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 a-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 a-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-a) assay. The real-time RT-PCR results revealed that the transcription
of genes encoding a-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-a production was suppressed by TTO treatment in
RAW264.7 cells stimulated by S. aureus supernatant containing staphylococcal enterotoxins (SEs).