ICEPmu1, an integrative conjugative element (ICE) of Pasteurella multocida: analysis of the regions that comprise 12 antimicrobial resistance genes

Background: In recent years, multiresistant Pasteurella multocida isolates from bovine respiratory tract infections have been identified. These isolates have exhibited resistance to most classes of antimicrobial agents commonly used in veterinary medicine, the genetic basis of which, however, is largely unknown. Methods: Genomic DNA of a representative P. multocida isolate was subjected to whole genome sequencing. Genes have been predicted by the YACOP program, compared with the SWISSProt/EMBL databases and manually curated using the annotation software ERGO. Susceptibility testingwas performed by broth microdilution according to CLSI recommendations. Results: The analysis of one representative P. multocida isolate identified an 82 kb integrative and conjugative element (ICE) integrated into the chromosomal DNA. This ICE, designated ICEPmu1, harboured 11 resistance genes, which confer resistance to streptomycin/spectinomycin (aadA25), streptomycin (strA and strB), gentamicin (aadB), kanamycin/neomycin (aphA1), tetracycline [tetR-tet(H)], chloramphenicol/florfenicol (floR), sulphonamides (sul2), tilmicosin/clindamycin [erm(42)] or tilmicosin/tulathromycin [msr(E)-mph(E)]. In addition, a complete bla(OXA-2) gene was detected, which, however, appeared to be functionally inactive in P. multocida. These resistance genes were organized in two regions of approximately 15.7 and 9.8 kb. Based on the sequences obtained, it is likely that plasmids, gene cassettes and insertion sequences have played a role in the development of the two resistance gene regions within this ICE. Conclusions: The observation that 12 resistance genes, organized in two resistance gene regions, represent part of an ICE in P. multocida underlines the risk of simultaneous acquisition of multiple resistance genes via a single horizontal gene transfer event.