RESUMO
The antimicrobial activity of extracts and constituents of Gomphrena martiana and Gomphrena boliviana (Amaranthaceae) were determined in order to identify the compounds responsible for the folk-medicinal use of these plants. Each extract was evaluated against 20 microorganisms, including Gram-positive and Gram-negative bacteria, spore-forming Gram-positive bacteria, an acid-fast bacterium, a fungus and two yeasts. Fractionation of each petroleum ether (PE) extract yielded five 5,6,7-trisubstituted flavones that were separately tested showing high activity against M. phlei (minimum inhibitory concentration (MIC) 15, 20 and 75 micrograms/ml) approaching that of commercial bactericides. Other natural and synthetic flavonoids with diverse structures were also tested to define structure-activity relationships. Each EtOH extract was subsequently fractionated and monitored by bioassays leading to isorhamnetin 3-O-beta-robinobioside (MIC 50 micrograms/ml) in both instances. This glycoside is reported here for the first time in G. boliviana.