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 M100,⁵ M02,⁷ and M07.⁸ Standards for testing the in vitro antimicrobial susceptibility of bacteria that grow anaerobically are found in CLSI M11.⁹ Guidelines for standardized antimicrobial susceptibility testing (AST) of bacterial pathogens from aquatic animals are available in CLSI VET03.² Guidelines for AST of infrequently isolated or fastidious bacteria that are not included in CLSI M100,⁵ M02,7 M07,⁸ or M11⁹ are available in CLSI VET06³ and CLSI M45.¹⁰ The AST methods provided in this standard can be used in laboratories around the world, including but not limited to:

• Veterinary and medical diagnostic laboratories

• Public health laboratories

• Research laboratories

• Food laboratories

• Environmental laboratories

This standard and its supplement (CLSI VET01S¹) are not intended to guide the use of antimicrobial agents that are used for production or disease prevention purposes.