Comparative core-genome MLST of vancomycin-resistant Enterococcus faecium supports the utility of wastewater-based surveillance: a pilot study

Wastewater-based surveillance (WBS) could complement clinical data and be used as an early warning tool for population-level monitoring of priority pathogens such as vancomycin-resistant Enterococcus faecium (VREfm). In this pilot study, we isolated VREfm using CHROMAgar VRE from 77 composite wastewater influent samples from ten wastewater treatment plants (WWTPs), assessed their phenotypic antibiotic susceptibility profiles using the broth microdilution assay, and used core genome MLST (cgMLST) to examine the genetic relatedness of human and wastewater isolates in Finland. VREfm was isolated from 17 samples (22%), with no significant difference in the isolation rate across the ten WWTPs (p = 0.407). The phenotypic antimicrobial susceptibility testing (AST) revealed that all isolates were resistant to ampicillin, ciprofloxacin, erythromycin, teicoplanin, and vancomycin and were susceptible to chloramphenicol, daptomycin, and linezolid. All 17 VREfm isolates belonged to CC17, and 12 of them were sequence type ST80. Altogether, 76% (n = 13) of the wastewater isolates harboured the vanA gene, 18% (n = 3) had the vanB gene, and 6% (n = 1) had both vanA and vanB genes. cgMLST revealed that most VREfm strains from wastewater clustered closely (< 10 allelic differences) with those from human surveillance isolates across the country. The ST80 isolates belonged to the clusters identified during 2022-2023 as part of national VRE surveillance of screening samples. Therefore, WBS closely aligns with what is observed through human surveillance. We believe that this pilot supports the utility of WBS as a sensitive tool for AMR surveillance. This study also contributes to the One Health approach, needed to fully understand the transmission dynamics of pathogenic bacteria and effectively manage the antimicrobial resistance (AMR) challenge.