**Radha Singh and Ashok K. Dubey \***

Department of Biological Sciences & Engineering, Netaji Subhas Institute of Technology, New Delhi 110078, India; dharana.radha@gmail.com

**\*** Correspondence: adubey.nsit@gmail.com or akdubey@nsut.ac.in

Received: 24 May 2020; Accepted: 5 June 2020; Published: 19 June 2020

**Abstract:** In view of the fast depleting armamentarium of drugs against significant pathogens, like methicillin-resistant *Staphylococcus aureus* (MRSA) and others due to rapidly emerging drug-resistance, the discovery and development of new drugs need urgent action. In this endeavor, a new strain of endophytic actinobacterium was isolated from the plant *Datura metel*, which produced secondary metabolites with potent anti-infective activities. The isolate was identified as *Streptomyces californicus* strain ADR1 based on 16S rRNA gene sequence analysis. Metabolites produced by the isolate had been investigated for their antibacterial attributes against important pathogens: *S. aureus*, MRSA, *S. epidermis*, *Enterococcus faecium* and *E. faecalis*. Minimum inhibitory concentration (MIC90) values against these pathogens varied from 0.23 ± 0.01 to 5.68 ± 0.20 μg/mL. The metabolites inhibited biofilm formation by the strains of *S. aureus* and MRSA (Biofilm inhibitory concentration [BIC90] values: 0.74 ± 0.08–4.92 ± 0.49 μg/mL). The BIC90 values increased in the case of pre-formed biofilms. Additionally, the metabolites possessed good antioxidant properties, with an inhibitory concentration (IC90) value of 217.24 ± 6.77 μg/mL for 1, 1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging. An insight into different classes of compounds produced by the strain ADR1 was obtained by chemical profiling and GC-MS analysis, wherein several therapeutic classes, for example, alkaloids, phenolics, terpenes, terpenoids and glycosides, were discovered.

**Keywords:** endophytic actinobacteria; *Streptomyces* sp., anti-*S. aureus*; anti-MRSA; anti-biofilm
