Identification of porcine-derived atypical intestinal pathogenic Escherichia coli reveals to a hidden threat of extraintestinal infection.

Abstract

Post-weaning diarrhea in piglets is primarily caused by enterotoxigenic Escherichia coli (ETEC) and Shiga toxin-producing E. coli (STEC). However, the diversity and combinations of virulence factors among clinical porcine isolates remain incompletely characterized. Here, we systematically screened 296 E. coli isolates from diarrheic piglets and identified nine isolates (3.0 %) that lacked classical ETEC/STEC virulence factors but co-harbored F18 fimbriae and a hemolysin gene cluster. These F18⁺&hly⁺ strains exhibited strong intestinal adhesion and colonization in vitro and in vivo and were designated enteroadherent hemolytic E. coli (EAHEC). Notably, EAHEC strains belonging to phylogroup D also displayed extraintestinal pathogenicity in mouse models, indicating cross-niche potential. Based on this finding, we identified 17 hybrid pathogenic E. coli (HyPEC) isolates (5.7 %) by PCR screening. These hybrid strains were classified based on two criteria: (i) the presence of both intestinal virulence markers (ETEC/EPEC/STEC/EAHEC) and ExPEC/UPEC-associated virulence genes, or (ii) the carriage of ExPEC/UPEC-associated virulence genes despite lacking classical intestinal pathotype markers, while nonetheless exhibiting intestinal pathogenic phenotypes. Phenotypic assays showed that most HyPEC strains retained strong intestinal colonization capacity, whereas systemic infection capacity varied by phylogroup, with B2 and D lineages exhibiting the highest virulence. Overall, our study documents the existence and virulence potential of atypical enteroadherent hemolytic and hybrid E. coli in pigs, highlights diverse virulence-module combinations in porcine isolates, and indicates that recognition of such atypical/hybrid strains may have implications for future diagnostics and surveillance of porcine diarrheal disease.