Virulence gene profiling and cytotoxicity of Vibrio spp. isolated from treated wastewater effluent and receiving surface waters in Durban, South Africa†

Untreated or partially treated wastewater often harbours virulent Vibrio species that threaten environmental and public health. This study aimed to characterize the virulence gene profiles and cytotoxic effects of Vibrio species isolated from treated effluents and downstream rivers at four wastewater facilities in Durban, KwaZulu-Natal province, South Africa. A total of 200 Vibrio spp., isolated from treated effluent and surface waters of four wastewater treatment facilities in Durban, KwaZulu-Natal, were screened, with Vibrio vulnificus isolates (n = 178) showing high prevalence of iron acquisition genes such as viuB (72.47%), feoB (56.74%) and fbpC (55.06%) while other virulence genes like ompU, apxIB, and hlyB were also detected. Vibrio alginolyticus isolates (n = 15) exhibited rtx (66.67%) and pvuA (46.67%), among others. Five representative isolates caused a progressive decline in cell viability in both HepG2 and HEK293 cells over 72 h, with final viability dropping below 3% in multiple instances. Morphological damage confirmed strong cytotoxic activity. Statistical analysis showed significant associations between specific genes detected among the isolates. These findings demonstrate that treated wastewater still contains highly virulent Vibrio strains capable of harming human cells, posing ongoing risks in regions with compromised water infrastructure.