Effects of agars on determination of potency of polymyxin B sulfate by the agar plate diffusion method

The effects of agars on the determination of the potency of polymyxin B sulfate by the agar plate diffusion method were investigated. The results showed that there were significant differences in the potency of polymyxin B sulfate between agars produced by different manufacturers. In order to investigate this phenomenon, two major components of polymyxin B sulfate, namely, the polymyxin B1 and B2 fractions, were isolated and purified by preparative high-pressure liquid chromatography, and the effects of agars on the potencies of these two components were individually examined. The results showed that there were significant differences in the relative potency of the B1 component versus that of the B2 component between the agar preparations produced by different manufacturers. On the basis of the relative potency of each of the components and the composition of the components, we developed a theoretical equation with which we could calculate the activity of polymyxin B sulfate on a certain agar. The values computed from the equation were consistent with those obtained from actual measurements. It was concluded that the effects of agars on the determination of polymyxin B sulfate activity will become evident only when the composition of the reference standard significantly differs from that of the samples. When the standard and samples have the same compositions, the sources of the agar preparation exert no effect on the determination of the potency by the agar plate diffusion method.