Aims: An actinomycete strain, designated KBS50, was isolated from a beach sediment sample collected from the Santubong area in Sarawak, Malaysia. This study reports on the identification, characterization and evaluation of the antimicrobial potential of this rare actinomycete. Methodology and results: KBS50 was identified as a potentially new species of Plantactinospora genus using the 16S rRNA gene sequence analysis. The rare actinomycete showed distinct morphological and physiological characteristics from other species of Plantactinospora. KBS50 exhibited strong antagonistic activities against Gram-positive bacteria (Staphylococcus aureus and Bacillus subtilis) and fungi (Aspergillus niger, Ganoderma boninense, and Rhizoctonia solani). The actinomycete also tested positive for proteolytic activity. Meanwhile, secondary screening of the cell-free culture broths and the ethyl acetate crude extracts detected antimicrobial activity against the Gram-positive bacteria only. The minimum inhibitory concentration of the crude extract against B. subtilis and S. aureus was 5.21±1.30 μg/mL and 15.63±0.00 μg/mL, respectively. Conclusion, significance and impact of study The results presented in this paper provided an insight into the capability of Plantactinospora sp. KBS50 as a potential source of bioactive secondary metabolites compounds. This study also showed that the marine-associated environment such as the coastal area in Sarawak can be a valuable source of unique actinomycetes that can be exploited for natural product discovery.

Aims: A rare marine-derived actinomycete, Plantactinospora sp. KBS50, has been identified as a potential source of bioactive secondary metabolites compounds. The present study aimed to evaluate the secondary metabolites biosynthetic capability of strain KBS50 using the One Strain Many Compound (OSMAC) fermentation strategy. Methodology and results: Strain KBS50 was fermented in a basal medium (ISP2) supplemented with selected biological and chemical elicitors, as well as cultivation at different pH value and incubation temperature. Statistical analysis revealed that the antimicrobial activities were significantly increased, as compared to the basal medium, ISP2. Similarly, the comparative High Performance Liquid Chromatography (HPLC) analysis showed an increase in secondary metabolites production, as well as the detection of potential new metabolites, particularly from the crude extracts of ISP2 medium supplemented with 1% (w/v) sodium chloride and with the culture filtrate of Aspergillus niger. The bioassay-guided fractionation showed that the extract of strain KBS50 contains multiple compounds with antibacterial activity against the Gram-positive strains. Further fractionation led to the isolation of two semi-pure compounds (compound 3 and 4) with bactericidal properties against Staphylococcus aureus. The Minimum Inhibitory Concentration (MIC) values of compound 3 and 4 were recorded at 7.81 μg/mL and 62.50 μg/mL, respectively. The minimum bactericidal concentration (MBC) for compound 3 was recorded at 15.63 μg/mL while the MBC for compound 4 was recorded as 125.00 μg/mL. Conclusion, significance and impact of study The OSMAC fermentation strategy used in this study had successfully enhanced the detection of antibiotics and secondary metabolites from Plantactinospora sp. KBS50. The bioassay-guided fractionation further established the capability of strain KBS50 as a source of bioactive secondary metabolite compounds with potent antimicrobial activity.