scope:

This standard describes reference agar disk diffusion techniques, as well as standard broth (macrodilution and microdilution) and agar dilution methods used to determine in vitro antimicrobial susceptibility of bacteria that grow aerobically. It includes:

• Agar plate preparation

• Broth and agar dilution test preparation

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

• Results interpretation and reporting considerations

• QC procedures

• Disk diffusion and dilution test method limitations

To assist the veterinary laboratory, suggestions are provided for selecting antimicrobial agents for routine testing and reporting. Additionally, a brief overview of the various antimicrobial classes, bacterial mechanisms of antimicrobial resistance (AMR), and specific tests for detecting AMR are included.

For additional resources, standards for testing the in vitro antimicrobial susceptibility of bacteria isolated from humans that grow aerobically using disk or dilution methods are found in CLSI documents M100,⁷ M02,² and M07,³ respectively. Standards for testing the in vitro antimicrobial susceptibility of bacteria that grow anaerobically are found in CLSI document M11.⁸ Guidelines for standardized antimicrobial susceptibility testing (AST) of infrequently isolated or fastidious bacteria that are not included in CLSI documents M100,⁷ M02,² M07,³ or M11⁸ are available in CLSI documents VET06⁹ and M45.¹⁰ The AST methods provided in this standard can be used in laboratories around the world, including but not limited to:

• Veterinary diagnostic laboratories

• Public health laboratories

• Research laboratories

• Food laboratories

• Environmental laboratories

This standard and its supplement (VET08¹) are not intended to guide the use of antimicrobial agents that are used for production or disease prevention purposes.

¹ CLSI. Performance Standards for Antimicrobial Disk and Dilution Susceptibility Tests for Bacteria Isolated From Animals. 4th ed. CLSI supplement VET08. Wayne, PA: Clinical and Laboratory Standards Institute; 2018.

² CLSI. Performance Standards for Antimicrobial Disk Susceptibility Tests. 13th ed. CLSI document M02. Wayne, PA: Clinical and Laboratory Standards Institute; 2018.

³ CLSI. Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically. 11th ed. CLSI document M07. Wayne, PA: Clinical and Laboratory Standards Institute; 2018.

⁷ CLSI. Performance Standards for Antimicrobial Susceptibility Testing. 28th ed. CLSI supplement M100. Wayne, PA: Clinical and Laboratory Standards Institute; 2018.

⁸ CLSI. Methods for Antimicrobial Susceptibility Testing of Anaerobic Bacteria; Approved Standard-Eighth Edition. CLSI document M11-A8. Wayne, PA: Clinical and Laboratory Standards Institute; 2012.

⁹ CLSI. Methods for Antimicrobial Susceptibility Testing of Infrequently Isolated or Fastidious Bacteria Isolated From Animals. 1st ed. CLSI document VET06. Wayne, PA: Clinical and Laboratory Standards Institute; 2017.

¹⁰ CLSI. Methods for Antimicrobial Dilution and Disk Susceptibility Testing of Infrequently Isolated or Fastidious Bacteria. 3rd ed. CLSI guideline M45. Wayne, PA: Clinical and Laboratory Standards Institute; 2016.