Antibacterial effect of essential oils of peppermint and pennyroyal on major bovine mastitis bacteria

Antibiotic therapy of bacterial diseases has some side effects including antibiotic resistance. There is need to investigate new and natural antibacterial agents because of various side effects of antibiotics. According to antibacterial effects, essential oils (EOs) are suitable alternatives. Escherichia coli, Staphylococcus aureus and Streptococcus agalactiae cause various diseases in human and animals. Also, these three bacteria are major bovine mastitis-causing bacteria. So, this study investigated antibacterial activity of Mentha piperita (peppermint) and Mentha pulegium (pennyroyal) essential oils in comparison with lincospectin, gentamicin and amoxicillin/clavulanate antibiotics on these bacteria. Gas chromatography/mass spectrometry was used for the analysis of EOs. Antibacterial effects of the EOs on bacteria were evaluated with minimum bactericidal concentration (MBC), minimum inhibitory concentration (MIC), disk diffusion method, and time- kill assay in tryptic soy broth (TSB). Major components of peppermint EO were carvone (63.02%) and limonene (24.48%), and those of pennyroyal EO were pulegone (48.16%), eucalyptol (14.57%). According to MIC and MBC results, the antibacterial activity of peppermint was the same or higher than lincospectin and pennyroyal effect on Streptococcus agalactiae was higher than lincospectin. The inhibition zone of peppermint and pennyroyal had no significant difference with gentamycin but the effects of two EOs against the other two bacteria were lower than gentamycin and amoxicillin/clavulanate. Sub-MIC concentrations of oils decreased Escherichia coli count at 24 h. Generally, the essential oil of peppermint and pennyroyal had a significant antibacterial effect and the effect of peppermint was higher than pennyroyal.