Abstract

A total of 192 different Streptomyces isolates were recovered from 34 soil samples collected from different localities in Egypt. The cytotoxic activities of culture supernatant of each of the recovered isolate were investigated against Caco-2 cell line using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The Streptomyces isolate coded S19, that showed the highest cytotoxic activity against Caco-2 cell line (CD₅₀ of 26.4 µg/mL), was identified using 16S rRNA gene sequence analysis as Streptomyces variabilis isolate MW091521. The influence of cultural parameters affecting the cytotoxic activity of the respective isolate including the different culture media, initial pH value, temperature and rotation speed was investigated. A maximum cytotoxic activity (CD₅₀ of 13.4 µg/mL) was obtained using the culture medium of soybean meal at an initial pH of 7.0, incubation temperature 28°C and 200 rpm. Thereafter, the fermentation was conducted in a 14 L laboratory fermenter under the previously stated optimum conditions with the aim of extraction, and purification of cytotoxic metabolite(s). Fractionation of the ethyl acetate extract followed by advanced spectroscopic analysis using LC/ESI/MS as well as 1D and 2D NMR experiments yielded the cytotoxic metabolite heliomycin. To the best of our knowledge, this is the first report on the production of heliomycin by Streptomyces variabilis isolate MW091521.