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CLSI Publishes Revised Standard on Methods for Antimicrobial Susceptibility Testing of Anaerobic Bacteria (M11-A8)


Wayne, Pennsylvania, USA—March, 2012—The Clinical and Laboratory Standards Institute (CLSI) recently published Methods for Antimicrobial Susceptibility Testing of Anaerobic Bacteria; Approved Standard—Eighth Edition (M11-A8). This standard provides reference methods for the determination of minimal inhibitory concentrations of anaerobic bacteria by agar dilution and broth microdilution.

Susceptibility testing is indicated for any organism that contributes to an infectious process warranting antimicrobial chemotherapy if its susceptibility cannot reliably be predicted from existing antibiograms. Antimicrobial resistance patterns for many anaerobic bacteria have changed significantly over the last 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.

“Antimicrobial resistance among anaerobic bacteria has been steadily increasing over that last two decades to both older and newer agents. However, most clinical laboratories do not test susceptibility of these organisms on a routine or surveillance basis. The updated edition of M11 provides instruction on when and how to test anaerobic bacteria in the clinical laboratory setting,” states David W. Hecht, MD, MS, MBA, Loyola University Medical Center, Maywood, Illinois, USA, and Chairholder of the working group that developed the document. “The working group expects that new studies using the methods recommended in this edition will result in greater consistency in testing and will serve as the gold standard for future comparisons and clinical studies.”

The objectives of this document are to:

  • Offer useful information about methods for antimicrobial susceptibility testing of anaerobic bacteria.
  • Provide reliable and reproducible methods that can be used for determining susceptibility for pathogens.
  • Present a step-by-step guide to susceptibility testing, including the number and species of organisms to test, frequency of testing, and selection of appropriate antimicrobial agents.

“Determining antimicrobial susceptibility for clinical anaerobic isolates becomes increasingly important as the level of resistance increases,” explains Hannah Wexler, PhD, Wadsworth Anaerobe Laboratory, UCLA School of Medicine, Los Angeles, California, USA, and member of the document development committee. “M11-A8 is designed to clearly explain to clinical laboratories how to assess testing needs and how to do the tests so that the results will be helpful to the ID staff. The pictures and tables included in the document make it very user-friendly and help standardize the interpretation of results.”

Two end-point–determining susceptibility testing methods for anaerobic bacteria are described in M11. The agar dilution method (Wadsworth) remains the reference standard, and is well suited for surveillance testing and research. It is also the standard to which other methods are compared. Broth microdilution is well suited for the clinical laboratory, but is currently limited to testing of the Bacteroides fragilis group of organisms and selected antimicrobial agents. Quality control (QC) criteria for each procedure are also described.

The tables in CLSI document Performance Standards for Antimicrobial Susceptibility Testing; Twenty-Second Informational Supplement (M100-S22), when used in conjunction with the standard M11-A8, represent the most current information for drug selection, interpretation, QC, and antibiogram reports.

CLSI is a volunteer-driven, membership-supported, nonprofit organization dedicated to developing standards and guidelines for the health care and medical testing community through a consensus process that balances the perspectives of industry, government, and the health care professions. For additional information, visit the CLSI website at www.clsi.org or call 610.688.0100.

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