Susceptibility testing for bovine respiratory and enteric disease

Area of Science:

• Veterinary Microbiology
• Antimicrobial Resistance
• Clinical Pathology

Background:

• Established interpretation guidelines exist for antimicrobial susceptibility testing (AST) using NCCLS-approved veterinary-specific breakpoints and methods.
• Applying these established breakpoints to non-standard applications can lead to unclear interpretations of susceptibility results.

Purpose of the Study:

• To clarify the interpretation of antimicrobial susceptibility testing results when veterinary-specific breakpoints are applied in various contexts.
• To highlight the importance of understanding the predictive value of different AST methods and interpretive criteria for veterinary clinicians.

Main Methods:

• Discussion of serial-dilution and disk-diffusion testing methodologies.
• Analysis of the relationship between minimum inhibitory concentrations (MICs) and zone diameters.
• Consideration of the application of NCCLS-approved breakpoints in different scenarios.

Main Results:

• Susceptible (S) results using serial-dilution breakpoints define a population with MICs at or below the breakpoint.
• Resistant (R) results indicate MICs at or above the breakpoint, with no upper limit.
• Disk-diffusion zone diameters correlate with MICs based on specific bacterial populations, and this correlation may change when applied to different isolates.

Conclusions:

• Veterinary clinicians need to be aware of the varying predictive value of AST results depending on the application and interpretive criteria used.
• When veterinary-specific interpretive criteria are unavailable, maintaining records of clinical response alongside susceptibility data is recommended.
• Consultation on the relationship between pathogen MICs and pharmacokinetic-pharmacodynamic (PK-PD) parameters is advised in such situations.