Assessment of Escherichia coli reactivation after photocatalytic water disinfection using flow cytometry: comparison with a culture-based method

The photocatalytic process generates highly reactive oxidative species, such as hydroxyl radicals, which enable mineralization of cellular compounds. Microorganisms often tend to lose their culturability after disinfection, but could remain viable to proliferate under optimum conditions. Estimation of bacterial counts using culture-based methods pose limitations in differentiating viable, non-viable, and viable but non-culturable (VBNC) cells. Presence of viable and VBNC state cells in disinfected water could pose a potential health risk and accurate estimation of these cells through a molecular method is critical. Assessment of live/dead states of an indicator waterborne pathogen, Escherichia coli (ATCC^®10798) after disinfection was conducted using flow cytometry. Photocatalysis was carried out under low pressure ultraviolet (LP UV) radiation alone and at four titanium dioxide (TiO₂) concentrations (1, 0.75, 0.5 and 0.1 g/L). During the repair period, flow cytometry showed 4–5 log₁₀ higher cell counts than the culture-based method. Photocatalysis using 0.1 g/L TiO₂ resulted in 50% cells with intact cell membrane during the repair period and lowered the repair rate of the ATCC^®10798, E. coli after disinfection.