Antimicrobial Susceptibility of Enterococcus Isolates from Cattle and Pigs in Portugal: Linezolid Resistance Genes optrA and poxtA

Enterococci are part of the commensal gut microbiota of mammals, with Enterococcus faecalis and Enterococcus faecium being the most clinically relevant species. This study assesses the prevalence and diversity of enterococcal species in cattle (n = 201) and pig (n = 249) cecal samples collected in 2017. Antimicrobial susceptibility profiles of E. faecium (n = 48) and E. faecalis (n = 84) were assessed by agar and microdilution methods. Resistance genes were screened through PCR and nine strains were analyzed by Whole Genome Sequencing. A wide range of enterococci species was found colonizing the intestines of pigs and cattle. Overall, the prevalence of resistance to critically important antibiotics was low (except for erythromycin), and no glycopeptide-resistant isolates were identified. Two daptomycin-resistant E. faecalis ST58 and ST93 were found. Linezolid-resistant strains of E. faecalis (n = 3) and E. faecium (n = 1) were detected. Moreover, oxazolidinone resistance determinants optrA (n = 8) and poxtA (n = 2) were found in E. faecalis (ST16, ST58, ST207, ST474, ST1178) and E. faecium (ST22, ST2138). Multiple variants of optrA were found in different genetic contexts, either in the chromosome or plasmids. We highlight the importance of animals as reservoirs of resistance genes to critically important antibiotics.