by Christine Carson, PhD

Australian postage stamp - TeaTreeAn experienced group of US-based researchers (Cuaron et al. 2014) have recently published a paper with the title “The Isolation of Staphylococcus aureus Tea Tree Oil-Reduced Susceptibility Mutants” in the journal Phytotherapy Research (impact factor 2.068). The abstract is available here. As the title says, they have managed to isolate bacteria, in this case Staphylococcus aureus, that have a reduced susceptibility to tea tree oil. So does this mean that tea tree oil is losing its ability to inhibit and kill bacteria?

There are several methods that you can use in the laboratory to try and make bacteria resistant to antimicrobial (bug-killing) compounds. These methods are used in research to find out if bacteria are likely to develop resistance to a test substance. Several independent research groups have used some of these methods to try and make bacteria resistant to tea tree oil. Whichever method they choose, the aim is to see if they can make bacteria grow in the presence of a tea tree oil concentration that previously inhibited or killed them.

The lowest concentration of oil that inhibits the growth of a bacterium is called the Minimum Inhibitory Concentration (MIC) while the lowest concentration of oil that kills a bacterium is called the Minimum Bactericidal Concentration (MBC). If you can increase the MIC and/or the MBC of a compound against a particular bacterium, then you could say you’ve made your bacteria resistant to that compound. It’s a bit more complicated than that depending on whether your antimicrobial compound is used topically or systemically, but that’s the gist of it. Tea tree oil is only recommended for topical use, so any increase in MICs or MBCs might be cause for concern.

Petri dishThe researchers used a couple of well-known methods to try and make their bacteria resistant to tea tree oil. Note, their tea tree oil complied with the international standard for the oil (ISO 4730). In one of the methods they used two different strains of S. aureus. One strain was methicillin-susceptible (MSSA) while the other was methicillin-resistant (MRSA). They managed to increase the MICs from 0.27% to 0.38% in the methicillin-susceptible strain and from 0.17% to 0.29% in the methicillin-resistant strain. When they took these less susceptible isolates and grew them again without any tea tree oil present, upon subsequent MIC testing, the MICs reverted back to their original values. So the change in susceptibility wasn’t permanent. It was just transient.

There are a few major points to come out of these results. Firstly, the increases in MIC are pretty minor in comparison to what happens when antibiotics are tested in this way. Usually, antibiotic MICs at least quadruple or more! These MICs haven’t even doubled. Secondly, when antibiotic MICs increase in these types of tests, they are often permanent. Even if you grow the bacteria afterwards in the absence of the antibiotic, the increased MICs often remain elevated. Finally, changes in the MIC of the magnitude seen here and in other studies are unlikely to compromise the ability of tea tree oil to kill bacteria in practice. Most products formulated to exploit the antimicrobial activity of tea tree oil contain sufficient tea tree oil to render such small changes in the MIC unimportant. That is, even if the MICs have gone up a bit, there is still enough tea tree oil there to inhibit or kill the bacteria.

As mentioned above, there have been several independent attempts to make bacteria resistant to tea tree oil. None has managed to increase the MICs much more than 2-fold and in no case, has any change been permanent. So, although the researchers did manage to isolate bacteria that have reduced susceptibility to tea tree oil, tea tree oil is not losing its bug-killing punch.

Christine Carson
Christine has a PhD in Microbiology and has been investigating the biological effects of natural products, including Melaleuca alternifolia (tea tree) essential oil, for over 20 years. She has published over 50 papers in the field and is interested in the antimicrobial, anti-inflammatory and other potentially useful medicinal properties of natural products, particularly plant-derived compounds, as well as the safety and toxicity associated with their use. More recently, she has joined a group working to address the growing problem of antimicrobial resistance. She is a Research Associate at the University of Western Australia and a Research Scholar at Edith Cowan University.