A strain of Streptomyces sp. isolated from rhizospheric soil of Crataegus oxycantha producing nalidixic acid, a synthetic antibiotic.

AIM: Exploration of microbes isolated from rhizospheric soil of Crataegus oxycantha for bioactive natural products. METHODS AND RESULTS: A strain of Streptomyces sp. (C-7) was isolated from rhizospheric soil of C. oxycantha. The 16S rRNA gene sequence of strain C-7 displayed 99% sequence similarity with different Streptomyces species. The highest score was displayed for Streptomyces sp. strain Chy2-8 followed by Streptomyces violarus strain NBRC13104 and Streptomyces arenae strain ISP5293. The position of C-7 in the phylogenetic tree suggested uniqueness of the strain. Nalidixic acid (1), a quinolone antibiotic, was isolated from Streptomyces sp. strain (C-7) for the first time and characterized by NMR and chemically analysed. Compound 1 exhibited antimicrobial activity against Escherichia coli and Pseudomonas aeruginosa. The production of compound 1 was also validated by repeating fermentation of strain C-7 and compound isolation in a separate natural product laboratory with no prior information. Furthermore, Compound 1 showed a cytotoxic effect against human prostate cancer cell line PC3 with an IC50 11 µg ml-1 . CONCLUSION: To the best of our knowledge, this is the first report showing production of nalidixic acid naturally by a strain of Streptomyces sp. SIGNIFICANCE AND IMPACT OF THE STUDY: In this study, we isolated a strain of Streptomyces sp. producing nalidixic acid, which was otherwise only obtained through chemical synthesis.