THE LINEAR REPRESENTATION OF DOSAGE-RESPONSE CURVES IN MICROBIAL-ANTIBIOTIC ASSAYS

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Abstract
A simple graphical procedure is described whereby the results of photometric growth assays of antibiotic activity may conveniently be represented by plotting the percentages of growth as ordinate on a normal curve-deviate scale, against the logarithms of the antibiotic concentrations. In many instances this representation will be linear from 2 to 98% of growth (98 to 2% inhibition). This linearity permits ready interpolation or extrapolation of data, or the assessment of a 50% endpoint. In favorable instances the entire dosage-response curve can be predicted from only 2 experimental measurements, or a single measurement and knowledge of the average slope for such systems. The slope of the line also affords a useful parameter which enters into the assessment of potency differences or serves as an indication of possible changes in mechanisms of action. Utilization of this plot also facilitates certain statistical examinations of the date.