Emergence of cfr-Mediated Linezolid Resistance in Staphylococcus aureus Isolated from Pig Carcasses

Altogether, 2547 Staphylococcus aureus isolated from cattle (n = 382), pig (n = 1077), and chicken carcasses (n = 1088) during 2010-2017 were investigated for linezolid resistance and were further characterized using molecular methods. We identified linezolid resistance in only 2.3% of pig carcass isolates. The linezolid-resistant (LR) isolates presented resistance to multiple antimicrobials, including chloramphenicol, clindamycin, and tiamulin. Molecular investigation exhibited no mutations in the 23S ribosomal RNA. Nevertheless, we found mutations in ribosomal proteins rplC (G121A) and rplD (C353T) in one and seven LR strains, respectively. All the LR isolates carried the multi-resistance gene cfr, and six of them co-carried the mecA gene. Additionally, all the LR isolates co-carried the phenicol exporter gene, fexA, and presented a high level of chloramphenicol resistance. LR S. aureus isolates represented 10 genotypes, including major genotypes ST433-t318, ST541-t034, ST5-t002, and ST9-t337. Staphylococcal enterotoxin and leukotoxin-encoding genes, alone or in combination, were detected in 68% of LR isolates. Isolates from different farms presented identical or different pulsed-field gel electrophoresis patterns. Collectively, toxigenic and LR S. aureus strains pose a crisis for public health. This study is the first to describe the mechanism of linezolid resistance in S. aureus isolated from food animal products in Korea.