Legionella jordanis inactivation in water by solar driven processes: EMA-qPCR versus culture-based analyses for new mechanistic insights

Abstract

In this contribution, the validation of EMA-qPCR method for the quantification of viable Legionella spp.in water after solar treatments was carried out. EMA-qPCR was used to evaluate the different effects ofseveral solar water disinfection processes over this bacterium, and furthermore their mode of action.Inactivation of Legionella jordanis in water by solar photocatalytic (TiO2and TiO2/H2O2) and solar pho-tochemical (solar/H2O2and solar disinfection) processes have been investigated under natural sunlight.Culture-based and molecular (EMA-qPCR) techniques were systematically compared for the analysisof treated water samples. Solar tests were done under natural solar radiation (clear sky) and ambienttemperature (20–35◦C) for 2 h, using H2O2/Solar (10, 20 and 50 mg/L), TiO2/Solar (100, 200, 300, 400,and 500 mg/L) and TiO2/H2O2/Solar (100/10, 200/10, 500/10 mg/L). According to culture-based method,the best results of bacterial inactivation were obtained for 500/10 mg/L of TiO2/H2O2. The order of effi-ciency to reach complete inactivation was: TiO2/H2O2/solar (5 min) > TiO2/solar (15 min) ≈ H2O2/solar(15 min) > Solar only disinfection (90 min). Moreover, EMA-qPCR and culturable counting results showeda direct correlation for samples treated with TiO2/solar for those catalyst concentrations that generate astrong oxidative attack over the cell wall. EMA-qPCR results demonstrated to be a good method to detectdamaged and dead cells when the treatment affects the integrity of the cell’s membrane, as occurs underphotocatalysis. Meanwhile for solar disinfection and solar/H2O2(at non-toxic concentrations, <1.5 mM),where membrane integrity remained unaltered, EMA-qPCR results couldn’t discriminate between aliveand dead cells, even when the bacteria were not culturable.