The tet39 Determinant and the msrE-mphE Genes in Acinetobacter Plasmids Are Each Part of Discrete Modules Flanked by Inversely Oriented p dif (XerC-XerD) Sites

Abstract

The tet39 tetracycline resistance determinant and the macrolide resistance genes msrE and mphE were found in an 18.2-kb plasmid, pS30-1, recovered from a global clone 2 (GC2) Acinetobacter baumannii isolate from Singapore, that conferred resistance to tetracycline and erythromycin. pS30-1 also contains mobA and mobC genes encoding MOB Q family proteins, but attempts to mobilize pS30-1 utilizing a coresident conjugative repAci6 plasmid were unsuccessful. Eight pdif sites, consisting of inversely oriented binding sites for the XerC and XerD recombinases separated by 6 bp, were detected in pS30-1. The tet39 determinant and the msrEmphE gene pair are each surrounded by two pdif sites in inverse orientation. Identical regions in different contexts and many previously unnoticed pdif sites were found in a number of different plasmids in GenBank, showing that the tet39 and msrE-mphE dif modules are mobile. A putative toxin/antitoxin system, a gene encoding a serine recombinase, and open reading frames of unknown function were also part of dif modules in pS30-1. In general, modules with internal XerC or XerD sites alternate. Two copies of ISAjo2-1 (94% identical to ISAjo2) in pS30-1 were inserted 5 bp from a XerC site, and this appears to be the preferred insertion site for this insertion sequence (IS) group. Apparently, Acinetobacter plasmids exploit the Acinetobacter XerC-XerD recombinases to mobilize DNA units containing resistance and other genes, via an uncharacterized mechanism. The tet39 and msrE-mphE dif modules add to the oxa24 module and the oxa58 module redefined here, bringing the total of resistance gene-containing dif modules in Acinetobacter plasmids to four. KEYWORDS tet39 tetracycline resistance, Acinetobacter baumannii, antibiotic resistance, dif modules, msrE-mphE macrolide resistance, plasmids A cinetobacter baumannii is an important nosocomial pathogen whose treatment is increasingly problematic due to high levels of extensive antibiotic resistance. In addition to resistance genes located in islands in the chromosomes of globally disseminated clones, global clone 1 and 2 (GC1 and GC2) isolates, a number of resistance genes can be carried by plasmids. Some of these genes are part of defined transposons, such as aphA6 (confers resistance to amikacin, kanamycin, and neomycin) in TnaphA6 (1) and oxa23 (carbapenem resistance) in Tn2006, Tn2008, or Tn2009 (2). These transposons have often been found in large conjugative repAci6 plasmids (1,3,4). However, in Acinetobacter species two other resistance genes encoding carbapenemases appear to be mobilized by a novel mechanism. The oxa24 gene and its single base pair variant encoding OXA-72 have been found flanked by inversely oriented XerC-XerD binding sites, or dif-like sites (referred to as pdif sites from here on), in plasmids from various Acinetobacter species (5-8). As this oxa24 module has several different sequences on

Molecular epidemiology and antimicrobial resistance features of Acinetobacter baumannii clinical isolates from Pakistan

“…The msr(E)-mph(E) macrolide resistance operon in Ab-Pak-cluster-7 was surrounded by two pdif sites creating a module with a probable movability mediated by the XerC-XerD system [56]. The msr(E)-mph(E) operon was co-located on a contig carrying the repAci25 gene, encoding for novel plasmid replication initiator protein from the Rep_3 protein family (pfam01051) and superfamily (cl19398).…”

Multidrug-resistant plasmids repress chromosomally encoded T6SS to enable their dissemination

“…LCPs Are Essential for SMP Dissemination. SMPs contribute to the MDR phenotype in many Ab clinical isolates around the world, as they encode resistance to tetracyclines, aminoglycosides, and carbapenems (7,10). The presence of putative oriT and mobA, a conserved relaxase, indicates that SMPs can be mobilized if the T4SS is provided in trans.…”

“…Many SMPs carry antibiotic resistance cassettes against tetracyclines, aminoglycosides, and carbapenems (8,9). Although these plasmids do not encode their own conjugation machinery, the presence of conserved oriT and mobA elements suggests SMPs are mobilizable if the conjugative machinery is provided in trans (10). Although SMPs and LCPs have divergent genetic origins, they contain a conserved T4SS machinery and/or elements that target them for mobilization, supporting that conjugation plays a critical role in their dissemination.…”

The plasmid diversity ofAcinetobacter bereziniaeHPC229 provides clues on the ability of the species to thrive on both clinical and environmental habitats

“…XerC/D sites are generally composed by two conserved motifs of 11 bp, separated by a less-conserved central region (cr) generally spanning 6 nt [69], although sites with cr regions of five, seven, and even more nucleotides have also been described [14,48,69,70]. The ubiquitous distribution of the XerC/XerD site-specif recombination system among bacteria has led to proposals that the XerC/D sites bordering these modules play roles in their mobilization and dissemination [13,15-18], although the exact mechanism is still obscure [17]. In this context, we have recently found [13] that at least some of the XerC/D sites located in a number of A. baumannii plasmids can in fact conform proficient pairs for site-specific recombination mediating the formation (and resolution) of plasmid co-integrates.…”