Novel Plasmid-Borne Multidrug Resistance Gene Cluster Including lsa (E) from a Linezolid-Resistant Enterococcus faecium Isolate of Swine Origin

Abstract

A novel nonconjugative plasmid of 28,489 bp from a porcine linezolid-resistant Enterococcus faecium isolate was completely sequenced. This plasmid harbored a novel type of multiresistance gene cluster that comprised the resistance genes lnu(B), lsa(E), spw, aadE, aphA3, and two copies of erm(B), which account for resistance to macrolides, lincosamides, streptogramins, pleuromutilins, streptomycin, spectinomycin, and kanamycin/neomycin. Structural comparisons suggested that this plasmid might have developed from other enterococcal plasmids by insertion element (IS)-mediated interplasmid recombination processes. During recent years, several ABC transporters were identified in staphylococci, streptococci, and enterococci that confer combined resistance to pleuromutilins, lincosamides, and streptogramin A antibiotics (PLS A ). The corresponding genes are, and lsa(E) (10). In contrast to the aforementioned genes, the gene lsa(B) confers only elevated MICs to lincosamides which, however, are below the clinical breakpoints for resistance (11). The lsa(E) gene has been identified as part of plasmid-borne or chromosomal multiresistance gene clusters in methicillin-resistant (MRSA) and methicillin-susceptible (MSSA) Staphylococcus aureus (12-14), coagulase-negative or -variable staphylococci (7, 15), and Enterococcus spp. (16) of human and animal origin in Europe and Asia; in human Streptococcus agalactiae from South America (17); and, most recently, in Erysipelothrix rhusiopathiae of swine origin in China (18). It is believed that the basic type of these multiresistance gene clusters, which comprises the resistance genes aadE, spw, lsa(E), and lnu(B), has developed in Enterococcus spp. (10,12,13). In the present study, a nonconjugative plasmid from Enterococcus faecium that harbors a novel lsa(E)-carrying multiresistance gene cluster was identified and completely sequenced to gain insight into its structure and the genetic environment of lsa(E).Thirty-five enterococcal strains, including Enterococcus faecalis (n ϭ 21), Enterococcus faecium (n ϭ 13), and Enterococcus gallinarum (n ϭ 1), were isolated from a pig farm in Guangxi province, China. These isolates were investigated for the presence of the lsa(E) gene by PCR using previously described primers (13). The lsa(E) gene was detected in five E. faecalis isolates and one isolate each of E. faecium and E. gallinarum. The lsa(E) nucleotide sequences of the seven isolates were identical to those of lsa(E) on plasmids pV7037 from MRSA ST9 and pXD4 from E. faecium (13, 16). The six E. faecalis and E. faecium isolates were analyzed by multilocus sequence typing (MLST; http://www.mlst.net/). Antimicrobial susceptibility testing by broth microdilution (19,20) revealed that except for the PLS A phenotype, all isolates were also resistant to erythromycin, tetracycline, streptomycin, gentamicin, kanamycin, and ciprofloxacin but susceptible to ampicillin and vancomycin. Additional resistance (or elevated MICs) to rifampin, florfenicol, or linezolid was seen in five or six of the is...

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