Abstract
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 Pasteurella multocida from a case of BRD, designated ICEMh1PM22 for its structure and host genome insertion site, and to identify consequences for host fitness and antimicrobial therapy. The modular structure of ICEMh1-like elements found in several related livestock pathogens was compared to ICEMh1PM22, 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 ICEMh1-like elements. Random PCR-based mapping of ICEMh1PM22-genome junctions in transconjugants provided evidence that ICEMh1PM22 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 ICEMh1-like-harboring livestock pathogens. Bacterial genera harboring receptive tRNA-leuUUG were identified to establish the potential host range of ICEMh1-like elements. ICEMh1PM22-carrying transconjugants in P. multocida and Mannheimia haemolytica 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, ICEMh1PM22 was retained in transconjugants in extended culture. To identify possible therapeutic efficiencies, antimicrobial combinations were screened for synergistic interactions against AMR ICEMh1PM22-carrying transconjugants. No antimicrobial combination tested exhibited synergistic interactions against AMR P. multocida or M. haemolytica harboring ICEMh1PM22. In conclusion, this study provided information on the structural variation of ICEMh1-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.
Highlights
Integrative and conjugative elements (ICEs) are mobile genetic elements that can transfer autonomously by conjugation in bacteria
The ICEMh1PM22 sequence may encode for ∼94 proteins that have been annotated with putative functions or as conserved hypothetical proteins
Similar to the ICEMh1 prototype in M. haemolytica strain 42548 (Eidam et al, 2014) and ICEPmu1 in P. multocida strain 36950 (Michael et al, 2011b), and other Pasteurellaceae ICE (Beker et al, 2018), ICEMh1PM22 harbors multiple antimicrobial resistance genes/gene cassettes interspersed with numerous putative transposases in two discrete ‘resistance regions’
Summary
Integrative and conjugative elements (ICEs) are mobile genetic elements that can transfer autonomously by conjugation in bacteria. Related ICE sequences have been described in other BRD Pasteurellaceae isolates (Klima et al, 2016; Beker et al, 2018) Of particular concern, these ICEs can harbor multiple resistance genes, including determinants capable of conferring resistance to most veterinary antimicrobials that are approved for the treatment of BRD (FDA, 2016). These ICEs can harbor multiple resistance genes, including determinants capable of conferring resistance to most veterinary antimicrobials that are approved for the treatment of BRD (FDA, 2016) In addition to those Pasteurellaceae, ICE ICEPmu1- and ICEMh1-like sequences have been detected in mortality-associated Histophilus somni (Bhatt et al, 2018), as well as in disease-associated Pasteurellaceae from swine suffering from respiratory infection Resistance genes found in these ICE are often highly related to AMR genes from other human and veterinary bacteria (Michael et al, 2011a,b; Eidam et al, 2014; Cameron et al, 2018)
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