Susceptibility testing of Rhodococcus equi: An interlaboratory test

Area of Science:

• Veterinary Microbiology
• Clinical Diagnostics
• Antimicrobial Resistance

Background:

• Antimicrobial susceptibility testing (AST) is crucial for veterinary diagnostics.
• A standardized broth microdilution protocol for Rhodococcus equi AST was recently developed by the Clinical and Laboratory Standards Institute (CLSI).

Purpose of the Study:

• To evaluate the fitness for use of the new standardized CLSI protocol for Rhodococcus equi AST in routine laboratory diagnostics.
• To assess the interlaboratory reproducibility of the protocol.

Main Methods:

• Eighteen laboratories performed broth microdilution to determine minimum inhibitory concentrations (MICs) for two R. equi strains against 24 antimicrobial agents.
• Modal MIC values were established, and acceptable ranges were defined as modal MIC ±1 dilution step.
• Performance was evaluated based on the percentage of MIC values falling within acceptable ranges.

Main Results:

• The protocol demonstrated high accuracy, with 75.9% to 100% of MIC values within acceptable ranges for the type strain and 85.2% to 100% for the field strain.
• Oxacillin and vancomycin yielded the most homogeneous results, while cefotaxime and ceftiofur showed more variability.
• Only one laboratory failed to meet the arbitrary 80% success rate cut-off for one strain, indicating overall protocol robustness.

Conclusions:

• The new standardized broth microdilution protocol is suitable for routine antimicrobial susceptibility testing of Rhodococcus equi in veterinary diagnostics.
• The protocol exhibits good interlaboratory reproducibility and accuracy, supporting its adoption in clinical settings.