keywords: Antimicrobial, Oxidative stress, Metabolites, Drug resistance, Drug discovery, Functional oligosaccharide
The aim of the current study was to evaluate the antioxidant and antibacterial activity of polyphenol, oligosaccharide, and polysaccharide extracts of Nigerian lichens~ Ochrolechia subpallescens Vers., Lecanora pinguis Tuck., Parmelia saxatilis (L.) Ach., Graphis scripta (L.) Ach., Verrucaria viridula (Schrad.) Ach. The polyphenol and polysaccharides were extracted with acidified methanol and hot water respectively, lyophilized, purified, and quantified. Oligosaccharides were extracted with hot water, treated sequentially with neutral and acid detergent solutions, and finally hydrolyzed with sulphuric acid solution, lyophilized, purified, and quantified. The 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity and Trolox equivalent antioxidant capacity were used to establish antioxidant potentials of lichen metabolites. The minimum inhibitory concentration of the metabolites was established to determine their antibacterial effects using the broth microdilution assay against gram-positive and gram-negative bacteria. The lichens~ O. subpallescens, L. pinguis, and G. scripta had the highest concentrations of oligosaccharide, polysaccharides, and polyphenols respectively. O. subpallescens oligosaccharide extracts showed the strongest antioxidant activity. There was positive correlation between the antioxidant activity and total phenolic content of the lichens. G. scripta oligosaccharide had the strongest antibacterial activity among all lichen species evaluated and inhibited the growth of all the test bacteria. P. saxatilis polyphenol extracts exhibited broad spectrum antibacterial activity inhibiting both gram-positive and gram-negative bacteria effectively. The oligosaccharides are low molecular compounds and were understandably more reactive than polysaccharides, having higher antioxidant and antibacterial activity. All tested lichen oligosaccharides extracts and polysaccharides of O. subpallescens and L. pinguis can be used as natural sources of antioxidants and antibacterial agents.