Phytochemical Screening, Antibacterial Efficacy and Acute Oral Toxicity Evaluation of Methanolic and Aqueous Vernonia amygdalina Leaf Extracts against Selected Bacterial Isolates

Abstract

Vernonia amygdalina Delile is widely used in African traditional medicine; however, comparative evidence regarding the antibacterial efficacy and safety of different extraction solvents remains limited. This study evaluated the phytochemical composition, antibacterial activity and acute oral toxicity of methanolic and aqueous leaf extracts of V. amygdalina against selected clinically important bacterial pathogens. Fresh leaves of V. amygdalina were extracted separately with 80% methanol and distilled water. Qualitative phytochemical screening was performed using standard methods. Antibacterial activity against Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa and Salmonella typhi was determined by agar disc diffusion, while minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined using broth microdilution. Acute oral toxicity was evaluated in Wistar rats administered graded doses (500–8000 mg/kg) and monitored for behavioural changes and mortality over 14 days. Both extracts contained alkaloids, flavonoids, tannins, saponins, terpenoids, cardiac glycosides and reducing sugars. The methanolic extract consistently produced significantly larger inhibition zones than the aqueous extract (P = 0.012), with the highest activity observed against S. aureus (28.00 ± 1.00 mm) at 200 mg/mL. MIC values for the methanolic extract ranged from 1.56–3.12 mg/mL compared with 3.12–6.25 mg/mL for the aqueous extract (P = 0.016). MBC values ranged from 50–100 mg/mL and 50–200 mg/mL for the methanolic and aqueous extracts, respectively, without significant differences (P > 0.05). No mortality occurred in any treatment group, indicating an oral LD₅₀ greater than 8000 mg/kg. However, transient restlessness and respiratory distress were observed only in animals receiving 8000 mg/kg. Vernonia amygdalina leaf extracts possess broad-spectrum antibacterial activity and a favourable acute safety profile. The methanolic extract demonstrated superior antibacterial efficacy, suggesting that methanol more effectively extracts bioactive antimicrobial constituents.