Analysis of the genetic and phylogenetic context of Escherichia coli O77g:H18 associated with clustered cases of HUS in France in 2025.

In January 2025, Santé publique France detected several cases of hemolytic uremic syndrome (HUS) in adults caused by a rare serotype of Shiga toxin-producing Escherichia coli, STEC O77g:H18. Epidemiological and traceability investigations linked the consumption of raw-milk cheese to these cases. The outbreak strain belonged to sequence type 69 (ST69) and harbored a Shiga toxin (stx) subtype stx2d located on a unique prophage. Putative virulence genes cdt-VABC, hlyE, fimH, and lpfA were detected; however, STEC virulence genes (eae and aggR) were absent. Phylogenetic analysis of E. coli O77g strains from public databases (Enterobase and NCBI) placed the outbreak strain within a cluster of clinical E. coli O77g:H18, ST69, stx2d strains isolated between 2012 and 2025, including one linked to a HUS case in France and three in Australia. This strain, distinct from O77g:H18 strains previously isolated from dairy products, is therefore not a new lineage. The closest cluster contains eight stx-negative strains isolated from poultry between 1990 and 2024 in Europe and North America. Differences in virulence gene composition between the "outbreak cluster" and the "poultry cluster" suggest a common ancestor that has diverged over time. The "outbreak cluster" is characterized by strains carrying an IncFIA/FIB plasmid encoding a putative enterotoxin gene (elt) absent in other E. coli strains in GenBank. We developed qPCR assays specific for the putative enterotoxin and O77g O-group. Combined with a PCR for stx2, these assays allow rapid screening of suspect unpasteurized cheese for the outbreak strain.IMPORTANCEThe emergence of hybrid pathotypes of Shiga toxin-producing Escherichia coli (STEC) has been of importance in recent years in public health. This outbreak, related to a rare serotype of STEC and involving an enterotoxigenic E. coli (ETEC)/STEC hybrid, highlights the necessity of phylogenetic analyses to investigate the origin of contamination with such strains. Our study suggests a common ancestor between the outbreak strain and strains isolated from poultry. We also provided molecular tools to target specifically this outbreak strain and help in future epidemiological investigations.