Tea tree oil might be the most essential of the essential oils. Native to Australia, the oil of the tea tree (Melaleuca alternifolia) was issued to infantry men in that country in the early 1900s to treat infections. And they were on to something. Scientific studies have shown when used in combination with other oils on wound dressings, tea tree oil can inhibit the growth of bacteria and fungus like Staphylococcus aureus and Candida albicans. And, when combined with geranium oil, tea tree oil inhibited the growth of antibiotic-resistant strains of bacteria.