Homologous Recombination within Large Chromosomal Regions Facilitates Acquisition of β-Lactam and Vancomycin Resistance in Enterococcus faecium.

Mónica García-Solache,James D Whiteaker,Michael S Gilmore,Louis B Rice,Robert E Mclaughlin,Francois Lebreton

doi:10.1128/aac.00488-16

Abstract

The transfer of DNA between Enterococcus faecium strains has been characterized both by the movement of well-defined genetic elements and by the large-scale transfer of genomic DNA fragments. In this work, we report on the whole-genome analysis of transconjugants resulting from mating events between the vancomycin-resistant E. faecium C68 strain and the vancomycin-susceptible D344RRF strain to discern the mechanism by which the transferred regions enter the recipient chromosome. Vancomycin-resistant transconjugants from five independent matings were analyzed by whole-genome sequencing. In all cases but one, the penicillin binding protein 5 (pbp5) gene and the Tn5382 vancomycin resistance transposon were transferred together and replaced the corresponding pbp5 region of D344RRF. In one instance, Tn5382 inserted independently downstream of the D344RRF pbp5 gene. Single nucleotide variant (SNV) analysis suggested that entry of donor DNA into the recipient chromosome occurred by recombination across regions of homology between donor and recipient chromosomes, rather than through insertion sequence-mediated transposition. The transfer of genomic DNA was also associated with the transfer of C68 plasmid pLRM23 and another putative plasmid. Our data are consistent with the initiation of transfer by cointegration of a transferable plasmid with the donor chromosome, with subsequent circularization of the plasmid-chromosome cointegrant in the donor prior to transfer. Entry into the recipient chromosome most commonly occurred across regions of homology between donor and recipient chromosomes.

Highlights

The transfer of DNA between Enterococcus faecium strains has been characterized both by the movement of well-defined genetic elements and by the large-scale transfer of genomic DNA fragments
Enterococcus faecium has emerged as one of the leading causes of health care-associated infections due to a combination of its high intrinsic levels of resistance to commonly used antibiotics, its remarkable genome plasticity that favors the ability to acquire de novo resistance when challenged with new antibiotics, and its ability to survive in diverse environments [1,2,3,4]
Individual transposons have been identified for both vanA and vanB elements [37,38,39], the transfer of these determinants between E. faecium strains has been associated with the movement of large segments of chromosomal DNA [10, 13, 15]

Summary

Introduction

The transfer of DNA between Enterococcus faecium strains has been characterized both by the movement of well-defined genetic elements and by the large-scale transfer of genomic DNA fragments. The Tn5382 transposon carrying vanB was first reported in E. faecium C68, a multiresistant clinical isolate in which it was located immediately downstream of a penicillin binding protein 5 (pbp5) allele conferring a high level of resistance to ampicillin. The genetic linkage of pbp and vanB has been identified in different strains of E. faecium isolated from different geographical regions [12, 14] This association is not universal, and the vanB element can insert in other regions of the chromosome or can be plasmid borne [15]. Transfer of the vanB transposon and pbp was observed in association with substantial quantities of C68 genomic DNA [13, 17] Those recipient strains, lacked pbp due to a large chromosomal deletion involving Tn916 [18].

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Results

Conclusion