Defining epidemiological cutoff values for Brucella melitensis: an European multicentre study

Area of Science:

• Microbiology
• Infectious Diseases
• Antimicrobial Resistance

Background:

• Brucellosis, primarily caused by Brucella melitensis, poses a significant risk for chronic infections and relapses.
• Standardized antimicrobial susceptibility testing (AST) procedures and clinical breakpoints are crucial for effective patient management and treatment outcomes.
• A Europe-wide network of Brucella reference laboratories collaborated with EUCAST to address the need for standardized AST.

Purpose of the Study:

• To establish standardized AST methods for Brucella melitensis.
• To determine wild-type (WT) minimum inhibitory concentrations (MICs) and zone diameter distributions.
• To set epidemiological cutoff (ECOFF) values for nine therapeutically relevant antimicrobial agents.

Main Methods:

• A total of 499 B. melitensis strains were tested across six European study centers.
• Broth microdilution (BMD) and disc diffusion (DD) methods were employed for AST.
• MICs and inhibition zone diameters were curated according to EUCAST standard operating procedure (SOP) 10.2 for ECOFF determination.

Main Results:

• Putative wild-type distributions were observed for tested antimicrobial agents using BMD and DD.
• MIC ECOFFs were determined for all agents, with six isolates showing slightly elevated MICs suggesting potential resistance mechanisms to rifampicin, streptomycin, and trimethoprim-sulfamethoxazole.
• ECOFFs were established for rifampicin and ceftriaxone, with tentative ECOFFs for ciprofloxacin, levofloxone, gentamicin, and streptomycin.

Conclusions:

• Standardized BMD and DD methodologies for B. melitensis were validated.
• EUCAST utilized the AST results to set ECOFFs, which were incorporated into the EUCAST clinical breakpoints table (v14.0).
• These advancements enable sensitive detection and monitoring of antimicrobial resistance development in B. melitensis.