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.

Escherichia albertii is a newly emerging foodborne pathogen causing diarrhea, and its distribution in environmental water and various animals as birds and raccoons has been reported. However, the infection routes of E. albertii to humans remain unclear. In this study, we aimed to determine the distribution of E. albertii particularly in shellfish. In 112 samples examined, a total of eight E. albertii strains were isolated from seven samples of retail shellfish, including three samples of Japanese basket clam called Shijimi in Japan, two samples of Japanese rock oyster, and two samples of pacific oyster. These isolates possessed major biochemical properties and sufficient tolerance of salinity equivalent to seawater. In these isolates, eae, paa, and Eacdt as virulence gene were detected, similarly to human-derived E. albertii strains. Additionally, Eccdt-I and stx2f were also detected only in one isolate from Japanese rock oyster. Furthermore, a certain E. albertii strain was highly susceptible to cefixime using as selective agent in enrichment broth and other beta-lactams, indicating that two-step enrichment method using two kinds of selective enrichment broth is an efficient approach for isolating various E. albertii strains. Pulsed-field gel electrophoresis analysis revealed that one of E. albertii isolates from shellfish exhibited a high degree of genetic similarity to E. albertii strains from human and environmental water. Our results suggest that shellfish can be a source of infection for humans through the transmission of E. albertii from other sources, such as contaminated environmental water.