title: MICs of 32 antimicrobial agents for Rhodococcus equi isolates of animal origin.
abstract: OBJECTIVE
  The aim of this study was to determine the MICs of 32 antimicrobial agents for 200 isolates of Rhodococcus equi of animal origin by applying a recently described broth microdilution protocol, and to investigate isolates with distinctly elevated rifampicin MICs for the genetic basis of rifampicin resistance.
  
  
  METHODS
  The study included 200 R. equi isolates, including 160 isolates from horses and 40 isolates from other animal sources, from the USA and Europe. MIC testing of 32 antimicrobial agents or combinations thereof followed a recently published protocol. A novel PCR protocol for the joint amplification of the three rpoB regions in which rifampicin resistance-mediating mutations have been reported was applied to isolates with elevated rifampicin MICs. The amplicons were sequenced and screened for mutations.
  
  
  RESULTS
  Susceptibility testing revealed a rather uniform distribution of MICs for most of the antimicrobial agents tested. The lowest MICs were seen for clarithromycin, rifampicin and imipenem. Six isolates (3%) exhibited distinctly higher MICs of rifampicin than the remaining 194 isolates. In five of these six isolates, single bp exchanges, which resulted in the amino acid exchanges Gln513Leu, Asp516Val, His526Asp or Ser531Leu, were detected in the rifampicin resistance-determining region 1 of the rpoB gene, with Gln513Leu representing a novel substitution for R. equi.
  
  
  CONCLUSIONS
  This study shows the MIC distribution of 32 antimicrobial agents for a large collection of R. equi isolates of animal origin from two continents. Isolates that exhibited distinctly elevated MICs of rifampicin were only rarely detected.
authors.affiliation: Institute of Farm Animal Genetics, Friedrich-Loeffler-Institut (FLI), Neustadt-Mariensee, Germany.
authors.forename: Anne
authors.initials: A
authors.lastname: Riesenberg
authors.forename: Andrea T
authors.initials: AT
authors.lastname: Feßler
authors.forename: Erdal
authors.initials: E
authors.lastname: Erol
authors.forename: Ellen
authors.initials: E
authors.lastname: Prenger-Berninghoff
authors.forename: Ivonne
authors.initials: I
authors.lastname: Stamm
authors.forename: Reinhard
authors.initials: R
authors.lastname: Böse
authors.forename: Anton
authors.initials: A
authors.lastname: Heusinger
authors.forename: Dieter
authors.initials: D
authors.lastname: Klarmann
authors.forename: Christiane
authors.initials: C
authors.lastname: Werckenthin
authors.forename: Stefan
authors.initials: S
authors.lastname: Schwarz
mesh_terms: D000193:Actinomycetales Infections / Q000382:microbiology / Q000662:veterinary*; D000818:Animals; D000890:Anti-Infective Agents / Q000494:pharmacology*; D012321:DNA-Directed RNA Polymerases / Q000235:genetics; D005060:Europe; D008826:Microbial Sensitivity Tests; D020125:Mutation, Missense; D016133:Polymerase Chain Reaction; D016929:Rhodococcus equi / Q000187:drug effects* / Q000235:genetics / Q000302:isolation & purification; D017422:Sequence Analysis, DNA; D014481:United States
chemical_list: D000890:Anti-Infective Agents; D012321:DNA-Directed RNA Polymerases; C105078:RNA polymerase beta subunit
keywords: broth microdilution; horses; rifampicin; rpoB