Medicinal plants can be a good alternative to antibiotics by producing antimicrobial compounds. The aim of this study was to investigate the antimicrobial effects of Robinia pseudoacacia L. and Phytolacca americana L. ethanolic leaf extract on several nosocomial pathogenic strains. Plant leaves were collected from the forests of Amol city in the spring of 2016. The extract was extracted by Soxhlet extractor method and then their antibacterial activity at concentrations of 6.25, 12.5, 25, 50, 100 and 200 mg/ml on Proteus mirabilis, Proteus vulgaris, Streptococcus sanguinis, and Staphylococcus aureus were studied by disk diffusion and well dilution methods. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were also determined. The highest inhibition zone diameter in the both disk diffusion and well dilution methods was found at concentrations of 200 mg/ml of each plant extract. In the disk diffusion method, R. pseudoacacia extract had highest inhibitory effect on P. mirabilis and P. vulgaris with a growth inhibition zone diameter of 24.67 mm, while P. Americana extract had the highest effect on S. aureus with 27.67 mm. In the well dilution method, R. pseudoacacia extract had the highest inhibition zone diameter on P. mirabilis (24.67 mm), whereas P. Americana showed highest inhibition zone diameter on S. aureus (24.3 mm). The MIC and MBC values of R. pseudoacacia extract was determined at 25 and 50 mg/ml concentrations for P. vulgaris, S. aureus and S. sanguis, and 50 and 100 mg/ml for P. mirabilis. The MIC and MBC values of P. Americana extract was 25 and 50 mg/ml for P. mirabilis, S. aureus and S. sanguis, and 50 and 100 mg/ml for P. vulgaris, respectively. Therefore, based on the results R. pseudoacacia and P. Americana ethanolic extracts had acceptable antimicrobial effect, especially at 200 mg/ml, against some important bacterial pathogens.
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