Susceptibility testing of Rhodococcus equi: an interlaboratory test

Due to the importance of antimicrobial susceptibility testing (AST) for veterinary diagnostics, a standardised protocol for AST of Rhodococcus (R.) equi by broth microdilution has recently been developed and approved by the Clinical and Laboratory Standards Institute (CLSI). The aim of the present study was to test this protocol in an interlaboratory comparative study for its fitness for use in routine laboratory diagnostics. All of the 18 participating laboratories determined the minimum inhibitory concentrations (MIC) of two R. equi strains against 24 antimicrobial agents. The modal MIC values were determined and the acceptable ranges were set as the modal MIC ±1 dilution step. The R. equi field strain Rh110 showed a slightly better performance than the type strain R. equi ATCC® 25729. For the different antimicrobial agents tested, the percentage of MIC values within the acceptable ranges varied from 75.9 to 100% for R. equi ATCC® 25729, and from 85.2 to 100% for R. equi Rh110. The most homogeneous MIC results (i.e. modal MIC ±1 dilution step) were obtained for oxacillin and vancomycin, while the most divergent results were seen with cefotaxime and ceftiofur. Using a success rate of at least 80% of the strain-specific MICs being within the acceptable ranges as an arbitrary cut-off, only one of the participating laboratories failed to reach this cut-off value for one of the two R. equi strains. Thus, we consider the new protocol fit for use in routine AST of R. equi.