Background: Due to increasing antibiotic resistance, it is important to identify the antimicrobial activity of herbs. This experimental study aimed to evaluate the antimicrobial activity of herbal extracts of common hop (Humulus lupulus) and oak (Quercus castaneifolia) against several microbial standard strains.

Materials and Methods: The alcoholic and aqueous extracts of H. lupulus and Q. castaneifolia were extracted. The inhibitory effects of herbal extracts were evaluated against the microbial standard strains (Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, and Candida albicans) by both agar diffusion and macrobroth dilution methods. In the agar diffusion method, concentrations of extracts were 125, 250, 500 and 1000 mg/ml. In order to determine the MIC, serial dilutions were prepared with a range from 1 to 512 mg/ml.

Results: Alcoholic extract of both hop and oak showed higher inhibitory effect against microbial standard strains, compared to the aqueous extract. Staphylococcus aureus ATCC 29213 and Candida albicans ATCC 76615 showed higher susceptibility to both alcoholic and aqueous extracts compared to Escherichia coli ATCC 25922 and Pseudomonas aeruginosa ATCC 27853. Statistically significant difference was found on the MIC of alcoholic and aqueous extracts (p<0.05).

Conclusion: This study revealed the considerable inhibitory effects of herbal hop and oak extracts on the various microorganisms, including bacteria and fungi. Although more research is needed in this field, they can be used as new antimicrobials in medicine.