Discovery of two new actinobacteria, Micromonospora palythoicola sp. nov. and Streptomyces poriticola sp. nov., isolated from marine invertebrates.

Marine invertebrates represent an underexplored reservoir for actinobacteria, which are known to synthesize novel bioactive compounds. This study isolated 37 actinobacterial strains from five distinct marine invertebrate hosts, namely Chondrilla australiensis, Palythoa sp., Favia sp., Porites lutea, and Acropora cervicornis, while no strains were obtained from Lissoclinum sp. and Lithophyllon sp. These isolates were taxonomically classified into six genera: Gordonia, Microbacterium, Micromonospora, Nocardia, Rhodococcus, and Streptomyces, with Streptomyces and Micromonospora being notably predominant. Comparative genomic analysis facilitated the identification of two novel species: Micromonospora palythoicola sp. nov. (strain S2-005 = TBRC 18343 and NBRC 116545) and Streptomyces poriticola sp. nov. (strain C6-003, =TBRC 17807 and NBRC 116425). Both species exhibited substantial genetic differences from their nearest known species as demonstrated by digital DNA-DNA hybridization and average nucleotide identity scores, which fell below the thresholds of 70% and 95%, respectively. Streptomyces poriticola C6-003 displayed significant antimicrobial activity and selective cytotoxicity against human breast cancer MCF-7 cells, with reduced toxicity towards human dermal papilla cells. Micromonospora palythoicola S2-005 manifested antimicrobial properties against Streptococcus mutans and Kocuria rhizophila. These findings highlight the considerable diversity of actinobacteria within marine invertebrates and underscore their potential as a source of new species with promising biological properties for therapeutic applications.