title: Novel transferable erm(46) determinant responsible for emerging macrolide resistance in Rhodococcus equi.
abstract: OBJECTIVE
  The objective of this study was to identify the molecular mechanism of macrolide resistance in the actinomycete Rhodococcus equi, a major equine pathogen and zoonotic agent causing opportunistic infections in people.
  
  
  METHODS
  Macrolide-resistant (n = 62) and macrolide-susceptible (n = 62) clinical isolates of R. equi from foals in the USA were studied. WGS of 18 macrolide-resistant and 6 macrolide-susceptible R. equi was performed. Representative sequences of all known macrolide resistance genes identified to date were used to search the genome assemblies for putative homologues. PCR was used to screen for the presence of the identified resistance determinant in the rest of the isolates. Mating experiments were performed to verify mobility of the gene.
  
  
  RESULTS
  A novel erm gene, erm(46), was identified in all sequenced resistant isolates, but not in susceptible isolates. There was complete association between macrolide resistance and the presence of erm(46) as detected by PCR screening of all 124 clinical isolates of R. equi. Expression of erm(46) in a macrolide-susceptible strain of R. equi induced high-level resistance to macrolides, lincosamides and streptogramins B, but not to other classes of antimicrobial agents. Transfer of erm(46) to macrolide-susceptible R. equi was confirmed. The transfer frequency ranged from 3 × 10(-3) to 1 × 10(-2).
  
  
  CONCLUSIONS
  This is the first molecular characterization of resistance to macrolides, lincosamides and streptogramins B in R. equi. Resistance was due to the presence of a novel erm(46) gene mobilizable likely by conjugation, which has spread among equine isolates of R. equi in the USA.
authors.affiliation: Microbial Pathogenesis Unit, School of Biomedical Sciences and The Roslin Institute, University of Edinburgh, Edinburgh, UK.
authors.forename: Elisa
authors.initials: E
authors.lastname: Anastasi
authors.affiliation: Department of Large Animal Medicine, University of Georgia, Athens, GA, USA gigueres@uga.edu.
authors.forename: Steeve
authors.initials: S
authors.lastname: Giguère
authors.affiliation: Department of Large Animal Medicine, University of Georgia, Athens, GA, USA.
authors.forename: Londa J
authors.initials: LJ
authors.lastname: Berghaus
authors.affiliation: Department of Infectious Diseases, University of Georgia, Athens, GA, USA.
authors.forename: Mary K
authors.initials: MK
authors.lastname: Hondalus
authors.affiliation: Department of Infectious Diseases, University of Georgia, Athens, GA, USA.
authors.forename: Jennifer M
authors.initials: JM
authors.lastname: Willingham-Lane
authors.affiliation: Microbial Pathogenesis Unit, School of Biomedical Sciences and The Roslin Institute, University of Edinburgh, Edinburgh, UK.
authors.forename: Iain
authors.initials: I
authors.lastname: MacArthur
authors.affiliation: Department of Large Animal Clinical Sciences, Texas A&M University, College Station, TX, USA.
authors.forename: Noah D
authors.initials: ND
authors.lastname: Cohen
authors.affiliation: Department of Environmental and Occupational Health Sciences, School of Public Health, University of Washington, Seattle, WA, USA.
authors.forename: Marilyn C
authors.initials: MC
authors.lastname: Roberts
authors.affiliation: Microbial Pathogenesis Unit, School of Biomedical Sciences and The Roslin Institute, University of Edinburgh, Edinburgh, UK Grupo de Patogenómica Bacteriana, Facultad de Veterinaria, Universidad de Léon, Léon, Spain.
authors.forename: Jose A
authors.initials: JA
authors.lastname: Vazquez-Boland
mesh_terms: D000193:Actinomycetales Infections / Q000382:microbiology / Q000662:veterinary; D000818:Animals; D000831:Animals, Newborn; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D003227:Conjugation, Genetic; D004269:DNA, Bacterial / Q000737:chemistry / Q000235:genetics; D024881:Drug Resistance, Bacterial; D022761:Gene Transfer, Horizontal; D005798:Genes, Bacterial; D006734:Horse Diseases / Q000382:microbiology; D006736:Horses; D055231:Lincosamides / Q000494:pharmacology; D018942:Macrolides / Q000494:pharmacology*; D016929:Rhodococcus equi / Q000187:drug effects* / Q000235:genetics* / Q000302:isolation & purification; D017422:Sequence Analysis, DNA; D025381:Streptogramin B / Q000494:pharmacology; D014481:United States
chemical_list: D000900:Anti-Bacterial Agents; D004269:DNA, Bacterial; D055231:Lincosamides; D018942:Macrolides; D025381:Streptogramin B