Page 257 - The Veterinary Laboratory and Field Manual 3rd Edition

226 Susan C. Cork and Roy Halliwell

occurs when cultured colonies are exposed to a benefit the treatment outcome, such as when
selection of antibiotics. This does not precisely a pathogenic strain is multiple-drug resistant,
mimic the conditions found in the animal host or when there are limited drugs that can be
but the results can still be useful when choosing used with that patient due to multiple-drug
a specific antibiotic treatment (Figure 4.13a). resistance, physiological limitations of the
For many clinical purposes, it is not necessary patient, or withdrawal time considerations. In
to accurately measure the minimum inhibitory these circumstances, measuring the MIC (mg/
and minimum bactericidal concentrations (MIC ml) will allow the consulting veterinarian to
and MBC) of selected antibiotics for an isolate. choose antibiotics for which the bacterial strain
It is usually sufficient to use a simple qualitative may have a MIC which falls in the ‘intermedi-
assay, which distinguishes isolates as ‘sensitive’ ate’ range of sensitivity. Bactericidal antibiotic
or ‘resistant’ to a particular antibiotic in line compounds may still be clinically effective when
with the probable outcome of therapy. dosed in such a manner that the antibiotic
There are circumstances when quantitative reaches a blood or tissue concentration in the
information about antibiotic sensitivity will ‘intermediate’ sensitivity range.

Figure 4.13a Factors affecting the choice of an antibacterial drug.

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