Adaptation to NaCl Reduces the Susceptibility of Enterococcus faecalis to Melaleuca alternifolia (Tea Tree) Oil

Curr Microbiol. 2015 Oct;71(4):429-33. doi: 10.1007/s00284-015-0871-1. Epub 2015 Jul 10.


This study investigated the hypothesis that the salt adaptation response of Enterococcus faecalis alters susceptibility to tea tree oil (TTO). Six E. faecalis isolates were adapted to 6.5 % NaCl, and then exposed to TTO in phosphate-buffered saline (PBS). One isolate was also exposed to TTO in Brain Heart Infusion Broth (BHIB). The viability of salt-adapted and non-adapted control cells was determined at 0, 45 and 90 min and compared. MICs for several antibiotics and TTO were also determined by E test and broth microdilution, respectively. Results showed that susceptibility to TTO in PBS was significantly reduced after salt adaptation for five isolates (83 %) (P < 0.05). Mean differences between salt-adapted and non-adapted cell counts were 2.51 log at 45 min and 2.13 log at 90 min. However, when E. faecalis ATCC 19433 was exposed to TTO in BHIB, no significant differences were seen. In conclusion, salt adaptation resulted in reduced susceptibility to TTO in PBS for the majority of isolates, indicating that cross protection had occurred. This effect was absent in BHIB, suggesting that the uptake of compatible solutes from the growth medium protected non-adapted cells from TTO. Whether this has implications for the clinical effectiveness of TTO remains to be determined.

MeSH terms

  • Adaptation, Physiological
  • Anti-Bacterial Agents / isolation & purification
  • Anti-Bacterial Agents / pharmacology*
  • Enterococcus faecalis / drug effects*
  • Enterococcus faecalis / physiology*
  • Melaleuca / chemistry*
  • Microbial Sensitivity Tests
  • Microbial Viability / drug effects
  • Sodium Chloride / metabolism*
  • Tea Tree Oil / isolation & purification
  • Tea Tree Oil / pharmacology*
  • Time Factors


  • Anti-Bacterial Agents
  • Sodium Chloride
  • Tea Tree Oil