Abstract
The search for novel bioactive metabolites continues to be of much importance around the world for pharmaceutical, agricultural, and industrial applications. Actinobacteria constitute one of the extremely interesting groups of microorganisms widely used as important biological contributors for a wide range of novel secondary metabolites. This study focused on the assessment of antimicrobial and antioxidant activity of crude extracts of actinobacterial strains. Western Ghats of India represents unique regions of biologically diverse areas called “hot spots”. A total of 32 isolates were obtained from soil samples of different forest locations of Bisle Ghat and Virjapet situated in Western Ghats of Karnataka, India. The isolates were identified as species of Streptomyces, Nocardiopsis, and Nocardioides by cultural, morphological, and molecular studies. Based on preliminary screening, seven isolates were chosen for metabolites extraction and to determine antimicrobial activity qualitatively (disc diffusion method) and quantitatively (micro dilution method) and scavenging activity against DPPH (2,2-diphenyl-1-picrylhydrazyl) and ABTS (2,2′-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) radicals. Crude extracts of all seven isolates exhibited fairly strong antibacterial activity towards MRSA strains (MRSA ATCC 33591, MRSA ATCC NR-46071, and MRSA ATCC 46171) with MIC varying from 15.62 to 125 μg/mL, whereas showed less inhibition potential towards Gram-negative bacteria Salmonella typhi (ATCC 25241) and Escherichia coli (ATCC 11775) with MIC of 125–500 μg/mL. The isolates namely S1A, SS5, SCA35, and SCA 11 inhibited Fusarium moniliforme (MTCC 6576) to a maximum extent with MIC ranging from 62.5 to 250 μg/mL. Crude extract of SCA 11 and SCA 13 exhibited potent scavenging activities against DPPH and ABTS radicals. The results from this study suggest that actinobacterial strains of Western Ghats are an excellent source of natural antimicrobial and antioxidant compounds. Further research investigations on purification, recovery, and structural characterization of the active compounds are to be carried out.
Highlights
The quest for novel biologically active secondary metabolites from microorganisms continues to rise due to emergence of drug resistance in pathogens causing life threatening diseases around the globe [1]
Soil microbes are a major source of a number of natural products including clinical important antibiotics, immunomodulators, enzyme inhibitors, antioxidants, anti-tumor and anticancer agents
Actinobacteria are abundant in soil, species of Streptomyces in particular represent the dominance over other microbes present in soil and play a vital role in recycling of materials and production of important metabolites [25]
Summary
The quest for novel biologically active secondary metabolites from microorganisms continues to rise due to emergence of drug resistance in pathogens causing life threatening diseases around the globe [1]. In order to withstand the oxidative stress caused, cells or organisms make use of antioxidants, that are able to block or delay the damage caused by several possible mechanisms such as halting chain reactions, preventing the formation of free radicals, neutralizing the singlet oxygen molecule, promoting anti-oxidant enzymes, and inhibiting pro-oxidative enzymes [8]. Natural antioxidants are found abundantly in metabolites produced by microorganisms These products have consistently been considered as mainstay for drugs with various interesting biological activities [10]. Actinomycetes have contributed nearly 45% of all the reported metabolites [14]
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