Natural and synthetic substituted chalcones, flavones and flavanones were tested for antibacterial activity. In order to determine synergism, new combinations of substituted flavonoids against Staphylococcus aureus, Escherichia coli and Enterobacter aerogenes were assayed. The results allow us to establish relationships between antimicrobial effect of the compounds and membrane structures of these microorganisms. When flavonoid combinations were employed a stronger effect was found against E. coli than against S. aureus. This fact is due to the existence of porins in the outer membrane of G(-)-bacteria. The compound that acts as enhancer acts by blocking the charges of amino acids in the porins and thus facilitates the passage of the other compound by diffusion into the bacterial cell.

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The bacteriostatic activity of 2’,4’,2-trihydroxychalcone; 2’,4’,3-trihydroxychalcone and 2’,4’,4-trihydroxychalcone, prepared by condensation of 2,4-dihydroxyacetophenone and benzaldehyde substituted, against Staphylococcus aureus ATCC 25923 was assayed by agar plate method. The three compounds presented important inhibition halos. In order to elucidate structure-activity relationships, the minimal inhibitory concentrations against S. aureus were determined by the broth dilution method and the results obtained were compared to that of 2',4'-dihydroxychalcone. The sequence observed was: MIC 2’,4’,3-(OH)3 > MIC 2’,4’-(OH)2 > MIC 2’,4’,4-(OH)3 > > MIC 2’,4’,2-(OH)3. These results showed that the introduction of an electron donating group (-OH) in the aromatic B-ring causes an increase in bioactivity, and that the intensity of action depends on the position of the OH substitute.

In order to determine the existence of synergism, the bacteriostatic action of flavonoids against Escherichia coli ATCC 25 922 between dihydroxylated chalcones and a clinically interesting conventional antibiotic, binary combinations of 2',3-dihydroxychalcone, 2',4-dihydroxychalcone and 2',4'-dihydroxychalcone with nalidixic acid and its ternary combinations with rutin (inactive flavonoid) were assayed against this Gram negative bacterium. Using a kinetic-turbidimetric method, growth kinetics were monitored in broths containing variable amounts of dihydroxychalcone alone, combinations of dihydroxychalcone variable concentration-nalidixic acid constant concentration and dihydroxychalcone variable concentration-nalidixic acid constant concentration-rutin constant concentration, respectively. The minimum inhibitory concentrations of dihydroxychalcones alone and its binary and ternary combinations were evaluated. All chalcones, and their binary and ternary combinations showed antibacterial activity, being rutin an excellent synergizing for the dihydroxychalcone-nalidixic acid binary combination against E. coli ATCC 25 922. Thus, this synergistic effect is an important way that could lead to the development of new combination antibiotics against infections caused by E. coli.