Evaluation of the antibacterial activity of selected Kenyan medicinal plant extract combinations against clinically important bacteria

BACKGROUND: Infectious diseases are a major global public health concern as antimicrobial resistance (AMR) currently accounts for more than 700,000 deaths per year worldwide. The emergence and spread of resistant bacterial pathogens remain a key challenge in antibacterial chemotherapy. This study aims to investigate the antibacterial activity of combined extracts of various Kenyan medicinal plants against selected microorganisms of medical significance. METHODS: The antibacterial activity of various extract combinations of Aloe secundiflora, Toddalia asiatica, Senna didymobotrya and Camelia sinensis against Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae and Methicillin Resistant Staphylococcus aureus was assessed using the agar well diffusion and the minimum inhibitory concentration in-vitro assays. The checkerboard method was used to evaluate the interactions between the various extract combinations. ANOVA test followed by Tukey’s post hoc multiple comparison test was used to determine statistically significant differences in activity (P < 0.05). RESULTS: At concentrations of 100 mg/ml (10,000 µg/well), the different combinations of the aqueous, methanol, dichloromethane and petroleum ether extracts of the selected Kenyan medicinal plants revealed diverse activity against all the test bacteria. The combination of methanolic C. sinensis and A. secundiflora was the most active against E. coli (14.17 ± 0.22 mm, diameter of zones of inhibition (DZI); MIC 2500 µg/well). The combination of methanolic C. sinensis and S. didymobotrya was the most active against S. aureus (16.43 ± 0.10 mm; MIC 1250 µg/well), K. pneumonia (14.93 ± 0.35 mm, DZI; MIC 1250 µg/well), P. aeruginosa (17.22 ± 0.41 mm, DZI; MIC 156.25 µg/well) and MRSA (19.91 ± 0.31 mm, DZI; MIC 1250 µg/well). The Minimum Inhibitory Concentration of the different plant extract combinations ranged from 10,000 µg/ well to 156.25 µg/well. The ANOVA test indicated statistically significant differences (P < 0.05) between single extracts and their combinations. The fractional inhibitory concentration indices (FICI) showed that the interactions were either synergistic (10.5%), additive (31.6%), indifferent (52.6%), or antagonistic (5.3%) for the selected combinations. CONCLUSION: This study findings validate the ethnopractice of selectively combining medicinal plants in the management of some bacterial infections in traditional medicine.