Background: Today, with raised use of antibiotics and prevalence of resistant strains, there is need for antimicrobial drugs that have fewer side effects than antibiotics. Rosmarinus officinalis is a medicinal plant which had many uses in traditional medicine. In this study, methanol leave extract of this plant is tested on various pathogens.

Materials and Methods: In this in vitro study, Rosmarinus officinalis was used to evaluate its antimicrobial effects. Methanol leave extract of this plant with concentrations of 400, 200, 100, and 50 mg/ml were prepared, and antibacterial activities were evaluated by well diffusion method on strains of Escherichia coli, Pseudomonas aeruginosa, Bacillus cereus and Staphylococcus aureus. Minimum inhibitory concentration and minimum bactericidal concentration were determined by the microplate method.

Results: In this study, the most efficacy of thanol extract of rosemary leaves was at concentration of 400 mg/ml against Pseudomonas aeruginosa and Escherichia coli. Minimum inhibitory concentration of the extract on the growth of these bacteria showed changes from 6.25 mg/ml to 100 mg/ml. Also MBC  of extract showed range  from 12.5 to 200 mg/ml respectively.

Conclusion: It was found that rosemary methanol extract inhibited growth of Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa but did not inhibit of Bacillus cereus growth.