Effect of tea tree oil on Staphylococcus aureus growth and enterotoxin production

Staphylococcus aureus (S. aureus), a major food-borne pathogen, causes disease in mammalian hosts by producing a wide variety of exoproteins, such as α-hemolysin and staphylococcal enterotoxins. Tea tree oil (TTO), an essential oil, has broad-spectrum antimicrobial activity. The objectives of this study were to evaluate the inhibitory effects of TTO on S. aureus growth and on α-hemolysin, enterotoxins A and B production. In this study, the effect of TTO on S. aureus growth in laboratory medium and pasteurized milk was determined by time-kill assays. Treatment with half of minimal inhibitory concentration (MIC) of TTO demonstrated very little or no reduction in numbers of viable ATCC 29213 cells; however, 1 × MIC of TTO reduced the viable cell count more noticeably, and 2 × MIC of TTO demonstrated an even greater reduction in the viable cell count, both in TSB and milk. The influence of TTO on enterotoxins was determined by real-time reverse transcriptase-PCR (real-time RT-PCR), a hemolysis assay, Western blot and a tumor necrosis factor alpha (TNF-α) assay. The real-time RT-PCR results revealed that the transcription of genes encoding α-hemolysin, staphylococcal enterotoxin A (SEA) and staphylococcal enterotoxin B (SEB) were down regulated after S. aureus was exposed to TTO (0.0625 mg/mL-0.5 mg/mL). The hemolytic assay showed that S. aureus hemolytic activity was inhibited by subinhibitory concentrations of TTO (0.0625 mg/mL-0.5 mg/mL). The Western blot assay identified that the production of the three virulence factors was inhibited by TTO (0.0625 mg/mL-0.5 mg/mL). Moreover, the enzyme linked immunosorbent assay (ELISA) demonstrated that TNF-α production was suppressed by TTO treatment in RAW264.7 cells stimulated by S. aureus supernatant containing staphylococcal enterotoxins (SEs).