Antibiotic Susceptibility Chart (Page 2 of 4) in pdf

Antibiotic Susceptibility Chart Page 2

What are break-points? Break-points are the bacteria/drug combination that

correlates with the interpretation guidelines established by CLSI. For example,

the break-points for florfenicol in cases of bovine pneumonia are ≤ 2 (S), 4 (I)

and ≥ 8 (R) ug/ml when Pasteurella multocida is tested.

What are efficacy ratios? Efficacy ratios are calculated by taking the resistant

break-point MIC and dividing it by the MIC obtained by the broth dilution

method. It is a tool that can be used to evaluate the relative efficacy of different

antimicrobial drugs. For example, recently the WVDL obtained the following

MIC results from an isolate of Mannheimia haemolytica.

Drug

MIC ug/ml

Interpretation

Resistant

Efficacy Ratio

Break-point

MIC ug/ml

≥ 8

Ceftiofur

0.5

≥ 8

Florfenicol

0.25

≥ 16

Oxytetracycline 0.5

≥ 128

Spectinomycin

≥ 32

Tilmicosin

≥ 64

Tulathromycin

In this case, the efficacy ratio for ceftiofur is 16. The efficacy ratio was calculated by

taking the resistant break-point MIC for ceftiofur (8 ug/ml) and dividing it by the

measured MIC of 0.5 ug/ml.

Therefore, the two antimicrobial drugs with the greatest predicted efficacy are florfenicol

and oxytetracycline. It is important to point out that the final decision of which

antimicrobial drug should be used lies with the practicing veterinarian.

The table below can be used for calculation of efficacy ratios.

Further information on antimicrobial drug use can be obtained at:

(veterinary antimicrobial decision support network).

Antibiotic Susceptibility Chart Page 2