Spread of oxazolidinone-resistant enterococci in an Italian pig slaughterhouse

The swine production chain represents a critical point for the spread of antimicrobial resistance, with slaughterhouses acting as potential hotspots for cross-contamination. The veterinary use of phenicols may promote resistance to oxazolidinones, a last-resort class of antibiotics. This study investigated the presence of oxazolidinone-resistant enterococci in a pig slaughterhouse in central Italy, analyzing 376 samples, 25.8% of which tested positive for Enterococcus spp. resistant to florfenicol. Environmental samples collected at the end of the day of slaughter showed higher contamination levels compared to those collected beforehand. PCR analysis of 42 isolates of Enterococcus faecalis and Enterococcus faecium revealed a high prevalence of the optrA gene, followed by poxtA and cfr(D). The data indicated high resistance rates to tetracycline and erythromycin, consistent with existing literature. Linezolid resistance was observed in 73.8% of isolates, while no vancomycin resistance was detected. Whole Genome Sequencing (WGS) analysis revealed diverse sequence types, complex resistomes, multiple virulence factors, and considerable variability in the genetic elements carrying cfr/cfr(D), optrA, and poxtA. These determinants were often plasmid-borne and co-localized with other antibiotic and heavy metal resistance genes, which could contribute to their persistence and dissemination through co-selection mechanisms. Overall, the findings highlight the widespread occurrence of oxazolidinone-resistant enterococci in the pig production chain and their potential circulation across different production stages. An integrated approach encompassing prudent antibiotic use in livestock, good hygiene practices at the slaughterhouses, and increased awareness of the problem will be essential to mitigate the spread of antimicrobial resistance along the food production chain.