Antimicrobial activity, brine shrimp cytotoxicity and phytochemical screening of Piliostigma thonningii (Schumach.) Milne-Redh aqueous and methanol stem bark extracts

Background: The stem bark of Piliostigma thonningii is acclaimed for managing various types of diseases by the Kikuyu community of Gaturi, Murang’a County. However, there is limited data on the antimicrobial efficacy and safety of its stem bark extracts. Aim and objectives: The study set out to examine the antimicrobial activities, qualitative phytochemical screening, and safety of Piliostigma thonningii stem bark extracts in brine shrimp (Artemia salina) both in aqueous and methanolic forms. Materials and Methods: Standard phytochemical screening techniques were used to study the phytochemicals present in both extracts. Antimicrobial susceptibility of Bacillus cereus, Escherichia coli, Candida albicans, Pseudomonas aeruginosa, Staphylococcus aureus and Klebsiella pneumoniae to the test extracts was assessed by Agar well diffusion method. Fluconazole and gentamicin were used as standard drugs. The cytotoxic profiles of the extracts were examined using the brine shrimp cytotoxic model, in which vincristine sulphate served as the standard cytotoxic drug. Results: According to phytochemical screening, all the extracts were found to possess saponins, glycosides, steroids, alkaloids, terpenoids, phenols, tannins, and anthraquinones. However, tannins were not found in aqueous extract while the methanol extract was devoid of anthraquinones. When evaluated against Bacillus cereus, the antibacterial effects of both extracts were notably lower (p < 0.0001) than gentamicin across various concentrations. When evaluated against Staphylococcus aureus, the aqueous extract exhibited significantly lower antibacterial effects (p = 0.0100) compared to gentamicin at 62.5 mg/mL, while the methanol extract exhibited no significant difference (p = 0.1551) relative to gentamicin at the same concentration. Neither extract showed antimicrobial effects against P. aeruginosa, E. coli and K. pneumoniae. Fluconazole exhibited superior activity compared to the methanol extract at 0.125 mg per ml (p = 0.0489) and 1.0 mg per ml (p = 0.0207). In the case of both extracts against test organisms B. cereus, S. aureus and C. albicans there was no difference in MIC and MBC values apart for methanol extract against B. cereus whose MIC (15.6mg/ml) and MBC (31.2mg/ml) were markedly lower than that for aqueous extract. It was not possible to identify the crude extracts’ minimum inhibitory concentration (MIC) or minimum bactericidal concentration (MBC) against bacteria that were gram negative namely; K. pneumoniae, E. coli, and P. aeruginosa at the tested concentrations, which suggests that the values may be higher than 500 mg/ml or completely ineffective. The standard drugs (gentamycin and fluconazole) were more effective against the tested organisms than both extracts. Gentamicin was more active against gram negative bacteria than gram positive bacteria as was portrayed by the lower MIC and MBC values. The LC50 in µg/ml of vincristine sulphate was 15.70, for methanol extract was 7425.46 and for aqueous extract was 1765.20. Conclusions: These findings suggest that against these organisms; Staphylococcus aureus, Candida albicans and Bacillus cereus, the antimicrobial effects of P. thonningii stem bark extracts are less than those of standard drugs. In addition, the results suggest that extracts of P. thonningii stem bark are not cytotoxic to brine shrimp larvae.