Isolation of <i>Escherichia coli</i> O8:H8 Suspected to be the Cause of a Diarrhea Outbreak in 2017

Food poisoning in Toyama City in 2021 was caused by milk contaminated with Escherichia coli lacking known virulence factors (atypical diarrheagenic E. coli). This case prompted us to suspect that atypical diarrheagenic E. coli was the causative agent of an outbreak of unexplained diarrhea in Niigata Prefecture in 2017. We thus conducted a reinvestigation for E. coli in stored samples. The results of Escherichia coli O-genotyping PCR and Escherichia coli H-genotyping PCR showed that serotype clusters were detected in samples, and E. coli O8:H8 clones were isolated from patient stools. E. coli O8:H8 caused outbreaks of diarrhea in Oita Prefecture in the past. The E. coli O8:H8 harbored colonization factor CFA/III, and LT2d, a novel variant of heat-labile enterotoxin LT2 produced by ETEC. E. coli O8:H8 from the 2017 diarrheal outbreak was confirmed to harbor the same virulence factors as E. coli O8:H8 from the Oita outbreaks, strongly suggesting that E. coli O8:H8 was the cause of the 2017 diarrheal outbreak. The infection was transmitted via a school lunch, with an average incubation period of approximately 38 h and a maximum of 3.0×103 bacteria ingested, including hypothetical factor. Assaying for serotype clusters by Og-/Hg-typing PCR when outbreaks are caused by atypical diarrheagenic E. coli has been proposed.