An endophytic Streptomyces sp. DARP-7 isolated from coastal lichen and enhancement of its antibacterial metabolite production using response surface methodology

Abstract

Actinomycete DARP-7 was isolated from the coastal foliose lichen Dirinaria aegilita (Afzel.ex Ach.) B.J.Moore and identified as Streptomyces sp. using 16S rRNA sequencing. Among the eight culture media used for antimicrobial metabolite production, the peptone yeast extract iron medium (ISP-6) showed higher production with the maximum zone of inhibition (12 mm) against Staphylococcus aureus . Further, the ISP-6 medium components were optimized through the Plackett-Burman design (PBD) followed by Response Surface Methodology (RSM). Yeast extract, sodium thiosulphate, and ferric ammonium citrate would play a significant role in antibiotic production. Further, the significant variables from PBD were optimized through Central Composite Design (CCD) of RSM for enhanced antibiotic production. The optimal medium components were found to be peptone - 15 (g/L), protease peptone - 5 (g/L), yeast extract - 2.915 (g/L), ferric ammonium citrate - 0.651 (g/L), K 2 HPO 4 – 1 (g/L) and Na 2 S 2 O 3 – 0.155 (g/L), respectively. The optimized result was validated and the antibacterial activity against S. aureus increased upto 19.9 mm (66.7%). The resazurin dye based quantification of minimum inhibitory concentration of the DARP-7 crude extract was found to be 41.34 µg and 46.44 µg against S. aureus and E.coli , respectively. Further, the compound profiling was done using UV-VIS, FTIR, EEM and GC-MS confirming the presence of polyene conjugates. Thus, the study delivers coastal lichen could be promising source for isolating bioactive metabolites producing rare actinobacteria. Furthermore, the use of a statistical optimization tool to improve bioactive metabolite yield is discussed.