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Summary

Induction of culturable Escherichia coli O157:H7 cells into a viable but non-culturable (VBNC) state by chloraminated tap water was carefully investigated; as many as 90% of initial cells entered into a VBNC state within 15 min, compared with 14% in river water within 14 weeks. To understand what specific stresses may induce E. coli O157:H7 into a VBNC state, chloraminated tap water, autoclaved river water, and media with known ingredients (PBS buffer and deionized water) at 4°C or 25°C were used to examine induction efficiency. Chloramination alone, or the combination of starvation with either low temperature or osmotic pressure, induced E. coli O157:H7 into a VBNC state, while starvation alone did not induce the bacteria into a VBNC state within 1.5 years. The mRNA of the rfbE and fliC genes was detected in the 10-month-old VBNC cells induced by river water, confirming the viability of E. coli O157:H7 VBNC cells. The VBNC cells induced by chloraminated water and the 10-month-old VBNC cells induced by river water were first resuscitated using autoinducers produced by E. coli O157:H7 itself in a serum-based medium; the VBNC cells of bovine isolates recovered more efficiently compared with those of clinical isolates. These results demonstrate a potential health risk of VBNC E. coli O157:H7 in environmental water and the utility of monitoring viable E. coli O157:H7 including VBNC cells based on the mRNA of the rfbE and fliC genes.