Pharmacognostic standardisation of the leaves of Costus afer Ker Gawl. (Zingiberaceae) and Palisota hirsuta (Thunb.) K. Schum. (Commelinaceae)

Abstract Background Microbes’ resistance to orthodox drugs and continued emergence of new infections call for search of new drugs that can mitigate such resistance and cure newly emerging infections. Despite the therapeutic importance of Costus afer and Palisota hirsuta, the detailed pharmacognostic characters of the two plants are still missing in the literature. Thus, this study evaluated the pharmacognostic characters of these plants and the antimicrobial activity of the plants’ methanol extract. Macroscopic, microscopic, fluorescence analysis, phytochemical screening, heavy metal analysis, proximate, antimicrobial assay and Gas Chromatography–Mass Spectrometry analyses were carried out on both plants. Results The microscopic evaluation showed that the epidermal layer of Costus afer is hypostomatic with polygonal shaped epidermal cells and cyclotic stomata on abaxial layer while the epidermal layer of Palisota hirsuta leaf is amphistomatic, having polygonal shape epidermal cell, trichomes and paracytic stomata. The proximate analysis showed that both plants have a high carbohydrate content (C. afer: 59.13% and P. hirsuta: 68.28%) but a low crude fat content (C. afer: 1.867% and P. hirsuta: 0.233%). The heavy metal analysis revealed a high content of Manganese (266.80 mg/100 g) in Palisota hirsua, and a high content of iron (43.94 mg/100 g) in Costus afer. Both plants are free of lead. The phytochemical screening showed that both plants have saponins, tannins, flavonoids, alkaloids and cardiac glycosides. Alkaloids are the highest constituents in Costus afer (9.1%) and Palisota hirsuta (7.1%). The antimicrobial assay showed both plant extracts had the highest inhibitory effect on both bacteria and fungi at 100 mg/kg. Six compounds and 23 compounds were identified by GC–MS in C. afer and P. hirsuta, respectively. Methyl stearate (40.43%) and hexadecanoic acid, methyl ester (25.62%) were the prevailing constituents identified in C. afer and P. hirsuta, respectively. Conclusion The pharmacognostic features of Costus afer and Palisota hirsuta shown in this study can be used to design a suitable monograph which would be useful in compilation of another edition of Nigeria Herbal Pharmacopoeial. The dominant compounds in both plants can serve as chemotaxonomic markers for correct identification of the two plants.