**Saket Siddharth and Ravishankar Rai Vittal \***

Department of Studies in Microbiology, University of Mysore, Manasagangotri, Mysore 570006, India; saketsiddharth@gmail.com

**\*** Correspondence: raivittal@gmail.com; Tel.: +91-821-241-9441

Received: 30 April 2018; Accepted: 13 July 2018; Published: 18 July 2018

**Abstract:** In the present study, marine actinobacteria *Streptomyces* sp.S2A was isolated from the Gulf of Mannar, India. Identification was carried out by 16S rRNA analysis. Bioactive metabolites were extracted by solvent extraction method. The metabolites were assayed for antagonistic activity against bacterial and fungal pathogens, inhibition of α-glucosidase and α-amylase enzymes, antioxidant activity and cytotoxic activity against various cell lines. The actinobacterial extract showed significant antagonistic activity against four gram-positive and two gram-negative pathogens. Excellent reduction in the growth of fungal pathogens was also observed. The minimum inhibitory concentration of the partially purified extract (PPE) was determined as 31.25 μg/mL against *Klebsiella pneumoniae*, 15.62 μg/mL against *Staphylococcus epidermidis*, *Staphylococcus aureus* and *Bacillus cereus*. The lowest MIC was observed against *Micrococcus luteus* as 7.8 μg/mL. MIC against fungal pathogens was determined as 62.5 μg/mL against *Bipolaris maydis* and 15.62 μg/mL against *Fusarium moniliforme*. The α-glucosidase and α-amylase inhibitory potential of the fractions were carried out by microtiter plate method. IC50 value of active fraction for α-glucosidase and α-amylase inhibition was found to be 21.17 μg/mL and 20.46 μg/mL respectively. The antioxidant activity of partially purified extract (PPE) (DPPH, ABTS, FRAP and Metal chelating activity) were observed and were also found to have significant cytotoxic activity against HT-29, MDA and U-87MG cell lines. The compound analysis was performed using gas chromatography-mass spectrometry (GC-MS) and resulted in three constituents; pyrrolo[1–a]pyrazine-1,4-dione,hexahydro-3-(2-methylpropyl)-, being the main component (80%). Overall, the strain possesses a wide spectrum of antimicrobial, enzyme inhibitory, antioxidant and cytotoxic activities which affords the production of significant bioactive metabolites as potential pharmacological agents.

**Keywords:** marine actinobacteria; *Streptomyces* sp.; enzyme inhibition; antimicrobial; antioxidant; cytotoxicity; GC-MS; pyrrolopyrazines
