A total of 168 actinomycete colonies were isolated from 14 sediment samples of the northern parts of the Oman Sea and were screened for cytotoxic and antimicrobial activity. Among four media and two treatments, the glucose arginine agar medium (18%) and heat treatment (28.3%) showed maximum isolation rate of actinomycetes. Preliminary characterization revealed that the members of Streptomycetaceae were widely distributed (66%) in the most of the sampling stations followed by Micromonosporaceae (14%), Nocardiaceae (6%), and Pseudonocardiaceae (4%), respectively. Approximately, 23.8% of the isolates inhibited the growth of at least one of the microbial test strains, while the majority of them belonged to the Streptomycetaceae family. Minimum inhibitory concentrations of the ethyl acetate culture extracts of the five most putative isolates varied from 64 μg/mL against Micrococcus luteus and Candida albicans to 1 mg/mL against Aspergillus niger. These extracts showed significant cytotoxic activity at18.74-193.5 μg/mL on the human breast (MCF7), colon (HCT 116), and liver (HepG2) tumor cell lines while exhibited less or no cytotoxicity on the normal cell line (HUVEC). Interestingly, IFSRI 193 extract selectively inhibited the growth of HCT 116 cell line and gram-positive bacteria. 16S rRNA gene sequencing revealed that the potent isolates have 97 to 99% similarity with S. chartreusis, S. cacaoi, S. sampsonii, S. qinglanensis, and S. diastaticus. These results suggested that the five Streptomyces strains could be considered candidates for discovering the antitumor antibiotics.
Keywords: Antimicrobial activity; Biodiversity; Cytotoxic activity; Marine actinomycetes; Oman Sea.