The antimicrobial activity of four commercial essential oils in combination with conventional antimicrobials

Lett Appl Microbiol. 2009 Apr;48(4):440-6. doi: 10.1111/j.1472-765X.2008.02548.x. Epub 2009 Feb 2.


Aims: Due to the emergence of multi-drug resistance, alternatives to conventional antimicrobial therapy are needed. This study aims to investigate the in vitro pharmacological interactions between essential oils (considered valuable as natural therapeutic treatments) and conventional antimicrobials (ciprofloxacin/amphotericin B) when used in combination.

Methods and results: Interactions of the essential oils (Melaleuca alternifolia, Thymus vulgaris, Mentha piperita and Rosmarinus officinalis) when combined with ciprofloxacin against Staphylococcus aureus indicate mainly antagonistic profiles. When tested against Klebsiella pneumoniae the isobolograms show antagonistic, synergistic and additive interactions depending on the combined ratio. The R. officinalis/ciprofloxacin combination against K. pneumoniae displayed the most favourable synergistic pattern. The interactions of M. alternifolia (tea tree), T. vulgaris (thyme), M. piperita (peppermint) and R. officinalis (rosemary) essential oils with amphotericin B indicate mainly antagonistic profiles when tested against Candida albicans.

Conclusion: While a number of interactions show complete antagonism, others show varied (synergistic, additive and/or antagonistic) interactions, thus the efficacy is dependent on the ratio in which the two components co-exist.

Significance and impact of the study: The predominant antagonistic interactions noted here, suggests that some natural therapies containing essential oils should be used with caution when combined with antibiotics.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amphotericin B / pharmacology
  • Anti-Bacterial Agents / pharmacology*
  • Candida albicans / drug effects
  • Ciprofloxacin / pharmacology
  • Drug Antagonism
  • Drug Interactions
  • Drug Synergism
  • Drug Therapy, Combination
  • Humans
  • Industrial Oils
  • Klebsiella pneumoniae / drug effects
  • Melaleuca / chemistry
  • Mentha piperita / chemistry
  • Microbial Sensitivity Tests
  • Oils, Volatile / pharmacology*
  • Rosmarinus / chemistry
  • Staphylococcus aureus / drug effects
  • Thymus Plant / chemistry


  • Anti-Bacterial Agents
  • Oils, Volatile
  • Ciprofloxacin
  • Amphotericin B
  • rosemary oil