Enterococcus thailandicus: genomic evidence of a potential new player in enterococcal virulence and antimicrobial resistance

Abstract

[Abstract] Background: Enterococcus thailandicus has been isolated from animals, fermented foods, wastewater, and humans, but its clinical relevance remains unclear. We investigated the pathogenic potential of E. thailandicus by phenotypic and genomic characterization of two human isolates from acute abdominal infections, complemented by a species-wide comparative genomic analysis of all publicly available E. thailandicus genomes.

Methods: Two E. thailandicus isolates were recovered from patients with acute cholecystitis and small bowel perforation. Species identification was performed by MALDI-TOF MS and ANI analysis, and antimicrobial susceptibility testing was conducted. Whole-genome sequencing (WGS) was performed, followed by in silico analysis of antimicrobial resistance genes, virulence factors, bacteriocins, and plasmid replicons. All available E. thailandicus genomes were included for phylogenetic comparison.

Results: Both clinical isolates were broadly susceptible to antimicrobials. WGS revealed the presence of the ptsD gene (linked to hospital-adapted E. faecium) and the bacteriocin gene enkB in both isolates. One isolate carried the tet(M) gene and a plasmid replicase associated with E. faecium plasmids. Among 46 genomes analyzed, 61 % carried ptsD, all carried enkB, and 48 % harbored acquired AMR genes, including functional optrA (n = 12) and poxtA (n = 5), both conferring linezolid resistance. Phylogenetic analysis showed wide genetic diversity with clustering by host or geography, consistent with ecological versatility.

Conclusions: This study reports the first genomic characterization of clinical E. thailandicus isolates, demonstrating its ecological adaptability, opportunistic pathogenic potential, and role as a reservoir of clinically relevant adaptive traits. Expanded genomic surveillance is warranted to clarify its evolutionary trajectory and potential impact on human health.