Indole: A novel signaling molecule and its applications

A variety of intercellular signal molecules, such as, the most studied N-acyl-homoserinelactones (AHLs) in Gram-negative bacteria, autoinducer (AI-2) in both Gram-negative and Gram-positive bacteria, and signal peptides in Gram-positive bacteria, have been discovered over the last 20 years. Although it has been known for over 100 years that many bacteria produce indole, the real biological roles of this molecule are only now beginning to be explored. Recently, indole has been identified as signaling molecule in diverse functions and those attributed to indole include extracellular signal, drug resistance, plasmid stability, virulence control and biofilm formation. Indole is widespread in the natural environment and a variety of both Gram-positive and Gram-negative bacteria produce large quantities of indole. To date, approx 85 bacterial species are known to produce indole. Both natural and synthetic indole derivatives are being used as antimicrobial agents. Understanding indole signaling will help to develop effective antimicrobial or antivirulence strategies and their biotechnological applications. Indole works at both high and low cell density. Though it does not behave as quorum sensing molecule, but some mode of its action is like quorum sensing molecule. Current postgenomic studies already indicate a role for IAA (indole acetic acid) signaling in bacteria and microorganism-plant interactions. Indole signaling appears to be important in microbial consortia and may influence the digestive and immune systems in humans. Recently, synthetic indole 7-fluoroindole (7FI) has been reported as a potential candidate for use in an antivirulence approach against persistent Pseudomonas aeruginosa infection. The present review aims to address the role of indole as novel signaling molecule in diverse bacterial genes.