Genomic analysis of an Erysipelothrix tonsillarum isolate from Orcinus orca indicates conservation of major Erysipelothrix virulence factors

The gram-positive bacterium Erysipelothrix tonsillarum is primarily a commensal resident of terrestrial animals that is infrequently isolated in clinical contexts. It is thought to lack most of the virulence factors described for the closely related pathogen E. rhusiopathiae, including the surface protective antigen (Spa) critical for host-microbe interactions. Despite this, E. tonsillarum was recently recovered from the internal organs of a killer whale (Orcinus orca) during postmortem examination. Whole genome sequencing confirmed the isolate's identity and provided an opportunity to screen for genetic features relevant to infection and disease. Homologs to major E. rhusiopathiae virulence factors were present in the E. tonsillarum isolate, including the full sequence of a proposed spa variant (SpaD). The SpaD variant had 53.3%-63.8% nucleotide identity to representative SpaA, SpaB, and SpaC sequences and 45%-54% amino acid conservation. Predictive three-dimensional models and structural alignments demonstrated the SpaD protein is highly similar to canonical types. While further functional characterization is necessary to determine the relevance of these homologs to pathogenicity, this study expands the known host range of E. tonsillarum and provides valuable insights into an understudied Erysipelothrix species.