Influence of organic matter, cations and surfactants on the antimicrobial activity of Melaleuca alternifolia (tea tree) oil in vitro

The effect of some potentially interfering substances and conditions on the antimicrobial activity of Melaleuca alternifolia (tea tree) oil was invest igated. Agar and broth dilution methods were used to determine minimum inhi bitory and cidal concentrations of tea tree oil in the presence and absence of each potentially interfering substance. Activity was determined against Gram-positive and -negative bacteria, and Candida albicans. Minimum inhibi tory or cidal concentrations differed from controls bs two or more dilution s, for one or more organisms, where Tween-20, Tween-80, skim-milk powder an d bovine serum albumin ere assessed. These differences were nor seen when a ssays were performed in anaerobic conditions, or in the presence of calcium and magnesium ions. The effect of organic matter on the antimicrobial acti vity of tea tree oil was also investigated by an organic soil neutralizatio n test. Organisms were exposed to lethal concentrations of tea tree oil ran ging from 1-10% (v/v), in the presence of 1-30%; (w/v) dry bakers' yeast. A fter 10 min contact time, viability was determined. At greater than or equa l to 1% , organic matter compromised the activity of each concentration of tea tl ee oil against Staphylococcus aureus and C. albicans. At 10% or more l organic matter compromised the activity of each tea tree oil concentratio n against Pseudomonas aeruginosa. Organic matter affected 1 and 2% tea tree oil, but not 4 and 8%, against Escherichia coli. In conclusion, organic ma tter and surfactants compromise the antimicrobial activity of tea tree oil, although these effects vary between organisms.