A bioassay-guided fractionation of methanol extract of Aristolochia bracteolata whole plant was carried out in order to evaluate its antimicrobial activity and to identify the active compounds in this extract. Antibacterial and antifungal activities of methanol extract against gram-positive, gram-negative, and fungal strains were investigated by the agar disk diffusion method. Among the strains tested, Moraxella catarrhalis and sea urchin-derived Bacillus sp. showed the highest sensitivity towards the methanol extract and hence they are used as test organisms for the bioassay-guided fractionation. From this extract, aristolochic acid 1 (AA-1) has been isolated and has showed the greatest antibacterial activity against both standard strain and clinical isolates of Moraxella catarrhalis with equal minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values of 25 and 50？μg/mL. Modification of the AA-1 to AA-1 methyl ester completely abolished the antibacterial activity of the compound and the piperonylic acid moiety of AA-1 which suggested that the coexistence of phenanthrene ring and free carboxylic acid is essential for AA-1 antibacterial activity. 1. Introduction Moraxella catarrhalis is a gram-negative, aerobic diplococcus human mucosal pathogen which causes middle ear infections in infants and children [1–3], and it is one of the three major causes of otitis media along with Streptococcus pneumonia and Haemophilus influenzae [4]. Although Moraxella catarrhalis is frequently found as a commensal of the upper respiratory tract, recently it has emerged as a genuine pathogen and is now considered an important cause of upper respiratory tract infections in healthy children and elderly people, lower respiratory tract infections in adults with chronic obstructive pulmonary disease [1, 5], and hospital-acquired pneumonia [6]. Amikacin, cefixime, fosfomycin, cefuroxime, cotrimoxazole, doxycycline, and erythromycin resistant strains of Moraxella catarrhalis were isolated and the widespread production of a -1actamase enzyme renders the bacterium resistant to the penicillin [7–9]. This has led to the search for new and effective therapeutic alternatives among natural compounds. Plants remain an important source of diverse chemical entities which have been used as drugs or provide scaffolds from which new drugs have been derived [10]. The selection of a suitable candidate species for investigations can be done on the basis of long-term use by humans. This approach is based on an assumption that the active compounds isolated from such
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