European multi-centre study to establish MIC and zone diameter epidemiological cut-off values for Bacillusanthracis

Area of Science:

• Microbiology
• Antimicrobial Resistance
• Clinical Diagnostics

Background:

• Bacillus anthracis clinical breakpoints are not yet established by the European Committee on Antimicrobial Susceptibility Testing (EUCAST).
• Lack of defined wild-type Minimum Inhibitory Concentration (MIC) distributions and epidemiological cut-off values (ECOFFs) hinders breakpoint establishment.
• Standardized methods are needed for accurate antimicrobial susceptibility testing (AST) of B. anthracis.

Purpose of the Study:

• To establish standardized AST methods, wild-type MIC distributions, and ECOFFs for ten antimicrobials against B. anthracis.
• To provide data for the determination of clinical breakpoints by EUCAST.
• To enable sensitive detection and monitoring of antimicrobial resistance in B. anthracis.

Main Methods:

• Testing of approximately 335 B. anthracis isolates using broth microdilution and disc diffusion.
• Curation of MIC and inhibition zone diameter data according to EUCAST SOP 10.2.
• Submission of results to EUCAST for ECOFF and clinical breakpoint determination.

Main Results:

• Putative wild-type MIC distributions were identified for tested antimicrobial agents.
• ECOFFs were defined for each antimicrobial agent.
• Three highly resistant strains to benzylpenicillin and isolates with resistance mechanisms to doxycycline, tetracycline, and amoxicillin were detected.

Conclusions:

• EUCAST utilized B. anthracis AST results to determine ECOFFs for ten antimicrobial agents.
• MIC distributions are crucial for determining clinical breakpoints.
• Defined ECOFFs facilitate sensitive detection of resistance mechanisms and monitoring of resistance trends.