Bacterial inactivation and study of damages in subcellular level during disinfection of aqueous samples

The objectives of the current study were i) to investigate the effectiveness of ozonation and UVA photocatalysis for the inactivation rate of representative bacterial strains, which are considered important for public health and ii) to study damages in subcellular level, in terms of disinfection effects on cellular components, namely, lipids, membrane and proteins. Disinfection experiments were conducted with the bacterial strains Escherichia coli, Pseudomonas aeruginosa and Bacillus cereus. Photocatalytic experiments were conducted in a batch type, laboratory scale photoreactor. UV-A irradiation was provided by 9 W lamps (Radium Ralutec, 9W/78, 350–400 nm) and the catalyst used was the commercially available TiO2. Ozone was produced by supplying dry air to an Aqua-Flo CD1B ozone generator. The impact of ozone and photocatalyst on bacteria survival was investigated by monitoring cell cultivability: counting on agar plate and by assessing lipid peroxidation (MDA), membrane permeability (ONPG) and protein release from the cell (SDS-PAGE). Ozonation proved to be more effective for the disinfection of aqueous samples compared with UV-A photocatalysis, as an almost 10 Log microbial reduction was achieved within 15 min of treatment in most cases of the selected bacterial species. However, the higher inactivation rates were recorded for Gram (-) bacteria (E. coli, P. aeruginosa), whose complete elimination was achieved after 15 min of treatment. The same bacterial reduction (8 Log) was obtained when B. cereus was tested (Gram +) but in longer period of time, i.e. 30 min. Regarding the subcellular level the lipid peroxidation and MDA destruction progressed at an exponential rate in the course of treatment, while ONPG hydrolysis assay showed negligible alterations. The latter indicates that the cell membrane may act as an effective barrier between the cytoplasm and the outer solution in each case. Nevertheless, not all bacteria exhibited comparable levels of damages in their structure and cellular format. The actual comparison which should be under consideration is between the thick wall of Gram (+) and the outer membrane of Gram (-), as each one represents the first line of defense. Cell wall complexity still remains a nebulous parameter and conclusions regarding its role in resistance during disinfection are still difficult to be definitive.