Quantitative determination of echinomycin by disc agar diffusion assay

Echinomycin is a peptide antibiotic of the quinoxaline group produced by Streptomyces echinatus. In propitious circumstances it can be determined by ultraviolet spectrophotometry, but for work with analogues and congeners a more specific assay is required. A disc agar diffusion technique has been developed and optimised for this purpose. It yields log dose-response curves which are linear over at least a 20-fold range of antibiotic concentration, and the variation of sensitivity with inoculum size, preincubation time, and temperature of incubation has been investigated. Other quinoxaline antibiotics can be assayed by the same technique; they yield quite different log dose-response curves indicative of lower potency associated with triostins as compared to quinomycins. The application of the technique to follow echinomycin production by S. echinatus A8331 in culture in maltose minimal medium is described.