| dc.description.abstract | The transmission mechanism of respiratory diseases includes routes such as direct or indirect contact, large droplets, and airborne transmission, apart from close contact transmission, which is complex. However, the transmission of bacteria causes infection predominantly via aerosols, and droplets are an integral part of aerosols. Infected droplets ejected from the host can either evaporate, forming 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 sessile mode. To assess the infection transmission through these three evaporation modes, deciphering the evaporation dynamics of bacterial-laden droplets is crucial. The study explores bacterial survival, virulence, and deposition patterns at different evaporation conditions and modes.
In the first part, using Salmonella Typhimurium (STM) as a model pathogen for the fomite mode of infection study, the investigation revealed how mechanical stresses and low moisture levels stress affect bacterial survival negatively but enhance the risk of infection through hyperproliferation. Also, the droplet medium's nutrient scale plays a pivotal role through spatio-temporally varying bacterial deposition patterns. The in-vivo infection study in mice and enhanced fold proliferation in organic and inorganic substrate samples confirm that STM WT maintains enhanced virulence and pathogenesis over hours and days, respectively.
In the second part, the study explores two modes of droplet evaporation to assess their significance in airborne disease transmission: droplets evaporating in a contact-free environment (levitated), forming droplet nuclei, and those settling on a hydrophilic substrate (sessile). The research involved a comparative study of levitated and sessile bacteria-laden droplets with identical initial volumes in a quiescent environment for four bacterial strains. This research examines mass transport, precipitate deposition patterns, and bacterial survival and virulence, exploring how evaporation rates affect bacterial viability, distribution in the precipitate, and virulence. Increasing osmotic pressure due to higher salt concentrations inactivates bacteria embedded in precipitates, particularly under accelerated evaporation. The comparison of bacterial survival and pathogenicity across both evaporation modes reveals striking differences influenced by evaporation rate, oxygen availability, and reactive oxygen species (ROS) generation. Results demonstrate that sessile precipitates contain more non-viable and dead bacteria than levitated droplets. It is attributed to the higher evaporation rate in the sessile mode, which leads to increased ROS production. Infection studies further confirm the reduced infection efficiency of bacteria in sessile precipitates.
In the third part, mass transport, micro-characterization of the samples, bacterial survival, and infectivity for three diameter reduction ratio-based stages of the levitated droplet for the aerosol and mixed evaporation modes of evaporation at two relative humidity (RH) conditions were analyzed. The low relative humidity (RH) condition simulates evaporation in arid regions like Delhi, while high RH conditions resemble those in colder climates like London. Klebsiella Pneumoniae (KP) is used as a model pathogen. Findings indicate that bacteria exhibit more remarkable survival in high RH environments than in low RH conditions across all diameter reduction ratio-based stages and modes of evaporation. In the aerosol mode, at a fixed RH level, evaporation time plays a crucial role, with bacteria in early-stage, partially dried samples showing higher viability than those in fully dried precipitates. A significant impact is wreaked upon the bacterial viability and infectivity by the droplet evaporation rate and the reactive oxygen species (ROS) generation in the bacterial cell. Therefore, these findings underscore the critical role of the evaporation history in determining bacterial survival and the subsequent transmission risk.
In the fourth part, the study investigates desiccation dynamics, pattern formations, and the viability and infectivity of STM WT-laden droplets on a hydrophilic substrate at varying temperatures. In industrial environments, particularly in food processing facilities, surfaces maintained at specific temperatures can serve as substrates for bacterial deposition, facilitating fomite-based infection. The findings reveal that bacterial deposition patterns are significantly affected by substrate temperature and the composition of the base fluid. Lower temperatures result in a ring-like deposit when Milli-Q water is used as the base fluid. In contrast, higher temperatures promote Marangoni convection, leading to a thinner ring with an inner deposit. Radial velocities at 50°C were an order of magnitude higher than at 25°C. Similarly, the dendritic deposition pattern in LB media varies with temperature, while the deposition pattern of the meat extract medium remains unchanged. Additionally, at 60°C, bacterial surface area is significantly reduced compared to 25°C while maintaining a constant aspect ratio. Although higher substrate temperatures decrease bacterial viability in precipitates, bacterial infectivity remains nearly unchanged across all three base fluids. This finding raises critical concerns about the potential transmission risks associated with fomite-based infections from heated surfaces in industrial settings.
This research has potential applications from public policy formation to drug development. Accounting for various stresses on the bacteria, public policies can be formed to curb the widespread infection. Developing indoor ventilation strategies to eliminate infectious droplets efficiently can be an area of research translation. Far-fetched goals like disease diagnosis and drug development will be a phenomenal outcome, creating a substantial social impact. Future research should investigate the genetic and molecular pathways that allow bacteria to regulate virulence under various stresses and include comparative studies across different matrices and conditions to refine decontamination strategies and improve the safety protocols. | en_US |
