Antimicrobial susceptibility testing (AST) is recommended to determine which antimicrobial agents should be considered for treating a bacterial pathogen. Many bacteria that cause disease in aquatic animals have growth conditions that vary substantially from routine terrestrial bacterial pathogens. It has thus become desirable to develop guidelines for standardizing AST methods for organisms isolated from aquatic animals that prefer or need certain conditions, such as lower temperatures, diluted media, or longer incubation times. Clinical and Laboratory Standards Institute guideline VET03—Methods for Antimicrobial Broth Dilution and Disk Diffusion Susceptibility Testing of Bacteria Isolated From Aquatic Animals describes broth dilution and disk diffusion, and it includes a series of procedures to standardize the way the tests are performed on Groups 1 and 3 aquatic bacteria. Group 1 nonfastidious bacteria grow readily in cation-adjusted Mueller-Hinton broth (CAMHB) and on Mueller-Hinton agar and are readily cultured at temperatures of 22°C ± 2°C and 28°C ± 2°C. Group 3 nonfastidious gliding bacteria grow in diluted CAMHB and are readily cultured at temperatures of 18°C or 28°C, depending on the species. The supplemental VET04 tables used with this guideline represent the most current information for antimicrobial agent selection, interpretation, and QC using the procedures described in VET03. The QC ranges for Escherichia coli ATCC® 25922 and Aeromonas salmonicida subsp. salmonicida ATCC® 33658 when tested at 18°C, 22°C, 28°C, and 35°C ± 2°C (ie, E. coli only) are listed in VET04 for different antimicrobial agents important to global aquaculture.

This guideline provides veterinary and aquatic animal disease diagnostic laboratories with currently recommended antimicrobial broth dilution and disk diffusion susceptibility testing methods for nonfastidious bacteria isolated from aquatic animals, including criteria for QC testing with two QC strains. It also provides suggestions as to the testing conditions for other fastidious aquatic animal bacterial pathogens. To avoid confusion, organisms relevant to aquaculture have been grouped (ie, Groups 1 through 5), and the organisms in each group and their corresponding numbers have not changed from previous editions of this guideline. This guideline also provides appendixes and tables describing media and disk preparation, methods for preparing stock solutions and dilutions of antimicrobial agents, and antimicrobial agents used in global aquaculture.

Clinical breakpoints and epidemiological cutoff values (ECVs) are included in VET04.1 Clinical breakpoints must be established using pharmacokinetic (PK) and pharmacodynamic (PD) data, in vitro antimicrobial susceptibility testing (AST) data, and clinical efficacy data. ECVs can be established from susceptibility data distributions alone. For information on how to develop these interpretive categories, consult CLSI document VET02.10 As more aquatic animal–specific information becomes available, this guideline and VET041 will be updated accordingly.

This guideline and its supplement (VET041) are not intended to guide the use of antimicrobial agents that are used for disease prevention or production uses.