Growth inhibiting activity of volatile oil from Cistus creticus L. against Borrelia burgdorferi s.s. in vitro
Borreliosis patients from self-help groups reported considerable pain relief after ingestion of Cistus creticus leaf preparations. C. creticus leaf extracts of different polarities such as aqueous, ethyl acetate, hexane extracts as well as the volatile oil fraction obtained by steam distillation were tested for their antibacterial activity against Borrelia burgdorferi sensu stricto (Bbss) in vitro using the antibiotic amoxicilline as standard and polysorbate 80 as solubilizer for lipophilic extracts. Comparison of the four plant preparations shows that the volatile oil exerts the strongest growth inhibitory effect. Even concentrations of 0.02% (w/v) volatile oil in cultivation media reduced the total number of bacteria to 2% in comparison to a growth control after an eight-day cultivation period. While the aqueous extract did not reduce bacterial growth, incubation with hexane and ethyl acetate extracts clearly inhibited microbial growth. The main volatile components of the three active extracts tested were analyzed by GC-MS. The number of different labdane-type diterpenes as well as the total relative amount of diterpenes in the samples tested was highest in the essential oil of C. creticus. Identification of ten different volatile labdane-type diterpenes was assigned to the essential oil of C. creticus. Among these, manoyl oxide, 13-epi-manoyl oxide, 3-acetoxy-manoyl oxide and the monoterpene carvacrol were determined to be major constituents, accompanied by minor amounts of 3-hydroxy-manoyl oxide, all of which are known to exert antimicrobial activity.