In vitro Antimicrobial Activity of Ethanolic and Methanolic Leaf Extracts of Ocimum gratissimum against Multidrug-Resistant Escherichia coli Clinical Isolates

Abstract

Background: The rapid emergence and spread of multidrug-resistant (MDR) Escherichia coli isolates have become a major global public health concern, limiting the effectiveness of conventional antibiotics and increasing treatment failure rates. Consequently, medicinal plants such as Ocimum gratissimum are being explored as potential alternative antimicrobial agents due to their reported antimicrobial properties. Objective: This study investigated the in vitro antimicrobial activity of ethanolic and methanolic leaf extracts of O. gratissimum against clinical multidrug-resistant E. coli isolates recovered from patients in Ado-Ekiti, Nigeria. Methods: Fifty clinical isolates of E. coli were re-characterized and confirmed using standard microbiological and biochemical identification techniques. Antimicrobial susceptibility testing was performed according to Clinical and Laboratory Standards Institute guidelines using the Kirby–Bauer disk diffusion method. Leaf extracts of O. gratissimum were prepared using ethanol and methanol by cold maceration extraction. Antibacterial activity was evaluated using the agar well diffusion technique, while minimum inhibitory concentrations (MICs) were determined by the broth dilution method. Data were analyzed using Student’s t-test at p < 0.05. Results: Among the 50 isolates, resistance was highest to ampicillin (44%) and lowest to ofloxacin (20%). Ten MDR isolates resistant to at least three antibiotic classes were selected for extract susceptibility testing. Ethanolic leaf extracts demonstrated significantly greater antibacterial activity, producing inhibition zones ranging from 7–20 mm, compared with methanolic extracts, which produced inhibition zones of 7–12 mm (p < 0.05; Cohen’s d = 0.85). Similarly, ethanolic extracts exhibited significantly lower MIC values (6.25–25 mg/mL) than methanolic extracts (12.5–50 mg/mL) (p < 0.05; Cohen’s d = 0.92), indicating superior antimicrobial potency against MDR E. coli isolates. Conclusion: The findings of this study demonstrate notable in vitro antibacterial activity of Ocimum gratissimum against multidrug-resistant E. coli isolates, with ethanolic extracts showing superior efficacy compared with methanolic extracts. The strong inhibitory activity observed suggests potential for further development as a source of antimicrobial compounds against resistant bacteria.