Australian Tea Tree (Melaleuca alternifolia) oil: an updated review of antimicrobial and other medicinal properties

The last comprehensive review of the effects of tea tree oil (steam distilled essential oil of Melaleuca alternifolia ; henceforth TTO) was conducted in 2006 by Carson and colleagues (Carson et al., 2006). Here, we provide an updated review focused on the antimicrobial (including antibacterial, antiviral, antifungal and anti-parasitic) and anti-inflammatory effects of TTO, as well as a discussion on novel TTO formulations for these specific actions. A systematic review of studies was conducted using PubMed and Scopus electronic databases with a defined search strategy and inclusion/exclusion criteria ensuring the selection of studies that investigated antimicrobial and anti-inflammatory actions of TTO in the context of pre-clinical research only. There has been a marked increase in the number of pre-clinical studies exploring the antimicrobial ( n = 78) and anti-inflammatory ( n = 14) actions of TTO since the Carson et al. (2006) review. These studies have extended our understanding of TTO as an antimicrobial agent, with an emphasis on biofilm-forming, clinical, and drug-resistant strains, investigation of the susceptibility of fungi (specifically Candida spp.), and investigation into different pathogenic classes to identify antiviral and anti-protozoan actions. Studies investigating the anti-inflammatory activity of TTO improved our understanding of TTO mechanisms of action and novel directions in formulation and delivery of TTO as a preventative and therapeutic agent showing promise in vitro and in vivo. Since the last review almost two decades ago, a significant body of pre-clinical research has been published concerning the antimicrobial and other therapeutic properties of TTO. These studies have expanded our understanding of the value of TTO in treating infections caused by different microorganisms, including more clinically relevant, health-related pathogenic states of bacteria and fungi (e.g. biofilms, clinical samples, and drug-resistant strains) as well as viruses and protozoa. Research into novel TTO delivery methods and isolated pathogenic drug targets is increasing and will be an ongoing requirement for further clinical research and applications.