Oxazolidinone resistance–associated genes cfr and optrA in methicillin- resistant coagulase negative staphylococci (MRCoNS) from healthy pigs in Italy

In the last two decades, oxazolidinones’ resistance genes cfr and optrA have been occasionally reported worldwide in Staphylococcus spp. of livestock origin. These genes can be chromosomic, but they are often transferable through mobile genetic elements. The objective of our study was investigating the presence of cfr and optrA genes in methicillin-resistant coagulase-negative staphylococci (MRCoNS) isolated from swine in a high farm-density area of northwestern Italy. Nasal swabs were taken from healthy pigs. After isolating pure cultures, selected MRCoNS (n=28) were phenotypically tested through Kirby-Bauer disk diffusion method (EUCAST guidelines for linezolid disk). Linezolid resistance was recovered in six samples, which were subjected to PCR targeting cfr and optrA genes. cfr gene was detected in one S. sciuri from a piglet, while optrA was amplified from S. sciuri, S.cohnii and S.pasteuri (n=3 samples) collected in different productive stages (sows, post-weaning and finishing). The detection of both genes from the same sample did not occur. In Italy, oxazolidinones’ resistance genes were recently detected in enterococci of swine origin, but this is the first report in staphylococci of animal origin. Our results highlight the importance of monitoring oxazolidinones’ resistance, even at farm level, to hinder the dissemination of these resistances to human community and hospitals, where oxazolidinones are used as last-resort antibiotics. In addition, they emphasize the need of surveillance of antibiotic usage in pigs, since both cfr and optrA genes mediate cross-resistance to other antibiotic classes, such as phenicols, that are routinely used in swine farming.