title: Changes in antibiotic susceptibility in staphylococci habituated to sub-lethal concentrations of tea tree oil (Melaleuca alternifolia).
abstract: OBJECTIVE
  To investigate the effect of sub-lethal challenge with tea tree oil (TTO) on the antibiotic resistance profiles of staphylococci.
  
  
  RESULTS
  Isolates of methicillin-resistant/-sensitive Staphylococcus aureus (MRSA and MSSA) and coagulase-negative staphylococci (CONS) were habituated to sub-lethal concentrations of TTO (72 h). Following habituation, the minimum inhibitory concentrations (MIC) of antibiotics and TTO were determined. Habituated MRSA/MSSA cultures had higher (P < 0.05) MIC values than control cultures for the examined antibiotics. Habituated MRSA/MSSA cultures also displayed decreased susceptibility to TTO. Although the MIC of habituated MRSA/MSSA for the examined antibiotics reverted to control values after subsequent culture in the absence of TTO, the increased MIC against TTO were maintained. When compared with control cultures, habituated CoNS cultures had higher (P < 0.05) MIC values against three-fifths of the antibiotics examined; no changes in TTO MIC were observed.
  
  
  CONCLUSIONS
  TTO habituation 'stress-hardens' MRSA and MSSA, evidenced by transient decreased antibiotic susceptibility and stable decreased TTO susceptibility. Although TTO habituation did not decrease susceptibility of CoNS to TTO, such cultures showed transient decreased antibiotic susceptibility.
  
  
  CONCLUSIONS
  Application of TTO at sub-lethal concentrations may reduce the efficacy of topical antibiotics used with TTO in combination therapies.
authors.affiliation: Food Microbiology Research Group, University of Ulster, Shore Road, Newtownabbey, Northern Ireland, UK. ma.mcmahon@ulster.ac.uk
authors.forename: M A S
authors.initials: MA
authors.lastname: McMahon
authors.forename: M M
authors.initials: MM
authors.lastname: Tunney
authors.forename: J E
authors.initials: JE
authors.lastname: Moore
authors.forename: I S
authors.initials: IS
authors.lastname: Blair
authors.forename: D F
authors.initials: DF
authors.lastname: Gilpin
authors.forename: D A
authors.initials: DA
authors.lastname: McDowell
mesh_terms: D000900:Anti-Bacterial Agents / Q000633:toxicity*; D000891:Anti-Infective Agents, Local / Q000494:pharmacology / Q000633:toxicity*; D024881:Drug Resistance, Bacterial / Q000187:drug effects*; D031650:Melaleuca / Q000737:chemistry*; D055624:Methicillin-Resistant Staphylococcus aureus / Q000187:drug effects; D008826:Microbial Sensitivity Tests; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000378:metabolism; D020947:Tea Tree Oil / Q000494:pharmacology / Q000633:toxicity*
chemical_list: D000900:Anti-Bacterial Agents; D000891:Anti-Infective Agents, Local; D020947:Tea Tree Oil