In vitro activity against multi-drug resistant bacteria and cytotoxicity of lichens collected from Mount Cameroon

Abstract

Natural products remain a promising source of new efficacious antimicrobials to counter increasing resistance and prevent emergence of multidrug and extensively resistant bacteria phenotypes which hinder successful chemotherapy. Lichens have been shown to possess significant antimicrobial activity. This study investigated the antibacterial properties of six lichens found on Mount Cameroon. Methanol extracts of the lichens were screened against nine multidrug resistant clinical bacteria isolates and 6 control strains using disc diffusion and microdilution assays. The phytochemical composition of the extracts was determined and active extracts evaluated for cytotoxicity on monkey kidney epithelial LLC-MK2 cells using microscopy and MTT-formazan assay. Three extracts showed intermediate to high activity with diameters of inhibition zones ranging from 15 to 30 mm against all nine multidrug resistant strains similar to gentamicin positive control (P = 0.1018–0.6699). Extracts of Usnea articulata and Usnea florida, were the most active with minimum inhibitory concentrations of 4–10 mg/mL and showed broad spectrum dose-dependent activity. All the extracts were not cytotoxic (CC50 from 56.58 to 278.50 µg/mL) and the most active were rich in alkaloids, flavonoids, terpenoids among others. The considerable broad spectrum bacteriostatic activity against multidrug resistant strains and lack of cytotoxicity of U. articulata and U. florida justifies further exploration of these lichens to identify the bioactive molecules for development into new efficacious antibacterials.