Essential oils are most commonly associated with aromatherapy and other alternative "medicines", but some researchers are investigating whether they actually have any potential medical benefits. Now a team at Johns Hopkins Bloomberg School of Public Health has found that some essential oils may have antibiotic properties, particularly against a persisting form of the bacteria that causes Lyme disease.

Borrelia burgdorferi is a tick-borne bacterium that causes Lyme disease when spread to humans. A round of antibiotics usually clears the infection right up, but in some cases the bacteria are known to stick around in a "stationary phase," where the cells either stop dividing or do so slowly. Unfortunately, that also makes them more resistant to antibiotics, so they can be harder to get rid of and may cause longer-lasting symptoms.

In the new study, the Johns Hopkins team treated lab dishes of stationary phase B. burgdorferi with 35 essential oils to find out if any had antibiotic properties. Of those, 10 types of essential oils were effective against the bacteria at concentrations of one part per thousand. Among them, oil from thyme leaves, cumin seeds, amyris wood and cinnamon bark performed well.

But the most effective oils were those from garlic bulbs, allspice berries, myrrh trees, spiked ginger lily blossoms and may chang fruit. Those oils managed to wipe out all of the stationary phase Lyme bacteria within seven days, and no bacteria grew back in the three weeks after that.

"We found that these essential oils were even better at killing the 'persister' forms of Lyme bacteria than standard Lyme antibiotics," says Ying Zhang, senior author of the study.

Of course, don't take those findings as an indication to start huffing essential oils out of your diffuser. They may smell nice, but essential oils can be harmful, irritating the skin and proving toxic if ingested in large amounts. There's even evidence that lavender and tea tree oil contain chemicals that potentially disrupt hormones.

The researchers on the current study say that in the future tests will need to be conducted on mouse models of persistent Lyme infection, to determine if effective doses are safe and how they might be administered.

The research was published in the journal Antibiotics.