Combined usage of serodiagnosis and O antigen typing to isolate Shiga toxin-producing Escherichia coli O76:H7 from a hemolytic uremic syndrome case and genomic insights from the isolate

Minor O-serogroups of Shiga toxin-producing Escherichia coli (STEC) can cause severe clinical complications in humans, including hemolytic uremic syndrome (HUS). However, detection and isolation of these minor serogroups of STEC are challenging due to the lack of specific isolation methods. Here, we present a case of HUS in which STEC was not isolated by routine diagnostic tests for the major serotypes. Therefore, we tried a new diagnostic and isolation method that combines PCR screening, immunomagnetic bead separation, and serum agglutination tests and successfully isolated STEC O76. Subsequent genomic analyses of the STEC O76 isolates revealed that several isolates of this serogroup carrying stx2 were related to severe infections. The complete genome of the HUS-derived isolates provided two important implications. First, using a complete genome as a reference in core genome single nucleotide polymorphism analysis leads to the highest resolution of the analysis. Second, the HUS-derived STEC O76:H7 possessed two copies of Stx2a prophages, and one of them showed a "prophage integrating into prophage" structure, as described in STEC O145:H28. These results demonstrate that our detection methods contribute to the diagnosis and isolation of minor serogroups of STECs and complete genomic analyses can illuminate the pathogenic potential of STECs.