (Controlled Electrodynamic Levitation and Extraction of Bioaerosols onto a Substrate) (36)(37)(38). In CELEBS (Fig.…”

“…Thus, assessment of viability of suspended microbes within droplets can be made after periods of suspension varying between less than 5 seconds to many hours. These longevity measurements can then be contextualised with detailed measurements of the dynamic changes in the physicochemical properties of droplets generated the exact same way in an instrument referred to as the Comparative Kinetic-Electrodynamic Balance (CK-EDB) (36,(39)(40)(41)(42)(43). The CK-EDB uses the same piezoelectric droplet-on-demand dispensers as the CELEBS to generate droplets, with particles captured in the path of a laser within a flow of humidity and temperature-controlled air (Fig.…”

Airborne virus transmission via respiratory droplets: Effects of droplet evaporation and sedimentation

“…In addition to the uncertainties associated with the droplet-size distribution, the sedimentation and evaporation processes of saliva droplets expelled from the mouth or nose are affected by a variety of different physical and chemical effects, which make modelling of airborne virus transmission even more complex. These effects include the evaporation-induced cooling of the droplet [ 28 , [44] , [45] , [46] ], airflows and ventilation effects for large droplets [ 21 , 25 , 47 ], finite evaporation-rate effects for small droplets [ 48 , 49 ], solar irradiation effect [ 50 , 51 ], and solute-induced effects, including water vapor-pressure lowering [ 52 , 53 ], local solute-concentration gradients [ [54] , [55] , [56] ], crust formation due to solute crystallization [ 54 , 57 , 58 ], liquid−liquid phase separation [ [59] , [60] , [61] ], and a possible solute-concentration dependence of the viscosity [ 62 , 63 ] and the water-diffusion coefficient [ 63 , 64 ] inside the droplet. These effects are themselves dominated by various parameters, such as the initial size of the droplet, the type and the initial volume fraction of solutes, the ambient temperature [ 47 , 50 , 65 , 66 ], the relative humidity [ 47 , 65 , [67] , [68] , [69] , [70] , [71] ], non-ideal effects due to inter-particle interactions inside the droplet [ 72 , 73 ], the internal morphology of droplets [ 59 , 74 , 75 ], and the initial height at which droplets are released into the air.…”