Antimicrobial activity of <i>Ziziphus mauritiana</i> and <i>Acacia nilotica</i> against multidrug-resistant <i>Escherichia coli</i> and <i>Klebsiella pneumoniae</i>

Multidrug-resistant (MDR) strains such as Escherichia coli and Klebsiella pneumoniae, resistant to broad-spectrum antibiotics, are a modern global issue. Plant extracts offer promising antimicrobial activity for such pathogens. This study evaluated the efficacy of aqueous and hydro-ethanolic extracts from leaves and bark of two cheaply and easily available plants, i.e., Ziziphus mauritiana (ber) and Acacia nilotica (gum Arabic or Kikar), against MDR E. coli and K. pneumoniae. Antibiotic susceptibility testing confirmed resistance in both strains to all tested antibiotics (ampicillin, ciprofloxacin, streptomycin, and tetracycline), with inhibition zones ≤ 10 mm. Antimicrobial assays revealed that hydro-ethanolic extracts of Z. mauritiana leaves exhibited the highest activity, with inhibition zones of 20 mm (E. coli) and 19 mm (K. pneumoniae) at 100 mg/mL, outperforming aqueous extracts. The minimum inhibitory concentration (MIC) for all the tested extracts was notably low (6 mg/mL for E. coli and K. pneumoniae). Growth curve analysis demonstrated significant suppression of bacterial proliferation by all extracts at MIC, with optical density (OD600) values remaining below 0.25 over 24 h compared to controls (OD600 ≥ 2.20). Additionally, biofilm formation was completely inhibited at MIC as well as sub-inhibitory concentrations (sub-MIC) of ½ MIC, highlighting that the antibiofilm activity of these extracts is not due to antimicrobial activity. Findings highlight the efficacy of extracts from different parts and solvents against MDR pathogens, suggesting their potential as an adjunct therapy. Findings suggest that hydro-ethanolic extracts perform significantly better than aqueous extracts, while no significant difference in antimicrobial activity was found in extracts from different parts of plants. Lastly, it was found that Z. mauritiana extracts have greater antimicrobial potential than A. nilotica extracts.