| Summary: | Sequence type 59 (ST59) is the dominant type of community-associated methicillin-resistant <i>Staphylococcus aureus</i> (MRSA) in Taiwan. Previously, we reported that ST59 MRSA harbors enterococcal IS<i>1216V</i>-mediated multidrug-resistant composite transposons MES<sub>PM1</sub> or MES<sub>6272-2</sub>. The MES were found to have a mosaic structure, largely originating in enterococci and partly native to <i>S. aureus</i>. The current study aimed to track the origin of the MES and how they disseminated from enterococci to ST59 <i>S. aureus</i>. A total of 270 enterococcal isolates were analyzed, showing that two ST64 <i>Enterococcus faecalis</i> isolated in 1992 and 11 clonal complex 17 <i>Enterococcus faecium</i> harbored MES<sub>PM1</sub>-like and MES<sub>6272-2</sub>-like structures, respectively. Sequence analysis revealed that ST64 <i>E. faecalis</i> strain N48 acquired the MES<sub>PM1</sub>-like structure on the plasmid pEflis48. The pEflis48 harbored the enterococci-originated region (erythromycin, kanamycin, and streptomycin resistances) and the <i>S.</i><i>aureus</i>-originated region (chloramphenicol resistance) of MES<sub>PM1</sub> but was separated by the replication region of the plasmid. Homologous recombination between the two direct repeats of IS<i>1216V</i> resulted in excision of the replication region of the plasmid to regenerate MES<sub>PM1</sub>. The p4780-1 and pV19 of <i>E. faecium</i> carried MES<sub>6272-2</sub>-like structures with IS<i>1216V</i>, albeit with multiple insertions by other insertion sequences. The findings show that IS<i>1216V</i> plays important roles in bidirectional gene transfer of multidrug resistance between enterococci and <i>S. aureus</i>.
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