Virulence determinants in nontoxinogenic Escherichia coli O157 strains that cause infantile diarrhea.
Ten sorbitol-fermenting Escherichia coli O157 strains that cause infantile diarrhea and are positive in the fluorescence actin staining test were determined to be negative for Shiga-like toxin (SLT) genes. We amplified their complete eae genes, contrasting them with those of SLT-producing E. coli O157 by restriction fragment length polymorphism analysis and nucleotide sequence analysis of a 400-bp stretch of the 3' end of eae. The data substantiated the presence of two eae genotypes within serogroup O157, one resembling eae of enteropathogenic E. coli (EPEC) strain E2348/69, found in nontoxinogenic E. coli O157 strains, and the other resembling eae of EHEC strain EDL 933, found in toxinogenic E. coli O157 strains. Another EPEC-specific virulence determinant was also shown to be large plasmids harboring EPEC adherence factor sequences. The SLT-negative E. coli O157 strains described here fall under the heading of EPEC, which serves as an explanation for their virulence in infants, and represent a third pathogroup within serogroup O157.