Study: Essential oils that attack biofilms in vitro

A 2012 study [1. Kavanaugh NL and Ribbeck K, 2012, Selected Antimicrobial Essential Oils Eradicate Pseudomonas spp. and Staphylococcus aureus Biofilms, Nicole L. Kavanaugh and Katharina Ribbeck, Appl Environ Microbiol. 2012 Jun; 78(11): 4057–4061.] has offered some insights into the impact of essential oils on biofilm formation, in particular regarding the biofilm of bacteria Pseudomonas spp. and Staphylococcus aureus.

The researchers set out to show that some essential oils could be more effective than antibiotics at eradicating bacteria within biofilms.

There is evidence that essential oils from plants have antimicrobial activity that damages the cell wall and membrane, which causes the cell to leak its contents and cause cell death. There is also evidence that essential oils do not cause resistance, unlike antibiotics. Essential oils are easy to get hold of, tend to have low toxicity to animals and humans, degrade quickly in water, and so are reasonably environmentally friendly.

The study into biofilm-attacking essential oils

Biofilms studied:

• Pseudomonas aeruginosa (PAO1)
• Pseudomonas putida (KT2440)
• Staphylococcus aureus SC-01

Essential oils studied:

• Cassia
• Clove (Syzygium aromaticum)
• Peru balsam (Myroxylon balsamum)
• Red thyme (Thymus vulgaris)
• Tea tree (Melaleuca alternifolia)

HOW THE ESSENTIAL OILS STACKED UP WITH P. PUTIDA AND P. AERUGINOSA

• Cassia oil at 0.2 per cent killed the vast majority of P. aeruginosa cells in a biofilm, but a slightly lower percentage (0.1 per cent) did not kill bacteria in solution or in biofilms.
• Cassia oil kills planktonic bacteria and biofilms comparably efficiently.
• Clove, Peru balsam, red thyme and tea tree were all effective at killing Pseudomonas.
• Just one antibiotic, ofloxacin, could eradicate planktonic and biofilm bacteria with almost equal efficiency.
• Antibiotics colistin and gentamicin were not effective at killing biofilms, even at high concentrations.
• Cassia and Peru balsam were effective against biofilms and planktonic bacteria at almost equal concentrations.
• Red thyme oil was effective against biofilms at less than 2 per cent strength, but was unable to kill planktonic bacteria at any concentration.
• P. putida bacteria is more sensitive than P. aeruginosa to clove, red thyme, and tea tree oils, but in particular clove oil, which was potent against P. putida (both forms) but not P. aeruginosa. 
• The effect of clove oil on P. putida is significantly different to that of P. aeruginosa.
• Red thyme oil was effective against planktonic bacteria at 5 per cent, the highest tested concentration.
• Tea tree is effective against biofilm cells.
• P. aeruginosa survived at the highest concentrations tested of tea tree.
• Clove oil was not effective against P. aeruginosa biofilms at 5 per cent solutions, eugenol was effective at 3.3 per cent.

ESSENTIAL OILS TESTED AGAINST ANTIBIOTIC-RESISTANT S. AUREUS (MRSA)

• Tea tree, thyme and peppermint are effective against planktonic and biofilm MRSA.
• Cassia, red thyme, and clove have not been tested against MRSA biofilms of any strain so far, and the strain used in this study has not previously been studied with essential oils.
• All the oils tested, including lavender oil, show a zone of inhibition.
• The biofilms were killed by the same or similar concentrations of cassia, Peru balsam, and red thyme as killed P. aeruginosa.

ACTIVE COMPONENTS OF THE ESSENTIAL OILS TESTED AGAINST S. AUREUS (MRSA)

• Cinnamaldehyde, eugenol and linalool (from cassia, clove, and lavender oils respectively) were tested.
• Cinnamaldehyde is as effective as complex cassia oil.
• Cassia, Peru balsam and red thyme essential oils are more effective at eradicating Pseudomonas and S. aureus (MRSA) biofilms than selected antibiotics.