Multicentre testing of the EUCAST broth microdilution reference method for MIC determination on Mycobacterium...

Area of Science:

• Microbiology
• Clinical Diagnostics
• Antimicrobial Resistance

Background:

• Standardized methods are crucial for antimicrobial susceptibility testing (AMST) of Mycobacterium tuberculosis.
• Existing protocols for determining minimum inhibitory concentrations (MICs) of antituberculous agents lack global consensus.
• The EUCAST subcommittee for AMST was formed to address this need.

Purpose of the Study:

• To establish a reference method for determining MICs of antituberculous agents.
• To evaluate and compare broth microdilution (BMD) and solid medium dilution (SMD) for Mycobacterium tuberculosis susceptibility testing.
• To provide a basis for setting microbiological breakpoints for antituberculous drugs.

Main Methods:

• A multicentre study evaluated BMD and SMD protocols for MIC determination of isoniazid, levofloxacin, and amikacin against Mycobacterium tuberculosis H37Rv.
• Two inoculum concentrations were used with Middlebrook 7H9 (BMD) and 7H10 (SMD) media.
• MICs were interpreted based on visual and 99% inhibition criteria after specific incubation periods.

Main Results:

• Both BMD and SMD demonstrated high reproducibility, with >95% intra- and inter-assay agreement within ±1 MIC dilution.
• MIC values for isoniazid, levofloxacin, and amikacin were determined using both methods.
• The study confirmed the eligibility of both BMD and SMD as reproducible reference methods.

Conclusions:

• Both solid medium dilution (SMD) and broth microdilution (BMD) are reproducible and suitable for Mycobacterium tuberculosis complex (MTBC) MIC determination.
• Broth microdilution (BMD) was selected as the official EUCAST reference method for AMST.
• This standardized method will be used to establish epidemiological cut-off values and clinical breakpoints for antituberculous agents.