AUTHOR=Cameron Andrew , Zaheer Rahat , McAllister Tim A. 

TITLE=Emerging Variants of the Integrative and Conjugant Element ICEMh1 in Livestock Pathogens: Structural Insights, Potential Host Range, and Implications for Bacterial Fitness and Antimicrobial Therapy

JOURNAL=Frontiers in Microbiology

VOLUME=10

YEAR=2019

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2019.02608

DOI=10.3389/fmicb.2019.02608

ISSN=1664-302X

ABSTRACT=<p>Horizontal gene transfer of integrative and conjugative elements (ICE) in bacterial pathogens of the bovine respiratory disease (BRD) complex has emerged as a significant cause of antimicrobial resistance (AMR) and therapeutic failure and mortalities in cattle. The aim of this study was to assess an AMR ICE occurring in <italic>Pasteurella multocida</italic> from a case of BRD, designated ICE<italic>Mh1</italic><sup>PM22</sup> for its structure and host genome insertion site, and to identify consequences for host fitness and antimicrobial therapy. The modular structure of ICE<italic>Mh1</italic>-like elements found in several related livestock pathogens was compared to ICE<italic>Mh1</italic><sup>PM22</sup>, and the repertoire of cargo genes in variable ICE modules was functionally categorized. AMR genes were identified as frequent additions to the variable modules of ICE<italic>Mh1</italic>-like elements. Random PCR-based mapping of ICE<italic>Mh1</italic><sup>PM22</sup>-genome junctions in transconjugants provided evidence that ICE<italic>Mh1</italic><sup>PM22</sup> integrates into the tRNA-leu for the UUG codon, and not into tRNA-leu for other codons. This was separately confirmed in the genomes of ICE<italic>Mh1</italic>-like-harboring livestock pathogens. Bacterial genera harboring receptive tRNA-leu<sup>UUG</sup> were identified to establish the potential host range of ICE<italic>Mh1</italic>-like elements. ICE<italic>Mh1</italic><sup>PM22</sup>-carrying transconjugants in <italic>P. multocida</italic> and <italic>Mannheimia haemolytica</italic> were less fit than isogenic strains without the ICE when grown without antimicrobial selection. This fitness cost was abrogated in the presence of subinhibitory concentrations of antimicrobials. Despite this cost, ICE<italic>Mh1</italic><sup>PM22</sup> was retained in transconjugants in extended culture. To identify possible therapeutic efficiencies, antimicrobial combinations were screened for synergistic interactions against AMR ICE<italic>Mh1</italic><sup>PM22</sup>-carrying transconjugants. No antimicrobial combination tested exhibited synergistic interactions against AMR <italic>P. multocida</italic> or <italic>M. haemolytica</italic> harboring ICE<italic>Mh1</italic><sup>PM22</sup>. In conclusion, this study provided information on the structural variation of ICE<italic>Mh1</italic>-like elements, refined the ICE insertion site and potential host range, and demonstrated the risk and consequences for AMR following horizontal transfer of ICE into BRD pathogens.</p>