Streptomyces mitragynae sp. nov., isolated from Mitragyna speciosa leaves in Thailand, with antimicrobial activity and secondary metabolite profiling

Two novel endophytic actinomycete strains, NRAIS3 T and NRAIS4, belonging to the genus Streptomyces , were isolated from Mitragyna speciosa leaves. Their taxonomic status was determined using a polyphasic approach. Both strains grew at temperatures ranging from 25 to 37 °C and at pH values between 6 and 10. Strain NRAIS3ᵀ tolerated up to 5% (w/v) NaCl, whereas strain NRAIS4 tolerated up to 7% (w/v) NaCl. Both strains contained ll -diaminopimelic acid in their cell-wall peptidoglycan, and their whole-cell hydrolysates contained ribose and glucose. The predominant cellular fatty acids were anteiso -C 15:0 , anteiso -C 17 : 0 , iso -C 16:0 and iso -C 15:0 . The polar lipids detected included diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine, phosphatidylinositol and phosphatidylinositol mannosides. Based on 16S rRNA gene sequence analysis, strains NRAIS3 T and NRAIS4 showed the highest similarity (99.4%) to the type strains Streptomyces fodineus TW1S1 T , Streptomyces puniciscabiei DSM 41929 T and Streptomyces filipinensis JCM 4369 T . The average nucleotide identity based on MUMmer and digital DNA–DNA hybridization values between the genomes of the two strains and those of related type strains were significantly below the Streptomyces species delineation thresholds of 96.7% and 70%, respectively. The ethyl acetate crude extract of NRAIS3 T exhibited antibacterial activity against Gram-positive and Gram-negative pathogenic bacteria. It showed the highest activity against Listeria monocytogenes ATCC 7644, followed by Staphylococcus epidermidis ATCC 12228, and Enterococcus faecium ATCC 35667. Liquid chromatography–mass spectrometry and gas chromatography–mass spectrometry analyses revealed diverse antimicrobial metabolites, including aminoquinolines, ergopeptines, isoflavones, hydroxycinnamic acids, N -alkyl piperazines, benzotriazoles, benzimidazoles, phenylacetamides, as well as volatile compounds such as 2,5,5-trimethylcyclohexane-1,3-dione, hexamethyl-cyclotrisiloxane and 1,4-bis(trimethylsilyl) benzene. Based on the taxonomic results, strains NRAIS3 T and NRAIS4 represent a novel species within the genus Streptomyces , for which the name Streptomyces mitragynae sp. nov. is proposed. The type strain is NRAIS3 T (=LMG 34056 T =TBRC 20459 T ).