Background: Several medicinal plants have been used by humans for their vast diversity in chemical compound content and antimicrobial activity. However, their appropriate use remains under investigation by many researchers. Objectives: This study aimed to determine the stage at which Escherichia coli O157 growth is more sensitive to certain concentrations of certain medicinal plants. Materials and Methods: Alcoholic extracts of the aerial parts and seeds of Peganum harmala and Teucrium polium were prepared using standard methods. The effective concentration (0.15-0.4 mg/ml) was chosen, based on their zones of inhibition on plates that were inoculated with E. coli O157 and plant extract and used to monitor bacterial growth by spectrophotometry. Results: All concentrations of the aerial parts of T. polium extract and the seeds of P. harmala decreased the absorbance in the first several hours after inoculation, after which bacterial growth increased to normal levels. Compared with control, no significant differences on the effect of the aerial parts of P. harmala extract on bacterial growth were observed. Conclusions: Based on the comparison of growth curves, higher concentrations effected greater inhibition. However, the effects of other concentrations of the plant extracts on bacterial growth should be investigated further, although their toxicity must be assessed before use.

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