, 1998). Hence, it is believed that P. aeruginosa

MVs are important to survive in microbial communities. Meanwhile, a number of bacteria secrete indole into the extracellular milieu. For other bacteria, it would be that these indole functions as inhibitors of PQS to escape predation by P. aeruginosa. To investigate the effect of indole on antimicrobial activities, we evaluated the ability of P. aeruginosa to inhibit the growth of actively dividing cells of the Gram-positive bacterium Bcl-2 protein family B. subtilis, which is known to be killed by P. aeruginosa (Park et al., 2005). As shown in Fig. 3, while a zone was clear around P. aeruginosa PAO1 on a lawn of B. subtilis cells, ΔpqsH did not kill B. subtilis. This result is consistent with published studies showing that the bactericidal activity is repressed in PQS-defective mutants (Park et al., 2005). The killing activity of PAO1 on the agar including indole was attenuated (Fig. 3), suggesting that indole also repress the killing ability of P. aeruginosa against B. subtilis. To determine whether indole oxidation products also affect MV production,

we tested the effect of oxidole, 4-hydroxyindole (4HI), 5-hydroxyindole (5HI), 6-hydroxyindole (6HI), isatin and indigo (Fig. Selleckchem Ganetespib 1). Oxidole exists in an equilibrium state with 2-hydroxyindole. The growth was not changed significantly with the addition of each molecule (Fig. 4a). 4HI, 5HI, 6HI and isatin inhibited MV production at the same level of indole, oxidole led to a 55% reduction of MVs, and no significant

changes were observed with indigo (Fig. 4b). PQS synthesis was also decreased in the presence of bicyclic compounds, including oxidole, 4HI, 5HI, 6HI and isatin, but not in the presence of indigo (Fig. 4c), suggesting that decreased MV production is caused by inhibition of PQS synthesis. In the same way, the activity of the pqsA promoter was also decreased in the presence of bicyclic compounds (Fig. 4d), indicating that these compounds inhibited PQS-stimulated transcription. Demeclocycline In addition, the results of pyocyanin synthesis showed a similar tendency (data not shown). To investigate whether structure is important for repression of MVs and PQS, we tested the effects of other cyclic compounds, such as catechol, naphthalene, naphthalene derivatives, 8-quinolinole and carbazole (Fig. 5a). In the growth curve assay, exogenous 8-quinolinole resulted in slightly decreased growth curve, whereas significant changes were not observed with other compounds (Fig. 5b). Exogenous catechol and carbazole did not inhibit MV production and PQS synthesis, whereas naphthalene led to a 44% reduction in them, and other naphthalene analogs used in this experiment, including 1-naphthol, 2-naphthol, 2,3-dihydroxynaphthalene and 1-aminonaphthalene and 8-quinolinol, significantly repressed both (Fig. 5c and d).