Inactivation of Escherichia coli by TiO2-mediated photocatalysis evaluated by a culture method and viability-qPCR

• E. coli photocatalytic inactivation was evaluated using culture, q-PCR and viability q-PCR.
• Photocatalytic effects on E. coli successively occurred with loss of culturability, loss of membrane integrity and DNA destruction.
• The photocatalytic treatment induced a VBNC state in E. coli.
• Two subpopulations of E. coli resistant and sensitive to the treatment were highlighted.

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 TiO2 photocatalysis. Culturable cells decayed faster (k1 = 0.66 h−1, T90 = 0.18 h) than viable cells (k1 = 0.18 h−1, T90 = 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.