Anaerobic susceptibility testing is sometimes performed, although may be time-consuming and difficult.
The MIC (minimum inhibitory concentration) methods currently in use by larger medical microbiology laboratories include:
- MIC (using special anaerobic panels), and
- MIC (using commercial predefined antibiotic gradient strips, e.g., bioMerieux Etest).
Always refer to the manufacturer's standard operating procedure. The method will likely include specific testing conditions (i.e., medium, inoculum, and incubation temperature), as well as routine quality control (QC) recommendations. Use the M100 table "MIC Breakpoints for Anaerobes" to interpret MIC breakpoints and review required QC.
Other methods
Agar and broth microdilution methods are usually found in research or reference laboratories and are quite labor-intensive. Agar dilution is considered the reference standard. CLSI guidelines are used to determine minimum inhibitory concentrations of anaerobic bacteria using agar dilution and broth microdilution methods.15
Antibiograms exist to aid clinicians by providing expected susceptibility results. Refer to the anaerobe cumulative antibiogram published in CLSI M100 for more information.16
When to perform susceptibility testing?
- Some anaerobes have unpredictable susceptibility patterns. Consider testing certain species of Clostridium, Bacteroides, Prevotella, and Fusobacterium.
- Persistence of severe infection in the presence of antimicrobial treatment.
- In cases of severe infections such as bacteremia, vascular grafts, brain abscesses, endocarditis, osteomyelitis, and joint and prosthetic sites.17
Antimicrobial agent selection should be based on the CLSI table found in M100. These agents are considered appropriate for clinical use and inclusion in the reporting of specific organism groups.
15. CLSI. Methods for Antimicrobial Susceptibility Testing of Anaerobic Bacteria. 9th ed. CLSI Standard M11. Clinical and Laboratory Standards Institute; 2018.
16. CLSI. Performance Standards for Antimicrobial Susceptibility Testing. 34th ed. CLSI Supplement M100. Clinical and Laboratory Standards Institute; 2024.
17. Nagy E, Boyanova L, Justesen US; ESCMID Study Group of Anaerobic Infections. How to isolate, identify, and determine antimicrobial susceptibility of anaerobic bacteria in routine laboratories. Clin Microbiol Infect. 2018;24(11):1139-1148. doi:10.1016/j.cmi.2018.02.008.
