Investigating the potential of Gunnera perpensa for the treatment of gonorrhoea

Abstract

Gonorrhoea, caused by Neisseria gonorrhoeae, is one of the most problematic sexually transmitted diseases (STDs) in the world as it negatively affects people’s health and livelihoods. Antibiotic resistance of N. gonorrhoeae puts pressure on available therapeutics; thus there is a need to find alternative sources of bioactive compounds. In this study, antigonococcal screening was conducted on ethanolic extracts from roots of five South African indigenous plants, namely: Gnidia kraussiana Meisn, Gunnera perpensa L, Pentanisia prunelloides (Klotzsch) Walp, Rhoicissus digitata (L.f.) Gilg & M.Brandt and Rhoicissus tridentata (L.f.) Wild & R.B.Drumm, which exhibited minimum inhibitory concentrations (MICs) of 97.5 μg/ml, 48.7 μg/ml and 195 μg/ml, respectively while the Rhoicissus spp. had MICs of 780 μg/ml. Gunnera perpensa L. (GP) was selected as the lead plant as it had the best activity against N. gonorrhoeae. Furthermore, the plant was not found to be cytotoxic to human keratinocytes (HaCaT), cervical cancer cells (HeLa) and human monocytes (THP-1) (IC50 >400 μg/ml). Gunnera perpensa was subjected to liquid-liquid partition chromatography which resulted in four semi-pure fractions. Butan-1-ol and water fractions showed the best antigonococcal activity (MIC=23.4 μg/ml) and showed no toxicity against the cell lines tested (IC50 >300 μg/ml). Five compounds were identified to be present in both bioactive fractions, namely: Z-venusol, ferulic acid glucoside, 4-O-β-D-glucopyranosyl-3,3’-tri-O-methylellagic acid, caffeic acid and 4-O-beta-D-glucosyl-trans-caffeate. While three compounds were classified as 1.4-benzoquinones. Z-venusol was identified as the most abundant compound present in both bioactive fractions. Gold nanoparticles (AuNPs) were synthesized using the ethanolic crude extract of G. perpensa. Characterization of the GP-AuNPs was conducted using Zetasizer, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM) and ultraviolet-visible spectrophotometry (UV-Vis). The nanoparticles had surface plasmon resonance (SPR) at 536 nm and a hydrodynamic size of 127.2±1.56 nm. At 24 h, 72 h and 1 week, the GP-AuNPs were found to be stable when subjected to various treatments (pH 4, pH 7, pH 10, 0.5 % cysteine, 0.5 % sodium chloride, 0.5 % phosphate-buffered saline (PBS), 0.5 % Bovine serum albumin (BSA), Dulbecco's Modified Eagle Medium (DMEM) and deionized water (control). The GP-AuNPs exhibited potent antigonococcal activity (MIC=10.4 μg/ml), which was found to be better than the crude extract (MIC=48.7 μg/ml). However, the nanoparticles were found to be ii cytotoxic to HaCaT, HeLa and THP-1 cells with IC50 values of 22.12±0.52 μg/ml, 41.98±10.65 μg/ml and 27.53±6.02 μg/ml, respectively. This study revealed that G. perpensa should be considered in future, for pre-clinical and clinical studies for investigating its potential anti-gonococcal activity due to the good antibacterial effects and moderate toxicity observed in the present study. There is a possibility that individual compounds in the plant may exhibit better bioactivity. The nanoparticles of the plants showed superior antigonococcal activity however, the cytotoxicity effects are of concern. In future, avenues must be explored to optimize the NPs to reduce toxicity.