Evaporation and pathogenesis of levitated bacteria-laden surrogate respiratory fluid droplets: At different relative humidity and evaporation stages

Aerosols are the principal cause of airborne infections and respiratory diseases. Droplets ejected from the host can evaporate and form a precipitate in the air (aerosol mode), evaporate for some time, and fall on the ground (mixed mode) or directly fall on the ground and evaporate in the sessile mode. Different evaporation modes, stages of evaporation, and the relative humidity (RH) conditions affect the survival and infectivity of the bacteria in the precipitate. We have investigated three droplet diameter reduction ratio-based stages of evaporation of a bacteria-laden levitated droplet at two different RH settings and evaporation modes (aerosol and mixed) mimicking real-life scenarios. The low RH condition mimics evaporation in arid regions, e.g., Delhi and the high RH conditions imitate cold areas like London. The study analyzes the mass transport, micro-characterizes the samples, and investigates the survival and infectivity of bacteria in the sample. Klebsiella pneumoniae survives more in high RH conditions than in low RH conditions for all diameter reduction ratio-based stages and modes of evaporation. For the aerosol mode, at a fixed RH condition, the evaporation time plays a vital role as the bacteria in early-stage partially dried samples are more viable than the full precipitate. The evaporation rate and the generation of reactive oxygen species cause a remarkable difference in the viability and infectivity of the K. pneumoniae samples. Therefore, our findings report that the evaporation history of an infected droplet is an indispensable factor in determining bacterial viability and subsequent infectivity.