The essential oil from aerial parts of Conyza bonariensis (L) Cronquist collected in Mérida was obtained by hydrodistillation and analysed by GC/MS. The major components were trans-β-farnesene (37.8%), trans-ocimene (20.7%) and β-sesquiphellandrene (9.8%). Cytotoxicity assay was also performed with the essential oil against HeLa (cervix carcinoma), A-459 (lung carcinoma) and MCF-7 (breast adenocarcinoma) human cell lines and against normal Vero cells (African green monkey kidney) with IC 50 values ranging from 1.4 to 45.8 µg/mL. Additionally, the essential oil presented a significant bactericidal effect against Bacillus cereus, while a moderate activity was observed against Staphylococcus epidermidis and Candida albicans.

The essential oils from the leaves of Lippia oreganoides collected in June 2003 (rainy season) and February 2004 (dry season) were analyzed by GC/MS. Thymol (61.9% and 44.7%, respectively) and carvacrol (7.9% and 16.8%, respectively) proved to be the major constituents of both collections. Linalool (0.4%) was present only in the collection made in June 2003, while carvacryl acetate (0.6% and caryophyllene oxide (1.6%) were only detected in the February 2004 collection.

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Antibacterial activity of the essential oil of Lippia oreganoides was evaluated against thirty-nine multiresistant bacterial strains of nosocomial origin and five reference bacterial strains using the disk diffusion agar method. The results obtained have revealed strong antibacterial activity against methicillin resistant Staphylococcus aureus, extended spectrum β-lactamase producing Klebsiella pneumoniae, and multiresistant Acinetobacter baumannii with MIC values ranging from 20 to 40 μg/mL. The low dose antibacterial activity observed in this investigation suggested that the essential oil of L. oreganoides could be used in pharmaceutical preparations for the treatment of infections caused by multiresistant bacteria. According to the literature consulted, this is the first time that antibacterial activity of this species against multiresistant bacterial strains of nosocomial origin has been reported.

A new lathyrane-type diterpene 8alpha,15beta-diacetoxy-7beta-benzoyloxy-3beta-(2-methylpropanoyloxy)-4alphaH,9alphaH, 11alphaH-lathyra-5E,12E-dien-14-one (latazienone) has been isolated from the latex of Euphorbia latazi Kunth. The structure of the new diterpene was determined by a combination of ID- and 2D-NMR techniques.