A Novel Transferable Resistance-Nodulation-Division Pump Gene Cluster, <i>tmexCD2-toprJ2</i> , Confers Tigecycline Resistance in Raoultella ornithinolytica

Abstract

We recently identified a novel plasmid-mediated resistance-nodulation-division (RND)-type efflux pump gene cluster, <i>tmexCD1-toprJ1</i>, in <i>Klebsiella pneumoniae</i> that conferred resistance to multiple antimicrobials, including tigecycline. While homologs of <i>tmexCD1-toprJ1</i> were found encoded in many other bacterial species in GenBank, their functions and transfer mechanisms remain unknown. This study identified another mobile gene cluster, <i>tmexCD2-toprJ2</i>, co-occurring on both a plasmid (pHNNC189-2) and the chromosome of a clinical <i>Raoultella ornithinolytica</i> isolate, strain NC189, producing KPC-2, NDM-1, and RmtC. <i>tmexCD2-toprJ2</i> shares high similarity at the nucleotide level with <i>tmexCD1-toprJ1</i>, with 98.02%, 96.75%, and 99.93% identities to <i>tmexC1</i>, <i>tmexD1</i>, and <i>toprJ1</i>, respectively. Phylogenetic analysis revealed that <i>tmexCD2-toprJ2</i> may have originated from the chromosome of a <i>Pseudomonas</i> species. The expression of <i>tmexCD2-toprJ2</i> in an <i>Escherichia coli</i> strain resulted in an 8-fold increase in the tigecycline MIC and decreased susceptibility to other antimicrobials. Genetic context analyses demonstrated that <i>tmexCD2-toprJ2</i>, together with the adjacent hypothetical site-specific integrase genes, was possibly captured and mobilized by a XerD-like tyrosine recombinase system, forming a putative transposition unit (<i>xerD</i>-like-<i>int3</i>-like-<i>thf2-ybjD-umuD-</i>Δ<i>umuC1</i>-<i>int1</i>-like-<i>int2</i>-like-<i>hp1-hp2-tnfxB2-</i>IS<i>Bvi2-tmexCD2-toprJ2-</i>Δ<i>umuC1</i>), which was inserted into <i>umuC-</i>like genes in both the NC189 plasmid pHNNC189-2 and the chromosome. Since <i>tmexCD1-toprJ1</i> and <i>tmexCD2-toprJ2</i> could confer multidrug resistance, the spread of these gene clusters, associated with the new recombinase system, calls for more attention.