Characterization of a Multiresistance Plasmid Carrying the optrA and cfr Resistance Genes From an Enterococcus faecium Clinical Isolate

Abstract

<i>Enterococcus faecium</i> E35048, a bloodstream isolate from Italy, was the first strain where the oxazolidinone resistance gene <i>optrA</i> was detected outside China. The strain was also positive for the oxazolidinone resistance gene <i>cfr</i>. WGS analysis revealed that the two genes were linked (23.1 kb apart), being co-carried by a 41,816-bp plasmid that was named pE35048-oc. This plasmid also carried the macrolide resistance gene <i>erm</i>(B) and a backbone related to that of the well-known <i>Enterococcus faecalis</i> plasmid pRE25 (identity 96%, coverage 65%). The <i>optrA</i> gene context was original, <i>optrA</i> being part of a composite transposon, named Tn<i>6628</i>, which was integrated into the gene encoding for the ζ toxin protein (<i>orf19</i> of pRE25). The <i>cfr</i> gene was flanked by two IS<i>Enfa5</i> insertion sequences and the element was inserted into an <i>lnu</i>(E) gene. Both <i>optrA</i> and <i>cfr</i> contexts were excisable. pE35048-oc could not be transferred to enterococcal recipients by conjugation or transformation. A plasmid-cured derivative of <i>E. faecium</i> E35048 was obtained following growth at 42°C, and the complete loss of pE35048-oc was confirmed by WGS. pE35048-oc exhibited some similarity but also notable differences from pEF12-0805, a recently described enterococcal plasmid from human <i>E. faecium</i> also co-carrying <i>optrA</i> and <i>cfr</i>; conversely it was completely unrelated to other <i>optrA</i>- and <i>cfr</i>-carrying plasmids from <i>Staphylococcus sciuri</i>. The <i>optrA</i>-<i>cfr</i> linkage is a matter of concern since it could herald the possibility of a co-spread of the two genes, both involved in resistance to last resort agents such as the oxazolidinones.