Viability assay of E. coli O157: H7 treated with electrolyzed oxidizing water using flow cytometry

Bacteria treated with chlorine-based sanitizer could enter into viable but non-culturable (VBNC) state, which may be underestimated its health risk. In this study, E. coli O157: H7 was treated with slightly acidic electrolyzed water (SAEW), acidic electrolyzed water (AEW) and sodium hypochlorite with different available chlorine concentrations (ACC). The reduction of population was determined, as well as the status of treated cells (viable and culturable, VBNC, dead) was assayed using flow cytometry combined with plate counting. Finally, the cell which cannot be detected using routine media was cultured in p-TSB to resuscitate. The results showed that the reduction of E. coli O157: H7 increased with increasing ACC of SAEW, AEW and sodium hypochlorite and the antimicrobial effect of electrolyzed oxidizing (EO) waters was much higher than sodium hypochlorite. SAEW, AEW and sodium hypochlorite with ACC of 30, 20 and 60 mg/L achieved E. coli O157: H7 colonies to non-detectable level on solid media. The results obtained from flow cytometric assay indicated that culturable cells after EO water and sodium hypochlorite treatments with increasing ACC firstly enter VBNC state and then transferred to dead status. VBNC E.coli was able to be resuscitated in p-TSB and therefore regarded as a potential risk for human health. E. coli O157: H7 in VBNC state after EO water treatment should be taken into consideration and complete inactivation could be achieved by elevated chlorine concentration.