Molecular basis of macrolide resistance among Pasteurella multocida and Mannheimia haemolytica isolates

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Aims of the study on macrolide-resistant Pasteurella (P.) multocida und Mannheimia (M.) haemolytica isolates were (1) the identification of the resistance mechanisms involved, (2) the characterization of the resistance genes or resistance-mediating mutations present, and (3) the analysis of the regulation and transferability of the resistance determinants, as well as the investigation of potential cross-resistances mediated by the respective resistance determinants. In the first period of the project SCHW382/10-1, three novel macrolide resistance genes, erm(42), msr(E) and mph(E), could be identified in P. multocida und M. haemolytica isolates from North America. The genes were located on a novel mobile genetic element, the integrative and conjugative element ICEPmu1. This ICE had a size of 82 kb and harbored in total twelve antimicrobial resistance genes in two resistance gene regions of approximately 15.7 and 9.8 kb in size. ICEPmu1 transferred with a high frequency of 2.9 x 10-6 to 1 x 10-4 from the original P. multocida isolate into P. multocida, M. haemolytica and Escherichia coli recipient strains where it conferred a multiresistance phenotype. PCRs were developed for a fast and reliable identification of these the novel resistance genes. Further studies showed that the genes also conferred resistance to tildipirosin or gamithromycin. Tildipirosin and gamithromycin represent the latest antimicrobial agents of the macrolide class which have been approved in 2011 for the treatment of respiratory tract infections in cattle and pigs. The co-localization of twelve resistance genes on a mobile element capable of high frequency self-transfer across genus boundaries represents a serious threat and drastically limits the options of an efficient antimicrobial therapy of respiratory tract infections in food-producing animals.The aims of the follow-up study are (1) to identify the macrolide resistance mechanism(s) in M. haemolytica isolates that do not harbor any of the new macrolide resistance genes, (2) to analyze ICEs in Pasteurellaceae for structural variabilities in their resistance gene regions and to investigate the potential of ICEs as accumulators and distributers of resistance genes, but also (3) to analyze the first macrolide-resistant Pasteurellaceae isolates from Germany.The results of the first study period provided important basic information on resistance to macrolide antibiotics in Pasteurellaceae. The follow-up study aims at the identification of other so far unknown resistance genes and/or mechanisms. While multiresistance-conferring ICEs were identified for the first time in Pasteurellaceae during the first study, the follow-up study will provide in-depth insight into the structural variability of these novel mobile elements as well as allow for a prediction of further developments towards pan-resistant Pasteurellaceae which cannot be treated anymore with antimicrobial agents.