Bactericidal Effect of Hot Water on Cucumbers Contaminated with <i>Escherichia coli</i>

In August 2016, a food-poisoning outbreak occurred across several nursing homes in Japan, which was caused by enterohemorrhagic Escherichia coli (EHEC) O157. The cucumbers added in perillas were speculated as the food causing the infection, due to the inefficient sterilization of cucumbers at these facilities. The present study aimed to investigate the bactericidal effects of sodium hypochlorite solution (NaClO) and hot water (80℃) on cucumbers attached with two E. coli strains: EH5487 (EHEC) and EC3207 (non- EHEC). The number of EC3207 did not reduce when treated by NaClO at 40 ppm or 200 ppm; whereas, the treatment of 2,000 ppm NaClO could significantly reduce the number of EC3207. Considering the bacterial count responsible for EHEC infection to be caused (10–100 CFU), the effect was insufficient. On the contrary, the number of EC3207 and EH5487 could be reduced to less than detective limit (1.0 log CFU/g) when treated with hot water. These results lead to the conclusion that the treatment with hot water at 80℃ is a useful sterilization method for E. coli including EHEC.