Southern African plants used as soap substitutes; phytochemical, antimicrobial, toxicity and formulation potential

A major proportion of the population in southern Africa relies on medicinal plants, commonly known as soapy plants, for bathing and washing; however, there is limited scientific evidence that validates the effectiveness of southern African soap plants. Therefore, this study aimed to investigate the phytochemistry, antimicrobial activity, and toxicity of plants used in southern Africa as soap substitutes. Thereafter, an effective antimicrobial herbal soap was formulated and assessed for antimicrobial efficacy. Plant selection was based on a comprehensive ethnobotanical literature review that was conducted to gather information on plants used as soap substitutes in southern Africa. A total of 29 plant extracts (26 plant species) were screened for antimicrobial activity against 16 relevant skin pathogens. The minimum inhibition concentration (MIC) assay was used to assess antimicrobial activity. The plant extracts were quantitatively screened for the presence of saponins. Furthermore, the plant extracts were also assessed for toxicity using the brine shrimp lethality assay (BSLA). Calodendrum capense (L.f.) Thunb., Pelargonium peltatum (L.) L'Hér. and Ptaeroxylon obliquum (Thunb.) Radlk. demonstrated notable broad-spectrum activity against most of the tested pathogens. Hermannia cuneifolia Jacq. displayed the highest saponin content (262.41 ± 1.90 mg/g), followed by Sideroxylon inerme L. subsp. inerme (71.34 ± 1.01 mg/ml), Acalypha glabrata Thunb. (70.48 ± 2.05 mg/g), and Noltea africana (L.) Endl. (68.53 ± 2.43 mg/g). Acalypha glabrata, Aloe maculata All., Bauhinia bowkeri Harv., Deinbollia oblongifolia (E. Mey. ex Arn.) Radlk., Ledebouria luteola Jessop., Pouzolzia mixta Sohm, and S. inerme subsp. inerme organic and aqueous extracts demonstrated the lowest toxic effects at 24 and 48 h. Calodendrum capense and Ptaeroxylon obliquum organic extracts were non-toxic at 500 μg/ml, and Pelargonium peltatum displayed low toxicity at a concentration of 125 μg/ml. Calodendrum capense, Pelargonium peltatum and Ptaeroxylon obliquum were selected as the three most favourable plants for soap formulation. Pelargonium peltatum and Calodendrum capense herbal soaps were categorized as first-grade soaps (84 and 80% total fatty matter, respectively). All the tested soaps having plant concentrations of 12.5 mg/ml inhibited all the tested micro-organisms except for Enterobacter cloacae. The findings (phytochemical, antimicrobial, toxicity) herein contribute to the knowledge gaps that exist in the ethnobotanical literature of some southern African soap plants and provide evidence for their potential to be incorporated into soap formulation.