Discovery of Streptomyces marinisediminis sp. nov., a new thiolutin producing actinomycete isolated from Thai marine sediment

Thiolutin and 2,2-dioxidothiolutin are sulfur-containing antibiotics discovered through a targeted high-throughput screening strategy designed to identify compounds with antimicrobial activity. The producing strain, JHD1^T, is an actinomycete isolated from marine sediment collected in Chonburi Province, Thailand. Chemotaxonomic characteristics and 16S rRNA gene sequence analysis indicated that strain JHD1^T belongs to the genus Streptomyces. Taxonomic analysis using a polyphasic approach further supported its placement within the genus Streptomyces. The strain exhibited low similarity to its closest known relatives, Streptomyces alkaliterrae OF1^T and Streptomyces chumphonensis KK1-2^T, with average nucleotide identity based on MuMmer ultra-rapid aligning tool (ANIm) values of 85.7-86.2%, average amino acid identity (AAI) values of 71.8-78.1%, and digital DNA-DNA hybridization (dDDH) values of 23.4-25.0%, all well below the established thresholds for species delineation. These results reveal that strain JHD1^T represents a novel species within the genus Streptomyces, for which the name Streptomyces marinisediminis sp. nov. is proposed. The crude extract obtained from the culture broth of strain JHD1^T exhibited strong antimicrobial activity against Gram-positive bacteria and led to the discovery of two promising secondary metabolites, thiolutin and 2,2-dioxidothiolutin, along with 8-amino-2H-benz[e][1,3]oxazine-2,4(3H)-dione, benadrostin, 2,3-dihydroxybenzamide, and 2-hydroxy-3-methoxybenzamide. In this study, 2,2-dioxidothiolutin was structurally confirmed for the first time by X-ray crystallographic analysis. Interestingly, thiolutin demonstrated strong anti-human small cell lung cancer (NCI-H187) activity, with an IC₅₀ value of 0.35 µg/mL, and also showed cytotoxicity against human breast cancer (MCF-7) cells, with an IC₅₀ value of 5.61 µg/mL.