Comparison between broth microdilution and agar disk diffusion methods for antimicrobial susceptibility testing of...

Area of Science:

• Veterinary Microbiology
• Antimicrobial Resistance
• Bovine Health

Background:

• Antibiotic resistance necessitates rational antimicrobial use in livestock, particularly for treating cow mastitis, a major driver of antibiotic consumption in cattle.
• Accurate antimicrobial susceptibility testing is crucial for selecting effective treatments and combating resistance.

Purpose of the Study:

• To compare the accuracy and concordance of the broth microdilution (BMD) method using Sensititre Custom Plates against the agar disk diffusion (ADD) method for bovine mastitis pathogens.
• To evaluate the agreement between BMD and ADD for various antimicrobials against common contagious and environmental mastitis-causing bacteria.

Main Methods:

• Evaluated 215 bovine mastitis isolates including Staphylococcus aureus, Streptococcus agalactiae, Streptococcus uberis, Escherichia coli, and others.
• Compared BMD results with Sensititre Custom Plates against ADD results for 12 antimicrobials.
• Applied CLSI and EUCAST breakpoints for MIC and zone diameter, analyzing discrepancies (VMD, MD, MiD).

Main Results:

• Overall agreement between BMD and ADD was 80.7% (Cohen's kappa = 0.397), indicating fair concordance.
• Satisfactory agreement was observed for Staphylococcus aureus and Serratia marcescens (89-100%).
• Lower agreement was noted for specific pathogen-antimicrobial combinations, such as Streptococcus uberis with rifampicin (20%) and Escherichia coli with ampicillin (20%).

Conclusions:

• The BMD and ADD methods show fair concordance for bovine mastitis pathogens, with BMD generally being more restrictive.
• Discrepancies highlight the need for method-specific or pathogen-specific breakpoints to optimize antimicrobial selection.
• Minimizing very major discrepancies (0.2%) is critical to prevent treatment failures in bovine mastitis cases.