Taxonomy, bioactivities, and secondary metabolites of endophytic actinomycetes isolated from medicinal plants

Abstract

Fifty endophytic actinomycetes isolated from six medicinal plants in Thailand were identified as Micromonospora (34 strains), Streptomyces (8 strains), Microbispora (3 strains), Nonomuraea (2 strains), Plantactinospora (2 strains), and Amycolatopsis (1 strain) based on phenotypic characteristics and 16S rRNA gene sequencing. Fifteen strains exhibited antimicrobial activity against Gram-positive bacteria. Strain CR1-09T, proposed as Microbispora cathalanthi sp. nov. was active against Staphylococcus aureus ATCC 6538 and Kocuria rhizophila ATCC 9341. Streptomyces sp. 3MP-10 and 3MP-14, novel species were active against S. aureus ATCC 6538, K. rhizophila ATCC 9341, and Bacillus subtilis ATCC 6633. Strains CR1-09T and PA1-07 showed cytotoxicity against HepG2 cells but not Vero cells based on MTT assay. Streptomyces sp. 3MP-10 and 3MP-14 produced ikarugamycin ([M+H]+ = 479.2912 m/z; UV 224, 326 nm) and fumaquinone ([M+H]+ = 303.1225 m/z; UV 220, 236, 304, 428 nm) while 3MP-14 produced a new structural compound, pudicin ([M+Na]+ = 600.2576 m/z; UV 204, 246, 344 nm). Nonomuraea sp. PA1-05 and PA1-10 produced dilactonmycin ([M+H]+ = 539.3223 m/z; UV 222, 242, 316 nm) and spithioneine A ([M+H]+ = 492.1833 m/z; UV 250, 274, 336 nm), respectively. Micromonospora sp. BL1-08 produced riboflavin ([M+H]+ = 377.1466 m/z; UV 222, 268, 370, 446 nm) based on liquid chromatography-mass spectrometry (LC/MS) with the dictionary of natural products. Moreover, all strains produced a siderophore as nocardamine ([M+H]+ = 565.1552 m/z; UV 214, 272, 336 nm). The secondary metabolite biosynthetic gene clusters (BGCs) identified in the draft genome of strain 3MP-14 were related to the biosynthesis of polyketides by polyketide synthase (PKS-I, II, and III) as 41.7 % of all putative BGCs and non-ribosomal peptides by non-ribosomal peptide synthetase (NRPS).