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A new flavonoid (1) was isolated from organic extracts of Dalea versicolor, along with six known phenolic compounds (2-7). The structures of the seven compounds were determined by NMR and HRMS methods. These compounds were evaluated for direct activity against a variety of organisms in vitro, including the Gram-positive bacteria Staphylococcus aureus and Bacillus cereus. In addition, the compounds were evaluated for their ability to potentiate the activity of known antimicrobials through inhibition of multidrug-resistance (MDR) pumps. Compounds 1, 2, 4, and 7 exhibited direct or synergistic activity toward the human pathogen S. aureus and the opportunistic pathogen B. cereus. Compounds 4 and 7 were also found to potentiate the activity of berberine and of prescribed antibiotics, with 4 demonstrating a mode of action consistent with inhibition of the NorA MDR efflux pump in S. aureus.
Belofsky et al. (2004) studied this question.