P16 Oxazolidinone resistance-associated genes cfr and optrA in MDR CoNS from healthy pigs in Italy
Authors: Bonvegna Miryam , Grego Elena , Tomassone Laura 
Published: February 2022

Abstract

Background
Oxazolidinones are relatively novel antibiotics used exclusively in human medicine as last resort drugs for resistant pathogens like MRSA, vancomycin-resistant enterococci and penicillin-resistant Streptococcus pneumoniae. However, in the last two decades, oxazolidinone resistance genes cfr and optrA have been sporadically reported worldwide in Staphylococcus spp. of livestock origin.1 These genes can be chromosomal, but they are often transferable through mobile genetic elements, especially plasmids.2 In Italy, they have been recently detected in enterococci of swine origin.3

Objectives
To uncover the presence of cfr and optrA genes in methicillin-resistant CoNS (MRCoNS) originating from swine nasal swabs sampled in a high farm-density area of northwestern Italy. Healthy pigs were sampled from three productive stages (finishing, weaners and sows). After isolating pure cultures, selected staphylococci (n = 27), resulted methicillin-resistant from previous mecA identification, were phenotypically tested through Kirby–Bauer disc diffusion method for the antibiotics clindamycin, doxycycline, erythromycin, enrofloxacin, florfenicol, gentamicin, linezolid, tetracycline, tiamulin and trimethoprim/sulfamethoxazole (EUCAST v.11.0 guidelines for linezolid disc, CLSI VET08 for the other antibiotics). MIC through Etest (Liofilchem®, Roseto degli Abbruzzi, Teramo, Italy) was used for the antibiotic ceftaroline.

Results
All the chosen MRCoNS were MDR (MDR CoNS), as they were phenotypically resistant to more than three antibiotic classes. No strain was positive for ceftaroline resistance. Since linezolid resistance was recovered in six samples, we decided to perform PCR for the cfr gene (746 bp), which was detected in Staphylococcus sciuri from a piglet (GenBank accession number OL412394), and optrA (1395 bp), which was recovered in Staphylococcus pasteuri from a finisher, S. sciuri from a sow and Staphylococcus cohnii from a weaner (GenBank accession numbers OM165030, OM165031 and OM165032). Sanger sequencing confirmed PCR result for cfr, with 100% identity with the cfr gene detected from a clinical Italian isolate of MRSA (MH746818), and for optrA gene, which had 100% identity with the optrA previously found in a swine Italian Enterococcus faecium strain (MT723958). As far as we know, this is the first time that a cfr gene has been detected in S. sciuri from a nasal sample of animal origin in Italy. Furthermore, optrA was never detected in S. pasteuri and S. cohnii strains.

Conclusions
These results are relevant from a One Health perspective, as they underline the need for oxazolidinone resistance monitoring, not only in human medicine, but also at farm level. In this way, it will be easier to prevent the dissemination of this resistance to human community and hospitals, where oxazolidinones are considered last-resort antibiotics. Furthermore, they remind the importance of surveillance of antibiotic usage in pigs, as cfr and optrA resistance in staphylococci can be elicited using certain antibiotics, like phenicols, due to cross-resistance to this antibiotic class.