[A new mathematical approach for determining antibiotic concentrations using the agar diffusion method]

A mathematical approach using the one-point intercept form of the equation of a straight line for the assay of antibiotic biological activity by the agar diffusion method is described. Insignificant alteration in the experimental conditions made it possible to markedly lower the volume of work and materials for the statistic processing. As distinct from the routine methods, the assay substance in three different concentrations is applied to three cavities on a dish and the standard substance in one control concentration is applied to the other three cavities on the same dish. The antibiotic biological activity is estimated from the equation lg cn = lg cst + (dn - dst)/bi when the relationship between the dose logarithm and the size of the diameters of the test microbe inhibition growth zones is direct. When the relationship is inverse e.g. when the dose is expressed as the dilution, the equation lg cn = lg cst + (dst - dn)/bi is used. Factor bi is determined experimentally for every sample by the mean value of the sizes of the inhibition growth zone diameter by the programs of the simple linear-regression.