Antibiotic Susceptibility Chart printable pdf download

Antimicrobial Susceptibility Testing

Donald C. Sockett DVM, PhD

Wisconsin Veterinary Diagnostic Laboratory

01-04-13

In the past, the WVDL used the Kirby Bauer disc diffusion testing method for

antimicrobial susceptibility testing. In January 2006, the WVDL switched to a broth

dilution method that provides not only susceptibility information but also minimum

inhibitory concentration (MIC) information that is not available with the Kirby Bauer

method. In this document, we attempt to answer many of the frequently asked questions

regarding antimicrobial susceptibility testing. Of particular importance is the concept of

efficacy ratios. Efficacy ratios can be used to calculate which antimicrobial drug has the

highest predicted efficacy or activity against a given bacterial isolate. It is our hope that

this document will be useful to our clients, the practicing veterinarians.

What is the Kirby Bauer disk diffusion test? The Kirby Bauer test is a

qualitative assay whereby discs of paper are impregnated with a single

concentration of different antibiotics. The discs are placed on the surface of an

agar plate that has been inoculated with test bacteria. During incubation, the

antibiotics diffuse outward from the discs creating a concentration gradient. After

18-24 hours, the zone diameter (zone of inhibition) is measured and reference

tables are used to determine if the bacteria are Sensitive (S), Intermediate (I) or

Resistant (R) to the antimicrobial drugs.

What is the broth dilution method? The broth microdilution method is a liquid

culture method whereby a standard amount of bacteria are inoculated into the

wells of a 96 well micro-titer plate that contain different dilutions of

antimicrobial drugs. For example in the standard bovine/porcine panel, five wells

contain the antibiotic ceftiofur with dilutions of 8, 4, 2, 1, 0.5 and 0.25 ug/ml and

four wells contain spectinomycin with dilutions of 64, 32, 16 and 8 ug/ml,

respectively. After 18-24 hours, the plates are examined either visually or with an

analytical instrument for evidence of bacterial growth. Results are recorded as

minimum inhibitory concentrations (MIC).

What is the MIC? The MIC is the highest dilution (lowest concentration) of

antimicrobial drug that completely inhibits bacterial growth. The MIC value is

reported with interpretation guidelines (S, I, R) that have been established by the

Clinical and Laboratory Standards Institute (CLSI). Occasionally, no

interpretation (NI) is reported with the MIC result because no interpretative

guidelines have been established for the bacteria/drug combination being tested.

Antibiotic Susceptibility Chart