title: Macrolide resistance conferred by rRNA mutations in field isolates of Mannheimia haemolytica and Pasteurella multocida.
abstract: OBJECTIVE
  To determine how resistance to macrolides is conferred in field isolates of Pasteurella multocida and Mannheimia haemolytica that lack previously identified resistance determinants for rRNA methylation, efflux and macrolide-modifying enzymes.
  
  
  METHODS
  Isolates of P. multocida and M. haemolytica identified as being highly resistant (MICs >64 mg/L) to the macrolides erythromycin, gamithromycin, tilmicosin, tildipirosin and tulathromycin were screened by multiplex PCR for the previously identified resistance genes erm(42), msr(E) and mph(E). Strains lacking these determinants were analysed by genome sequencing and primer extension on the rRNAs.
  
  
  RESULTS
  Macrolide resistance in one M. haemolytica isolate was conferred by the 23S rRNA mutation A2058G; resistance in three P. multocida isolates were caused by mutations at the neighbouring nucleotide A2059G. In each strain, all six copies of the rrn operons encoded the respective mutations. There were no mutations in the ribosomal protein genes rplD or rplV, and no other macrolide resistance mechanism was evident.
  
  
  CONCLUSIONS
  High-level macrolide resistance can arise from 23S rRNA mutations in P. multocida and M. haemolytica despite their multiple copies of rrn. Selective pressures from exposure to different macrolide or lincosamide drugs presumably resulted in consolidation of either the A2058G or the A2059G mutation.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark.
authors.forename: Anders S
authors.initials: AS
authors.lastname: Olsen
authors.affiliation: MSD Animal Health Innovation GmbH, Zur Propstei, D-55270 Schwabenheim, Germany.
authors.forename: Ralf
authors.initials: R
authors.lastname: Warrass
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark srd@bmb.sdu.dk.
authors.forename: Stephen
authors.initials: S
authors.lastname: Douthwaite
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology; D024881:Drug Resistance, Bacterial; D020459:Genes, rRNA; D016680:Genome, Bacterial; D006801:Humans; D018942:Macrolides / Q000494:pharmacology*; D016978:Mannheimia haemolytica / Q000187:drug effects* / Q000235:genetics* / Q000302:isolation & purification; D008826:Microbial Sensitivity Tests; D009154:Mutation; D009876:Operon; D016979:Pasteurella multocida / Q000187:drug effects* / Q000235:genetics* / Q000302:isolation & purification; D012338:RNA, Ribosomal, 23S / Q000235:genetics
chemical_list: D000900:Anti-Bacterial Agents; D018942:Macrolides; D012338:RNA, Ribosomal, 23S
keywords: Pasteurellaceae; antimicrobial resistance; veterinary macrolides