Synergistic activity of extracts of three South African alien invasive weeds combined with conventional antibiotics against selected opportunistic pathogens.
Plants are known to play a major role in the treatment of microbial infections, as they contain important bioactive components that may act in synergism with conventional antibiotics, which are presently under threat of microbial resistance. Synergistic activity between dichloromethane, acetone, 70% ethanol, 50% methanol and hot water extracts of three southern African alien invasive plant species, namely Chromolaena odorata, Gomphrena celosioides and Tithonia rotundifolia and gentamicin, streptomycin, rifampicin and amphotericin B was studied using the checkerboard method. Antioxidant activity of the extracts was determined using the quantitative 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging and 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) decolourisation methods. The anti-inflammatory activity was determined with the 15-lipoxygenase enzyme assay. Synergistic to additive effects were observed among all tested extracts in combination with the conventional antibiotics against most of the tested strains. This was evident when the extracts were combined with gentamicin or amphotericin B against Klebsiella pneumoniae, Enterococcus faecalis, Salmonella Typhimurium, Cryptococcus neoformans and Candida albicans. The extracts of C. odorata had the best antioxidant activity in the ABTS assay with the 70% ethanol extract showing a minimum inhibitory concentration (IC50) of 1.960 μg/mL, better than that of vitamin C (IC50 = 2.027 μg/mL). The dichloromethane extract of T. rotundifolia displayed promising radical scavenging activity in the DPPH assay with an IC50 of 16.231 μg/mL. Most of the extracts of C. odorata (except the acetone extract) had anti-inflammatory activity better than that of the positive control quercetin (IC50 = 24.601 μg/mL). Results from this study indicate that extracts of these weedy plant species contain bioactive compounds that may act in synergy with conventional antibiotics in the fight against resistant microbial strains. Additionally they may reduce oxidative stress and inflammation.