MLS_B-Resistant Staphylococcus aureus in Central Greece: Rate of Resistance and Molecular Characterization

The aim of the present study was to determine the rate and mechanisms of resistance to macrolides, lincosamides, and streptogramin B (MLS_B) antibiotics of Staphylococcus aureus collected in Central Greece. Of the 2,893 S. aureus collected during 2012-2017, 1,161 isolates (40.2%) exhibited resistance to at least one of the MLS_B agents. The rate of erythromycin resistance was statistically significantly higher in methicillin-resistant S. aureus (MRSA) (58.6%) than in methicillin-sensitive S. aureus (MSSA) isolates (20.7%) (p = 0.002). Two hundred seventy-five representative MLS_B-resistant S. aureus, including 81 MSSA and 194 MRSA isolates, were further studied. Thirty-eight MSSA isolates carried ermC, 26 MSSA were positive for ermA, whereas 17 isolates carried msrA gene. Among MRSA, the ermA gene was identified in the majority of the isolates (n = 153). Thirty-seven MRSA isolates carried ermC; three isolates carried msrA, whereas the remaining MRSA was positive for two genes (ermA and ermC). Phylogenetic analysis showed that ST225, which belongs to CC5, was the most prevalent, accounting for 137 MRSA isolates. Higher genetic diversity was found in the group of MSSA isolates, which comprised of 13 sequence types. Whole-genome sequencing data showed that all ermA-positive S. aureus, with the exception of one ST398 isolate, harbored the ermA-carrying Tn554 transposon integrated into their chromosomes. Furthermore, Illumina sequencing followed by polymerase chain reaction screening identified that ermC, which was identified in a polyclonal population of MSSA and MRSA isolates, was carried by small plasmids, like pNE131. These findings highlighted the important role of high-risk clones and of mobile elements carrying resistance genes in the successful dissemination of MLS_B-resistant staphylococci.