Antimicrobial susceptibility testing of Mycobacterium tuberculosis complex isolates - the EUCAST broth microdilution
Thomas Schön1, Jim Werngren2, Diana Machado3
1Department of Clinical Microbiology and Infectious Diseases, Kalmar County Hospital, Linköping University, Linköping, Sweden.
Summary
Standardizing antibiotic susceptibility testing (AST) for Mycobacterium tuberculosis complex (MTBC) is crucial. The EUCAST reference method, a broth microdilution technique, establishes reliable minimal inhibitory concentration (MIC) values for MTBC isolates.
Area of Science:
- Microbiology
- Infectious Diseases
- Clinical Diagnostics
Background:
- Antibiotic susceptibility testing (AST) for Mycobacterium tuberculosis complex (MTBC) currently lacks standardization, leading to variable results.
- This variability hinders the establishment of epidemiological cut-off (ECOFF) values and clinical breakpoints according to EUCAST guidelines.
- Standardized methods for determining minimal inhibitory concentrations (MICs) are essential for both existing and novel anti-tuberculosis agents.
Purpose of the Study:
- To establish a standardized reference method for MIC determination in MTBC isolates.
- To address the need for consistent and reliable AST results for tuberculosis treatment and surveillance.
Main Methods:
- The EUCAST antimycobacterial susceptibility testing subcommittee (AMST) evaluated and compared various MIC determination protocols.
- Key aspects assessed included medium, inoculum preparation, drug preparation, incubation conditions, result reading, and interpretation.
- The standardized method involves broth microdilution using Middlebrook 7H9-10% OADC medium with a specific inoculum preparation and incubation protocol.
Main Results:
- The EUCAST reference method for MIC determination in MTBC is the broth microdilution method.
- It utilizes Middlebrook 7H9-10% OADC medium, a standardized inoculum (10^5 CFU/mL), and specific incubation and reading procedures.
- The method includes quality control using Mycobacterium tuberculosis H37Rv ATCC 27294 with defined MIC ranges for key drugs like isoniazid, levofloxacin, and amikacin.
Conclusions:
- The EUCAST reference method for MTBC has been endorsed and will be used for defining EUCAST ECOFFs and clinical breakpoints.
- This method provides a standardized approach for MIC determination, crucial for accurate tuberculosis diagnostics.
- Routine AST methods will require calibration against this reference method to align with EUCAST breakpoints.
