Discovery of antimalarial drugs from secondary metabolites in actinomycetes culture library.

BACKGROUND: Natural products play a key role as potential sources of biologically active substances for the discovery of new drugs. This study aimed to identify secondary metabolites from actinomycete library extracts that are potent against the asexual stages of Plasmodium falciparum (P. falciparum). METHODS: Secondary metabolites from actinomycete library extracts were isolated from culture supernatants by ethyl acetate extraction. Comprehensive screening was performed to identify novel antimalarial compounds from the actinomycete library extracts (n = 28). The antimalarial activity was initially evaluated in vitro against chloroquine/mefloquine-sensitive (3D7) and-resistant (Dd2) lines of P. falciparum. The cytotoxicity was then evaluated in primary adult mouse brain (AMB) cells. RESULTS: Out of the 28 actinomycete extracts, 17 showed parasite growth inhibition > 50% at a concentration of 50 µg/mL, nine were identified with an IC50 value < 10 µg/mL, and seven suppressed the parasite significantly with an IC50 value < 5 µg/mL. The extracts from Streptomyces aureus strains HUT6003 (Extract ID number: 2), S. antibioticus HUT6035 (8), and Streptomyces sp. strains GK3 (26) and GK7 (27), were found to have the most potent antimalarial activity with IC50 values of 0.39, 0.09, 0.97, and 0.36 µg/mL (against 3D7), and 0.26, 0.22, 0.72, and 0.21 µg/mL (against Dd2), respectively. Among them, Streptomyces antibioticus strain HUT6035 (8) showed the highest antimalarial activity with an IC50 value of 0.09 µg/mL against 3D7 and 0.22 µg/mL against Dd2, and a selective index (SI) of 188 and 73.7, respectively. CONCLUSION: Secondary metabolites obtained from the actinomycete extracts showed promising antimalarial activity in vitro against 3D7 and Dd2 cell lines of P. falciparum with minimal toxicity. Therefore, secondary metabolites obtained from actinomycete extracts represent an excellent starting point for the development of antimalarial drug leads.

Characterization of the surugamide biosynthetic gene cluster of TUA-NKU25, a Streptomyces diastaticus strain isolated from Kusaya, and its effects on salt-dependent growth.

Kusaya, a traditional Japanese fermented fish product, is known for its high preservability, as it contains natural antibiotics derived from microorganisms, and therefore molds and yeasts do not colonize it easily. In this study, the Streptomyces diastaticus strain TUA-NKU25 was isolated from Kusaya, and its growth as well as the production of antibiotics were investigated. Strain TUA-NKU25 showed advantageous growth characteristics in the presence, but not in the absence, of sodium chloride (NaCl). Antimicrobial assay, high-performance liquid chromatography, and electrospray ionization-mass spectrometry analysis showed that this strain produced surugamide A and uncharacterized antimicrobial compound(s) during growth in the presence of NaCl, suggesting that the biosynthesis of these compounds was upregulated by NaCl. Draft genomic analysis revealed that strain TUA-NKU25 possesses a surugamide biosynthetic gene cluster (sur BGC), although it is incomplete, lacking surB/surC. Phylogenetic analysis of strain TUA-NKU25 and surugamide-producing Streptomyces showed that sur BGC formed a clade distinct from other known groups.