Comparison between the photocatalytic inactivation of Gram-positive E. faecalis and Gram-negative E. coli faecal contamination indicator microorganisms

Photocatalytic inactivation of two different faecal contamination indicator microorganisms, the Gram-negative Escherichia coli and the Gram-positive Enterococcus faecalis, has been studied using TiO2 in suspension and immobilized onto the reactor wall. The effect of the main variables of the photocatalytic process on the disinfection efficiency has been analyzed using deionized water and a simulated effluent of wastewater treatment plant (WTP). Noticeable differences were observed between both types of bacteria during photolytic experiments without TiO2 in deionized water, probably due to the higher sensibility of E. coli to the osmotic stress, which leads to a higher cell membrane permeability and consequently a lower amount of hydroxyl radical attacks required to overcome the inactivation threshold. In contrast, despite their structural differences, Gram-positive and Gram-negative bacteria seem to follow the same inactivation mechanism, showing no significant differences in the experiments carried out with TiO2 in suspension either in deionized water or in WTP simulated effluent, with similar responses to changes in the concentration of catalysts and irradiation power (both variables involved in the hydroxyl radical generation). Similar results are observed using immobilized TiO2 in deionized water, although disinfection experiments of WTP simulated effluent using immobilized TiO2 showed much longer initial delays before the beginning of the inactivation for E. faecalis, suggesting a critical effect of the water composition of the bacteria-catalyst interaction. In any case, the irradiation time required to achieved the inactivation below the experimental bacterial detection limit is similar for both microorganisms, and experiments with mixtures of E. faecalis and E. coli in WTP simulated effluent show no significant differences. Therefore, it can be concluded that the results of photocatalytic disinfection experiments using E. coli as model bacteria could be reasonably extrapolated to other types of bacteria or bacterial mixtures.

Figure optionsDownload as PowerPoint slideResearch highlights▶ E. coli is more sentitive to mechanical and radiation stress in hypothonic solutions. ▶ E. coli and E. faecalis are similarly inactivated with TiO2 in suspension. ▶ E. coli and E. faecalis interact in a different way with immobilized TiO2. ▶ E. coli and E. faecalis are indistinctly inactivated in wastewater plant effluents. ▶ E. coli is a good model microorganism for photocatalytic disinfection studies.