Solar inactivation of E. coli DSM 1103

Wastewater has been implicated as a source of NDM-harboring bacteria and blaNDM-1 genes. This is of particular relevance in countries like Saudi Arabia where the pressure on water sources is high due to its scarcity, and there are plans to employ treated wastewater in agricultural irrigation. Sunlight can be used to serve as a final barrier to dissemination of contaminants in the environment. However, information is lacking on the effect of solar photoinactivation on ARBs, particularly that of an NDM-harboring bacteria. There is also a lack of information pertaining to the molecular response induced by these bacterium upon solar irradiation. This study examines the decay kinetics of the blaNDM-1-positive E. coli PI-7 upon solar irradiation in buffer and wastewater matrix so as to provide understanding on its fate and persistence upon dissemination into the natural environment. This study revealed differences in the solar inactivation kinetics between the blaNDM-1-positive E. coli PI-7 and a less-virulent blaNDM-1-negative commensal E. coli DSM 1103 in the buffer solution and treated wastewater. The results showed that a minimum of half a day exposure to solar irradiation is required to overcome the lag phase of E. coli prior to achieving inactivation. Although solar irradiation remains useful and effective in reducing E. coli strains by more than 5-log cell, transcriptomics revealed differences in the overall upregulation of protective and repair mechanisms between both E. coli strains. Moreover, subpopulations of the E. coli PI-7 expressed genes related to dormancy and persister cells formation during the late decay phase, which may have accounted for the prolonged persistence of E. coli PI-7. More importantly, this study showed that both E. coli strains displayed upregulation of horizontal gene transfer, antibiotic resistance and virulence functions, with a wider arsenal of such genes being upregulated in E. coli PI-7. The current guidance on the quality pertaining to treated wastewater for use in agricultural irrigation remains limited to assessing the abundance of fecal coliforms. Our findings suggest a need to expand efforts to monitor for the presence and abundance of ARB in the wastewater, particularly for those bacterial pathogens that are commonly associated with foodborne diarrheal diseases. Alternative strategies such as the combination of bacteriophage and solar irradiation can be utilized to minimize potential risks associated with ARB in treated wastewater.