Indole Inhibition of AHL-Mediated Quorum Signaling is Widespread in Gram-Negative
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The luxI/R quorum-sensing system and associated N-acylated homoserine lactone (AHL) signals is a widespread mechanism of gene regulation in gram negative bacteria. Quorum inhibition can be exploited as a strategy of bacterial competition. Although indole inhibition of Pseudomonas aeruginosa quorum signaling has been reported by other investigators, it has not been documented in other organisms. Co-culture with Escherichia coli BW25113 inhibited quorum-regulated pigmentation in Chromobacterium violeaceum, Serratia marcescens and Pseudomonas aureofaciens (Pseudomonas chlororaphis). We investigated whether indole was responsible for this effect. qRT-PCR of genes under the AHL-regulated system exhibited up-regulation in untreated cells compare with cells treated with 1.0 mM indole. Loss of pigmentation did not occur during co-culture with E. coli tnaA (lacking tryptophanase and indole production). In the absence of E. coli, the presence of physiologically relevant indole concentrations (0.5-1.0mM) caused a significant reduction of C. violaceum biofilm formation, pigment (violacein) production, elastase and chitinase activity. Similar indole-mediated inhibition of pigmentation was also observed in pure cultures of S. marcescens and P. aureofaciens. At indole concentrations above 1.0 mM, virulence of C. violaceum, S.marcescens, and P. aureofaciens was reduced in a Caenorhabditis elegans bioassay. Overall, indole-mediated inhibition of AHL quorum signaling appears to be a general phenomenon and not exclusive to P. aeruginosa.