Synergy between Essential Oils of Calamintha Species (Lamiaceae) and Antibiotics

The subject of the study was the investigation of the chemical composition and antimicrobial activity of the essential oils (EOs) isolated from Calamintha sylvatica, C. vardarensis, C. nepeta and C. glandulosa, as well as their antibacterial activity in combination with antibiotics. The quantitative and qualitative analysis of EOs was performed using the GC/FID and GC/MS methods. The antimicrobial activity of EOs against six standard bacterial strains and one strain of yeast was tested using the broth microdilution method, while the antimicrobial activity of a combination of essential oils and gentamicin/ciprofloxacin was tested by the checkerboard method. The dominant components (> 10%) of the essential oils were: cis-piperitone epoxide and menthone (C. sylvatica), pulegone and menthone (C. vardarensis), pulegone and piperitenone (C. nepeta), pulegone, piperitenone, menthone and piperitone (C. glandulosa). EOs did not exhibit significant antimicrobial activity except the essential oil of C. vardarensis which was selectively active against Staphylococcus aureus (MIC - 21.25 mu g/mL). The overall effect of essential oil-antibiotic combinations varied from synergistic (FICI lt = 0.5) to antagonistic (FICI >= 2) depending on the bacterial strain tested