The antimicrobial investigation of 40 Southern African medicinal plants against some selected gut bacterial pathogens

Abstract

Intra–abdominal infections (IAI) are the infections of the abdomen and the major cause of morbidity and mortality especially in developing countries. In these countries especially in rural areas, most people rely on traditional medicine for their primary health care due to factors such as culture, availability and affordability. The popular use of Southern African medicinal plants has already been reviewed, however, the scientific evidence against neglected pathogens of the gut including Helicobacter pylori, Fusobacterium varium as well as Bacteroides, and Clostridium species has not been adequately studied. The aim of this study was thus to evaluate the antimicrobial activity of extracts from selected medicinal plants traditionally used to treat stomach ailments, against neglected gut pathogens responsible for IAI, to investigate activity against biofilms (using Clostridium perfringens as a model) and to determine toxicity of antimicrobially active plant extracts. To test the antimicrobial effects of plant extracts against neglected gut pathogens, a total of 102 aqueous and organic extracts were prepared from 40 different plant species. These plant samples were screened for antimicrobial efficacy against anaerobes (Bacteroides fragilis (ATCC 25285), Bacteroides ovatus (ATCC 8483) Bacteroides thetaiotaomicron (ATCC 29741), Bactericides vulgatus (29327), C. perfringens (ATCC 13124), C. difficile (ATCC 43593), Fusobacterium nucleatum (ATCC 25586) F. varium (ATCC 27725), and microaerophiles (H. pylori clinical and H. pylori (B8) (reference strain) of the gut using the minimum inhibitory concentration (MIC) assay. The results from MIC assay revealed that the organic extracts were more active when compared to the aqueous extracts. The organic extract of Lippia javanica leaf showed the best antimicrobial activity overall with an MIC value of 0.5 µg/ml against C. perfringens, this value was comparable to that of positive control ciprofloxacin (0.2 µg/ml). Aqueous extracts of Gunnera perpensa leaf and rhizome were the most active with MIC values of 130 µg/ml against the Clostridium species. Leaf extracts from seven out of ten plants showed better activity when compared to the other plant parts. Plant extracts that displayed noteworthy antimicrobial activity against C. perfringens were further evaluated for their antibiofilm activity using the crystal violet (CV) staining assay. Approximately 90% of the extracts were active on one or more stages of biofilm development. Plant extracts disrupted biofilm development stage (24 h) where 57% of the extracts reduced the development of biofilms by at least 50%. The organic extract of Salvia africana–caerulea displayed the best antibiofilm activity overall, where it prevented the cell attachment (4 h) with inhibition percentage of 83%. The toxicity profiles of plants that displayed noteworthy antimicrobial activity were further evaluated using the brine shrimp lethality (BSLA) assay to confirm some degree of safety. The BSLA results revealed that 82% of the tested plant extracts were non–toxic. Only 18% of the organic extracts exhibited toxic effects to the brine shrimp with >50% mortality. All the aqueous extracts tested possessed no toxic effects. The results obtained from this study favours the traditional use of some of the tested plants for the treatment of IAI. This study demonstrates that studies should not only focus on common pathogens, but also on neglected pathogens which may yield excellent results not previously reported. The study contributes to the knowledge of antimicrobial properties of plants commonly found in Southern Africa.