Mobile genetic element diversity across Shiga toxin-producing Escherichia coli lineages in French cattle

Shiga toxin-producing Escherichia coli (STEC) represent a genetically diverse group of pathogens whose virulence is largely driven by mobile genetic elements (MGEs), including plasmids and bacteriophages. While horizontal gene transfer is central to STEC evolution, the extent to which virulence-associated MGEs circulate within natural reservoirs remains poorly understood. In this study, we investigated the diversity, distribution and lineage associations of MGEs in a collection of 73 E. coli strains isolated from cattle in France, a major reservoir for pathogenic STEC. Using both short-read and long-read whole-genome sequencing, we characterized plasmid content, prophage repertoires and stx-encoding phages and examined their relationships with strain phylogeny. We observed a high diversity of plasmids, with individual strains carrying up to four large plasmids, alongside an even greater diversity of prophages. Despite this diversity, some associations were identified between specific virulence plasmid groups, Stx phage types and defined pathogroups or lineages. These patterns were supported by the congruence between core-genome and accessory-genome phylogenies, suggesting long-term evolutionary coupling rather than frequent exchange of entire MGEs. In contrast, some non-virulence plasmids were broadly distributed, consistent with more general selective advantages. Notably, we identified enterohaemorrhagic E. coli strains (stx- and eae-positive strains) in atypical phylogenetic backgrounds, highlighting the capacity for virulence determinants to emerge in diverse lineages, while underscoring the constraints that limit their stable establishment, as their long-term persistence appears limited to specific genetic backgrounds. Together, our findings indicate that the circulation of virulence-associated MGEs in the bovine reservoir is constrained by ecological and evolutionary factors.