Short note Inactivation of Escherichia coli by TiO 2 -mediated photocatalysis evaluated by a culture method and viability-qPCR Majdi Kacem a, *, Valérie Bru-Adan b , Vincent Goetz a , Jean Philippe Steyer b , Gael Plantard a,c , Daniel Sacco a,c , Nathalie Wery b a CNRS, UPR 8521, PROMES, Rambla de la thermodynamique, F-66100 Perpignan, France b INRA, UR0050, Laboratoire de Biotechnologie de l'Environnement, Av. des Etangs, F-11100 Narbonne, France c University of Perpignan Via Domitia, 52 Av. Paul-Alduy, F-66100 Perpignan, France A R T I C L E I N F O Article history: Received 5 October 2015 Received in revised form 17 November 2015 Accepted 21 November 2015 Available online xxx Keywords: Photocatalysis q-PCR Viability-q-PCR VBNC Modeling E. coli A B S T R A C T The efficiency of photocatalysis treatment in bacteria is usually evaluated using culture techniques. However, cells under environmental stress can lose their ability to grow on culture media, becoming “viable but non-culturable” (VBNC). Thus, quantifying bacterial density by culture could be misleading and unhelpful for controlling public health risk. Here, culture, q-PCR and q-PCR combined with propidium monoazide treatment were used to evaluate E. coli inactivation during TiO 2 photocatalysis. Culturable cells decayed faster (k 1 = 0.66 h 1 , T 90 = 0.18 h) than viable cells (k 1 = 0.18 h 1 , T 90 = 0.85 h), which may be explained by the VBNC state induction during the photocatalytic treatment. The combination of culture and the molecular tools was useful to describe the photocatalytic effects on the cells that successively occurred with increasing irradiation times (loss of culturability, membrane integrity, followed by nucleic damage). Finally, the presence of biphasic decay kinetics was highlighted and may be linked to the presence of an E. coli subpopulation resistant to the treatment. The viability-q-PCR technique covered all the viable cells, elucidated the underlying mechanisms and made better estimates of the efficiency of the photocatalytic treatment. This molecular tool highlighted the need of combinatorial research to tackle the threat posed by VBNC bacteria with regard to public health. ã 2015 Elsevier B.V. All rights reserved. 1. Introduction With global population increase and climate change, many communities are facing water shortages. Water reuse is therefore attracting increasing attention. According to the intended use of the water, water managers can choose from a portfolio of treatment processes to design a wastewater reclamation system that meets specific water quality objectives. Among the alternative processes currently being developed, photocatalysis has emerged as a very attractive and environment- friendly technology for wastewater disinfection, especially as solar light can be used to drive the process [1,5,24,37]. Literature reports on photocatalytic disinfection highlight the inactivation of various kinds of microorganisms [34,42]. Most of the reports use titanium dioxide (TiO 2 ) in slurries [4,15], reaching a high efficiency in inactivating bacteria. The great majority of the reported studies concern the bactericidal effects of TiO 2 photocatalysis, and especially the inactivation of E. coli suspensions [24,28]. There is still much debate over what process or set of processes destroys this fecal indicator exposed to photocatalytic action. However, most research now indicates that the destruction of the cell membrane is the most important process in inactivation [24,35]. The efficiency of photocatalysis treatment on the bacteria has most often been evaluated using culture techniques. However, bacteria subjected to environmental stress can lose their ability to grow on culture media. This apparent dormant state in which bacterial cells are metabolically active but cannot be cultured by known laboratory methods has been called the “viable but non- culturable” (VBNC) state [40]. Since its description 30 years ago, controversy has arisen regarding the ecological significance of this state. Bacteria in this state demonstrate metabolic activity and maintain their pathogenic features [10]. A VBNC state has been shown for various bacteria, both Gram-positive and Gram- negative, both pathogenic and non-pathogenic [32,38]. Concerning E. coli, the presence of VBNC cells has been demonstrated in drinking water [3], in river waters [47] and during chlorination [33]. In this context, quantifying density as culturable cells can be misleading, especially in the context of “die-off” or “decay”, and * Corresponding author. E-mail address: kacemejdi@yahoo.com (M. Kacem). http://dx.doi.org/10.1016/j.jphotochem.2015.11.020 1010-6030/ ã 2015 Elsevier B.V. All rights reserved. Journal of Photochemistry and Photobiology A: Chemistry 317 (2016) 81–87 Contents lists available at ScienceDirect Journal of Photochemistry and Photobiology A: Chemistry journal homepa ge: www.elsev ier.com/locate/jphotochem