Mechanism of Decreased Susceptibility for Gram-negative Bacteria and Synergistic Effect with Ampicillin of Indole-3-carbinol

Indole-3-carbinol (I3C) is a natural compound found in a wide variety of plant food substances including members of the family Cruciferae with antioxidant and potential chemopreventive properties. In a previous study, I3C exhibited broad spectrum antibacterial activities. Particularly, it showed a more potent antibacterial activity against Gram-positive bacteria than Gram-negative bacteria. To elucidate this disparity of antibacterial activity between Gram-positive and Gram-negative bacteria, we investigated the actions of the efflux pumps and the lipopolysaccharide (LPS) barrier of the outer membrane of Gram-negative bacteria. The results showed that the antibacterial activity of I3C was affected by the barrier action of LPS in the outer membrane rather than by the efflux pumps. To assess its potential for combination therapy in treating bacterial infections, we investigated its synergy effects in combination with conventional antibiotics. The results demonstrated that I3C showed considerable synergistic activity in combination with ampicillin against drug-resistant isolates.