The aim of this work was to evaluate the antimicrobial activity of Xanthium strumarium L. leaf extracts against Staphylococcus aureus, Escherichia coli, Salmonella typhimurium, Pseudomonas aeruginosa and Clostridium perfringens, as well as to investigate the presence of the toxic compound carboxyatractyloside in different plant parts. S. aureus and C. perfringens were more sensitive to non-polar than to polar fractions, and there was no difference between extracts for the remaining bacteria. All extracts had strong antimicrobial activity against the evaluated microorganisms. Carboxyatractyloside was found in cotyledons and seeds but not in adult leaves and burrs. Thus, only Xanthium strumarium leaves in adult stage can be used for medicinal purposes.112159163Bisht, N.P.S., Singh, R., Chemical investigation of the leaves of Xanthium strumarium L. (1977) Journal of the Indian Chemistry Society, 54, pp. 797-798Bisht, N.P.S., Singh, R., Chemical investigation of the leaves of Xanthium strumarium L. (1978) Journal of the Indian Chemistry Society, 55, pp. 707-708Bozsa, R.C., Oliver, L.R., Shoot and root interference of common cocklebur (Xanthium strumarium) and soybean (Glycine max) (1993) Weed Science, 41, pp. 34-37Chang, S.T., Chen, P.F., Chang, S.C., Antibacterial activity of leaf essential oils and their contituents from Cinnamomum osmophloeum (2001) Journal of Ethnopharmacology, 77, pp. 123-127Cole, R.J., Isolation and redefinition of the toxic agent from cocklebur (Xanthium strumarium) (1980) Journal of Agricultural and Food Chemistry, 28, pp. 1330-1332Colodel, E.M., Driemeier, D., Celso, P., Intoxicação experimental pelos frutos de Xanthium cavanillesii (Asteraceae) em bovinos (2000) Pesquisa Veterinária Brasileira, 21, pp. 31-38Duarte, M.C.T., Anti-Candida activity of Brazilian medicinal plants (2005) Journal of Ethnopharmacology, 97, pp. 305-311Favier, L.S., Anti-ulcerogenic activity of xanthanolide sesquiterpenes from Xanthium cavanillesii in rats (2005) Journal of Ethnopharmacology, 100, pp. 260-267Ginesta-Peris, E., Garcia-Breijo, F.J., Primoyufera, E., Antimicrobial activity of xanthatin from Xanthium spinosum L (1994) Letters in Applied Microbiology, 18, pp. 206-208Han, T., New thiazinediones and other components from Xanthium strumarium (2006) Chemistry of Natural Compounds, 42, pp. 567-570Hsu, F.L., Chen, Y.C., Cheng, J.T., Caffeic acid as active principle from the fruit of Xanthium strumarium to lower plasma glucose in diabetic rats (2000) Planta Medica, 66, pp. 228-230Huber, T., Klingenberg, M., Beyer, K., Binding of nucleotides by the mitochondrial ADP/ATP carrier as studied by 1H nuclear magnetic resonance spectroscopy (1999) Biochemistry, 38, pp. 762-769Kim, I.T., Methanol extract of Xanthium strumarium L. possesses anti-inflammatory and anti-nociceptive activities (2005) Biological and Pharmaceutical Bulletin, 28, pp. 94-100Kim, Y.S., Two cytotoxic sesquiterpene lactones from the leaves of Xanthium strumarium and their in vitro inhibitory activity on farnesyltransferase (2003) Planta Medica, 69, pp. 375-377Lavault, M., Antileishmanial and antifungal activities of xanthanolides isolated from Xanthium macrocarpum (2005) Fitoterapia, 76, pp. 363-366Loretti, A.P., Intoxicação experimental pelos frutos de Xanthium cavanillesii (Asteraceae) em ovinos (1999) Pesquisa Veterinária Brasileira, 19, pp. 71-78Mandal, S.C., Neuropharmacological activity of Xanthium strumarium Linn. extract (2001) Journal of Herbs, Spices and Medicinal Plants, 8, pp. 69-77(2003) Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically, , NATIONAL COMMITTEE FOR CLINICAL LABORATORY STANDARDS. Approved Standard. 6.ed. Wayne: NCCLSNieves, L.J., Efecto diurético del Xanthium strumarium L. (Guizazo de Caballo) (1999) Revista Cubana de Plantas Medicinales, 1, pp. 22-25Qin, L., A new thiazinedione from Xanthium strumarium (2006) Fitoterapia, 77, pp. 245-246Riscala, E.C., Xanthanolides and a bisnorxanthanolide from Xanthium cavanillesii (1994) Phytochemistry, 35, pp. 1588-1589Scherer, R., Sismotto, M., Godoy, H.T., Antioxidant activity and related compounds of Xanthium strumarium extracts (2009) Phytochemistry, , Accepted nov. 2008Scott, J.S., Lapidus, R., Sokolove, P.M., Use of carboxyatractylate and tight-binding inhibitor theory to determine the concentration of functional mitochondrial adenine nucleotide translocators (1993) Analytical Biochemistry, 210, pp. 69-76Sharma, S.R., Anthelmintic activity of Xanthium strumarium against Haemonchus contortus infection in sheep (2003) Indian Journal of Animal Sciences, 73, pp. 342-344Sheu, S.J., Determination of xanthii constituents by high-performance liquid chromatography and capillary electrophoresis (2003) Journal of Food and Drug Analysis, 11, pp. 67-71Stuart, B.P., Cole, R.J., Gosser, H.S., Cocklebur (Xanthium strumarium var. strumarium) intoxication in swine: Review and redefinition of the toxic principle (1981) Veterinary Pathology, 18, pp. 368-383Taher, H.A., Ubiergo, G.O., Talenti, E.C.J., Constituents of the essential oil of Xanthium strumarium (1985) Journal of Natural Products, 48, pp. 857-857Talakal, T.S., Dwivedi, S.K., Sharma, S.R., In vitro and in vivo antitrypanosomal activity of Xanthium strumarium leaves (1995) Journal of Ethnopharmacology, 49, pp. 141-145Turgut, M., Carboxyatractyloside poisoning in humans (2005) Annals of Tropical Pediatrics, 25, pp. 125-134Yadava, R.N., Jharbade, J., Novel biologically active triterpenoid saponin from the leaves of Xanthium strumarium Linn (2007) Asian Journal of Chemistry, 19, pp. 1224-1230