<b>Identification of porcine-derived atypical intestinal pathogenic </b><b><i>Escherichia coli</i></b><b> alerts to a hidden threat of extraintestinal infection</b>

Post-weaning diarrhea in piglets is primarily caused by enterotoxigenic <i>Escherichia coli</i> (ETEC) and Shiga toxin-producing <i>E. coli </i>(STEC). However, the diversity and combinatorial patterns of virulence factors among clinical isolates remain poorly characterized. In recent years, hybrid pathogenic <i>E. coli</i> strains that possess both intestinal and extraintestinal virulence traits have gained increasing attention, posing emerging threats to swine health and food safety. Here, we systematically screened 296 <i>E. coli</i> isolates from diarrheic piglets and identified a subset of strains lacking classical ETEC/STEC virulence factors but co-harboring F18 fimbriae and hemolysin gene clusters. These strains exhibited strong intestinal adhesion and colonization abilities and may represent a novel pathotype, which we designated as enteroadherent hemolytic <i>E. coli </i>(EAHEC). Notably, EAHEC strains belonging to phylogroup D also displayed significant extraintestinal pathogenicity, suggesting their potential for cross-niche infections. Based on this finding, we further identified 17 hybrid strains and classified them into genotype-based and phenotype-based hybrid strain groups according to their virulence factor profiles, indicating the presence of hybrid strains even among atypical intestinal pathotypes. <i>In vitro</i> and <i>in vivo</i> assays showed that most hybrid strains possessed strong intestinal colonization capacity, whereas systemic infection potential varied significantly, with B2 and D phylogroup strains exhibiting the highest virulence. In conclusion, our study reveals the diverse combinations of virulence modules in porcine intestinal <i>E. coli</i> isolates and highlights the emergence and pathogenic potential of hybrid strains, providing novel insights into the mechanisms of atypical intestinal pathotypes and informing future strategies for swine disease surveillance and control.