Antibacterial Efficacy of Selected Medicinal Plants Extract against Multidrug-resistant Bacteria Isolated from Crude Oil-contaminated Soils in Obi-Ayagha, Niger Delta, Nigeria

Abstract

Environmental reservoirs of antimicrobial resistance contribute significantly to the global burden of multidrug-resistant (MDR) pathogens, particularly in petroleum-contaminated ecosystems that exert strong selective pressure on microbial populations. This study investigated the phytochemical composition and antibacterial activities of the plants Senna alata, Ocimum gratissimum, and Bryophyllum pinnatum against MDR bacteria isolated from crude oil-contaminated soils in Obi-Ayagha, Niger Delta, Nigeria. Seventeen bacterial isolates, including Proteus spp., Pseudomonas spp., Acinetobacter spp., Salmonella spp., Shigella spp., Klebsiella spp., and Staphylococcus aureus, were identified using conventional and molecular methods. The plant leaves were extracted with ethanol and methanol, followed by sequential fractionation using aqueous, ethyl acetate, and butanol solvents. Phytochemical screening revealed flavonoids and steroids in all extracts, while tannins were abundant in O. gratissimum and B. pinnatum. Alkaloids were detected only in S. alata, whereas cardiac glycosides were absent in all three plant extracts. Antibacterial activity was concentration-dependent, with S. alata exhibiting the broadest spectrum by inhibiting 17 of 17 isolates, followed by O. gratissimum (13 isolates) and B. pinnatum (9 isolates). Maximum inhibition zone diameters at 100 mg/mL were 20.67, 17.67, and 14.33 mm, respectively. Minimum inhibitory concentrations ranged from 6.25–100 mg/mL, while minimum bactericidal concentrations ranged from 25–100 mg/mL. The findings demonstrate that these medicinal plants possess appreciable antibacterial activity against MDR environmental bacteria, likely due to the synergistic effects of bioactive phytochemicals. These results support their potential as promising sources of novel antimicrobial agents and warrant further phytochemical characterisation and pharmacological evaluation.