Presence of the optrA gene in methicillin-resistant Staphylococcus sciuri of porcine origin
A total of 57 methicillin-resistant Staphylococcus aureus (MRSA) isolates and 475 methicillin-resistant coagulase-negative staphylococci (MRCoNS) collected from pigs in the Guangdong province of China in 2014 were investigated for the presence of the novel oxazolidinone-phenicol resistance gene optrA. The optrA gene was detected in 6.9% (n=33) of the MRCoNS, all of which were MR-Staphylococcus sciuri, but in none of the MRSA. Five optrA-carrying MR-S. sciuri also harbored the multiresistance gene cfr. Pulsed-field gel electrophoresis (PFGE) and dru typing of the 33 optrA-carrying MR-S. sciuri revealed 25 patterns and 5 sequence types, respectively. S1-nuclease PFGE and Southern blotting confirmed that optrA was located in the chromosomal DNA of 29 isolates, including one cfr positive isolate. The remaining four isolates harbored a ∼35 kb pWo28-3-like plasmid on which optrA and cfr were co-located together with other resistance genes as confirmed by sequence analysis. Six different types (I-VI) of the genetic environments of the chromosome-borne optrA genes were identified, which had the optrA gene and its transcriptional regulator araC in common. Tn558 was found to be associated with araC-optrA in types II-VI. The optrA gene in types II and III was found in close proximity to the ccr gene complex of the respective SCCmec cassettes. Since oxazolidinones are last resort antimicrobial agents for the control of serious infections caused by methicillin-resistant staphylococci in humans, the location of optrA gene close to the ccr complex is an alarming observation.