Clinical and Laboratory Standards Institute standard M11—Methods for Antimicrobial Susceptibility Testing of Anaerobic Bacteria describes the reference standard agar dilution method (Wadsworth) and the alternative broth microdilution method. Antimicrobial resistance patterns for many anaerobic bacteria have changed significantly over the past several years, resulting in a lack of predictability for many species. Susceptibility testing of anaerobes is recommended for surveillance purposes and for specific clinical situations. The agar dilution method is well suited for surveillance testing and research. It is also the standard with which other methods are compared. The alternative method, broth microdilution, is well suited for the medical laboratory but is currently limited to testing Bacteroides spp. and Parabacteroides spp. organisms and selected antimicrobial agents. QC criteria for each procedure are also described. This standardized procedure, when used in conjunction with the M1001 tables, includes the most current information for drug selection, interpretation, QC, and antibiogram reports. When new problems are recognized or improvements in these criteria are made, changes will be incorporated into future editions of this standard and in M100.1

This standard replaces the previous edition of the approved standard, M11-A8, published in 2012. Several changes were made in this edition, including:

• General: – Reorganized to fit the CLSI quality management system and path of workflow format – Made minor text revisions throughout for improved clarity and consistency with other CLSI documents – To align with the International Organization for Standardization, changed the name of the inoculum preparation method from growth method to broth culture method and changed direct colony suspension to colony suspension – Updated nomenclature for Bacteroides fragilis to Bacteroides spp. and Parabacteroides spp. per current standards

• Subchapter 1.4, Terminology: – Added definitions for epidemiological cutoff value, quality assurance, and saline – Revised definitions for breakpoint, interpretive category, and quality control for consistency with other CLSI documents – Added new abbreviations for consistency with other CLSI documents

• Chapter 3, Antimicrobial Susceptibility Testing Process for Anaerobic Organisms: – Added an anaerobic susceptibility testing process overview flow chart

• Appendixes and tables: – Reformatted Appendixes A, B, and C for improved clarity and workflow – Deleted Appendix D (Cumulative Antimicrobial Susceptibility Report for Anaerobic Organisms) and moved it to M1001 – Deleted Table 1 (Suggested Groupings of Antimicrobial Agents Approved by the US Food and Drug Administration for Clinical Use That Should Be Considered for Testing and Reporting on Anaerobic Organisms by Microbiology Laboratories in the United States) and moved it to M1001

This standard describes the CLSI antimicrobial susceptibility testing (AST) reference agar dilution method as well as the alternative broth microdilution method for Bacteroides spp. and Parabacteroides spp. organisms used to determine in vitro susceptibility to antimicrobial agents of bacteria that grow anaerobically. A method for ß-lactamase testing on anaerobic bacteria is also described. This standard includes:

• Preparation of broth and agar dilution tests

• Testing conditions (including inoculum preparation and standardization, incubation time, and incubation temperature)

• Reporting minimal inhibitory concentration (MIC) results

• QC procedures

• Limitations of the dilution test methods 

• A step-by-step guide to AST (see Appendix A)

To assist the medical laboratory, suggestions are provided for selecting antimicrobial agents for routine testing and reporting. The disk diffusion method has not been standardized for use with anaerobic organisms and is not included. Methods for culturing and identifying anaerobic bacteria are not discussed. See CLSI document M56 for information on methods for culturing and identifying anaerobic bacteria. Methods for AST of aerobic bacterial species are also not discussed. The AST methods provided in this standard can be used in laboratories around the world, including but not limited to:

• Medical laboratories

• Public health laboratories

• Research laboratories

• Food laboratories

• Environmental laboratories