Assessing the cytotoxicity and apoptosis-inducing ability of solar irradiated Salmonella Typhimurium in the RAW264.7 cell line in vitro

Solar water disinfection (SODIS) reduces diarrheal disease in resource-limited settings. However, the host cell interactions of solar-inactivated pathogens remain ambiguous. This study assessed the effects of natural solar radiation on the viability, intracellular survival, cytotoxicity, and apoptotic responses of Salmonella enterica serovar Typhimurium in RAW 264.7 macrophages. Bacteria were exposed to (i) natural solar ultraviolet radiation (SUVR), (ii) identical conditions but shielded from SUVR, and (iii) heat-chemically inactivated with 0.5% phenol and 65 °C. Solar-irradiated S. Typhimurium for 4 h and 8 h was completely inactivated and showed no intracellular recovery in macrophages, whereas the non-irradiated controls showed a high intracellular replication rate. Solar-treated S. Typhimurium induced significantly lower cytotoxicity and fewer necrotic macrophages, while the non-irradiated bacteria triggered high LDH release and extensive cell death. Flow cytometric analysis showed that solar-inactivated S. Typhimurium induced attenuated necrotic responses compared to the substantial necrosis caused by viable bacteria. The findings demonstrated that solar irradiation neutralises S. Typhimurium infectivity and reduces its capacity to damage or kill macrophages, supporting the safety of SODIS-treated water.