Indole Production Promotes Escherichia coli Mixed-Culture Growth with Pseudomonas aeruginosa by Inhibiting Quorum Signaling

Abstract

Indole production by Escherichia coli, discovered in the early 20th century, has been used as a diagnostic marker for distinguishing E. coli from other enteric bacteria. By using transcriptional profiling and competition studies with defined mutants, we show that cyclic AMP (cAMP)-regulated indole formation is a major factor that enables E. coli growth in mixed biofilm and planktonic populations with Pseudomonas aeruginosa. Mutants deficient in cAMP production (cyaA) or the cAMP receptor gene (crp), as well as indole production (tnaA), were not competitive in coculture with P. aeruginosa but could be restored to wild-type competitiveness by supplementation with a physiologically relevant indole concentration. E. coli sdiA mutants, which lacked the receptor for both indole and N-acyl-homoserine lactones (AHLs), showed no change in competitive fitness, suggesting that indole acted directly on P. aeruginosa. An E. coli tnaA mutant strain regained wild-type competiveness if grown with P. aeruginosa AHL synthase (rhlI and rhlI lasI) mutants. In contrast to the wild type, P. aeruginosa AHL synthase mutants were unable to degrade indole. Indole produced during mixed-culture growth inhibited pyocyanin production and other AHL-regulated virulence factors in P. aeruginosa. Mixed-culture growth with P. aeruginosa stimulated indole formation in E. coli cpdA, which is unable to regulate cAMP levels, suggesting the potential for mixed-culture gene activation via cAMP. These findings illustrate how indole, an early described feature of E. coli central metabolism, can play a significant role in mixed-culture survival by inhibiting quorum-regulated competition factors in P. aeruginosa. In nature, bacteria normally occur in polymicrobial communities. Interactions between community members typically involve several mechanisms, including responses to antimicrobial compounds, nutritional interactions, and signaling (9,12,42). Chemical signaling is widespread in bacteria, and in Gramnegative bacteria it involves several compounds, including N-acyl derivatives of homoserine lactone (AHLs), furans, small peptides, quinolones, and indole (37). In Pseudomonas aeruginosa, many genes involved with virulence and competition are regulated by AHL-and quinolone-based quorum signaling (35,50). Quorum signal disruption (quenching) has been shown to alter bacterial competition and reduce virulence (18). In one animal model study of P. aeruginosa lung infections, administration of a ginseng extract caused a reduction in AHL levels and AHL-regulated elastase without affecting bacterial growth (44). Microbial nutrition is also important in species' interactions. Central metabolism, long regarded as reflecting a number of routine housekeeping functions, is now being reexamined for its role in mixed-culture interactions within biofilm and planktonic populations (9).One component of central metabolism in Escherichia coli involves purine and pyrimidine nucleic acid synthesis (reviewed in reference 36). Pathways for de novo synthesis and salvage pathways...

Indole inhibition of N-acylated homoserine lactone-mediated quorum signalling is widespread in Gram-negative bacteria

“…Indole production was also shown to promote E. coli survival during co-culture with P. aeruginosa (Chu et al, 2012). Based on this current study, we conclude that indole production plays an important role in the competition between E. coli and AHL-producing Gram-negative bacteria, and that indole is a general inhibitor of AHLbased quorum signalling.…”

Indole Biodegradation in Acinetobacter sp. Strain O153: Genetic and Biochemical Characterization

“…Indole, produced mainly by E. coli in the intestine, can decrease the virulence of P. aeruginosa (53). This effect was later attributed to the indole-caused inhibition of AHL-based quorum-sensing signaling by altering the folding of the AqsR regulator (14,54). Such an effect was reported for a number of Gram-negative bacteria, including Chromobacterium violaceum, Pseudomonas chlororaphis, Serratia marcescens (55), and Acinetobacter oleivorans DR1 (14), a strain which was identified here to possess the iif-ant operon combination (Fig.…”

Indole inhibits bacterial quorum sensing signal transmission by interfering with quorum sensing regulator folding

“…Escherichia coli can excrete considerable amounts of indole and acetate in the stationary phase (Helling et al, 2002;Kobayashi et al, 2006). Indole has received great attention because of its extensive effects on various biological functions in the bacterial population, such as biofilm formation, antibiotic resistance and virulence (Lee et al, 2007(Lee et al, , 2009Chu et al, 2012;Nikaido et al, 2012;Vega et al, 2012). Indole has been known to be exported by the AcrEF-TolC pump (Kawamura-Sato et al, 1999), although others have reported that indole freely diffuses across membranes and AcrEF was not required for indole export (Piñero-Fernandez et al, 2011).…”

“…Highly expressed genes coding for proteases, molecular chaperones and TCA cycle enzymes in the presence of indole might play crucial roles in indole-induced stress conditions. Indole has been reported to act as an antivirulence compound against E. coli, Pseudomonas aeruginosa and Staphylococcus aureus (Lee et al, 2007(Lee et al, , 2009(Lee et al, , 2013Chu et al, 2012). Indole reduced virulence factors by modulating the expression of virulence and regulatory genes, which were reportedly regulated by the QS system in P. aeruginosa (Lee et al, 2009).…”