Linezolid-Resistant Enterococcus faecalis of Chicken Origin Harbored Chromosome-Borne <i>optrA</i> and Plasmid-Borne <i>cfr</i> , <i>cfr</i> (D), and <i>poxtA2</i> Genes

Abstract

The aim of this study was to investigate the transferability of acquired linezolid resistance genes and associated mobile genetic elements in an Enterococcus faecalis isolate QZ076, cocarrying <i>optrA</i>, <i>cfr</i>, <i>cfr</i>(D), and <i>poxtA2</i> genes. MICs were determined by broth microdilution. Whole-genome sequencing (WGS) was performed using the Illumina and Nanopore platforms. The transfer of linezolid resistance genes was investigated by conjugation, using E. faecalis JH2-2 and clinical methicillin-resistant Staphylococcus aureus (MRSA) 109 as recipients. E. faecalis QZ076 harbors four plasmids, designated pQZ076-1 to pQZ076-4, with <i>optrA</i> located in the chromosomal DNA. The gene <i>cfr</i> was located on a novel pseudocompound transposon, designated Tn<i>7515</i>, integrated into the 65,961-bp pCF10-like pheromone-responsive conjugative plasmid pQZ076-1. Tn<i>7515</i> generated 8-bp direct target duplications (5'-GATACGTA-3'). The genes <i>cfr</i>(D) and <i>poxtA2</i> were colocated on the 16,397-bp mobilizable broad-host-range Inc18 plasmid pQZ076-4. The <i>cfr</i>-carrying plasmid pQZ076-1 could transfer from E. faecalis QZ076 to E. faecalis JH2-2, along with the <i>cfr</i>(D)- and <i>poxtA2</i>-cocarrying plasmid pQZ076-4, conferring the corresponding resistant phenotype to the recipient. Moreover, pQZ076-4 could also transfer to MRSA 109. To the best of our knowledge, this study presented the first report of four acquired linezolid resistance genes [<i>optrA</i>, <i>cfr</i>, <i>cfr</i>(D), and <i>poxtA2</i>] being simultaneously present in the same E. faecalis isolate. The location of the <i>cfr</i> gene on a pseudocompound transposon in a pheromone-responsive conjugative plasmid will accelerate its rapid dissemination. In addition, the <i>cfr</i>-carrying pheromone-responsive conjugative plasmid in E. faecalis was also able to mobilize the interspecies transfer of the <i>cfr</i>(D)- and <i>poxtA2</i>-cocarrying plasmid between enterococci and staphylococci. <b>IMPORTANCE</b> In this study, the simultaneous occurrence of four acquired oxazolidinone resistance genes [<i>optrA</i>, <i>cfr</i>, <i>cfr</i>(D), and <i>poxtA2</i>] was identified in an E. faecalis isolate of chicken origin. The association of the <i>cfr</i> gene with a novel pseudocompound transposon Tn<i>7515</i> integrated into a pCF10-like pheromone-responsive conjugative plasmid will accelerate its dissemination. Moreover, the location of the resistance genes <i>cfr</i>(D) and <i>poxtA2</i> on a mobilizable broad-host-range Inc18 family plasmid represents the basis for their intra- and interspecies dissemination with the aid of a conjugative plasmid and further accelerates the spreading of acquired oxazolidinone resistance genes, such as <i>cfr</i>, <i>cfr</i>(D), and <i>poxtA2</i>, among Gram-positive pathogens.