Use of organic solvents in cylinder-plate assays.

A new antibiotic, aterrimin (Lewis et al., 1953), is unstable in aqueous solution at pH 7, and insoluble at lower pH levels. Aterrimin can be dissolved in absolute alcohol and the solutions can be used satisfactorily in a cylinder-plate type of assay (Heatley, 1944). Further investigation has established that sevbral other organic solvents are nontoxic to various bacteria when used in cylinders. This information may prove useful to others who wish to assay antibiotics which are not readily soluble or which are unstable in aqueous solutions. The literature suggests that it has been generally assumed that high concentrations of organic solvents would interfere with cylinder-plate assays. No referlarly of crude preparations which contain viable spores from the parent culture, the paper-disc method affords less protection against contamination than the cylinderplate method. The purification of aterrimin has been followed by cylinder-plate assays with Bacillus megaterium NRRL B-938 on nutrient agar (Difco) plates incubated for 16 hours at 35 C. Samples were evaporated when necessary and dissolved in absolute ethyl alcohol; or they were diluted with ethyl alcohol so that the concentration of any other solvent originally present was reduced to less than 3 per cent. Standards were prepared in absolute ethyl alcohol. Inconsistent results may arise if all samples and standards are not added as solutions