Introduction Diterpenes are an important family of natural products because of their wide range of different structural types and biological properties. These compounds possess antifungal, antiviral, cyto- toxic and antibacterial properties [1], [2], [3], [4]. As a rule, the antibacterial compounds are more active against Gram-positive thanGram-negativebacteria[2],[3],[4].Oneoftheseantibacter- ial diterpenes, 13-epi-sclareol Fig. 1), is a labdane-type com- poundisolatedfromtheresinousexudatesofthemedicinalplant species Pseudognaphalium cheiranthifolium Lam.) Hilliard et Burtt. and P. heterotrichium Phil.) A. Anderb. [5]. The diterpene 13-epi-sclareol constitutes the principal component of the resin and in previous experiments the antibacterial activity against Gram-positivebacteriawasreported[5].Thiscompoundshowed a higher activity against Bacillus cereus, B. subtilis and B. coagulans, than against Staphylococcus aureus. In liquid media, 13-epi-sclareol effects the lysis of B. cereus and B. subtilis, as de- termined by a decrease in cell density and in cell viability [5]. In general,antibacterialactivityhasbeenreportedformanynatural compounds without particular interest in the mechanism of ac- tion. One possible mechanism of action is the interaction with the bacterial cytoplasmic membrane, thus interfering with the respiratory chain or the energy metabolism. As mentioned above, Gram-positive bacteria are more sensitive to antimicro- bial agents than Gram-negative bacteria, due to the different Effectof13-epi-SclareolontheBacterialRespiratory Chain LorenaTapia 1 JanetTorres 1 Leonora Mendoza 2 Alejandro Urza 3 Jorge Ferreira 4 Mario Pavani 4 Marcela Wilkens 1 Affiliation 1 Departamento de Biología, Facultad de Química y Biología, Universidad de Santiago de Chile, Chile 2 Departamento de Química de los Materiales, Facultad de Química y Biología, Universidad de Santiago de Chile, Chile 3 Laboratorio de Química Ecológica, Facultad de Química y Biología, Universidad de Santiago de Chile, Chile 4 Departamento de Bioquímica y Medicina Experimental, Facultad de Medicina, Universidad de Chile, Chile Correspondence Marcela Wilkens ´ Facultad de Quimica y Biologia ´ Universidad de Santiago de Chile ´ Casilla 40 Correo ´ 33 Santiago ´ Chile ´ Phone: +56-2-681-1366 ´ Fax: +56-2-681-2108 ´ E-mail: mwilkens@lauca.usach.cl Received February 6, 2004 ´ Accepted July 10, 2004 Bibliography Planta Med 2004; 70: 1058±1063 ´  Georg Thieme Verlag KG Stuttgart ´ New York DOI10.1055/s-2004-832647 ISSN 0032-0943 Abstract 13-epi-Sclareol is a labdane-type diterpene isolated from the resi- nous exudates of the medicinal plant species Pseudognaphalium cheiranthifolium Lam.) Hilliard et Burtt. and P. heterotrichium Phil.) A. Anderb. This compound has antibacterial activity only against Gram-positive bacteria, showing a bactericidal and lytic action. The interaction of 13-epi-sclareol with the bacterial re- spiratory chain was analyzed. The compound inhibited oxygen consumption of intact Gram-positive cells, but not with Gram- negativebacteria.ThecompoundinhibitedNADHoxidaseandcy- tochrome c reductase activities, while coenzyme Q reductase and the cytochrome c oxidase activities were not affected. These re- sults suggest that the target site of 13-epi-sclareol is located be- tween coenzyme Q and cytochrome c. Using cytoplasmic mem- branefractions,theresultsoftheanalysisoftheenzymeactivities associated with the respiratory chain complexes were the same for both Gram-positive and Gram-negative bacteria, indicating that the compound has no access to the cytoplasmic membrane of intact Gram-negative bacteria. Thus, the Gram-negative envel- ope may act as a physical barrier that prevents the access of this compound to the site of action. Key words Diterpene ´ 13-epi-sclareol ´ antibacterial ´ respiratory chain ´ oxy- gen consumption Original Paper 1058