7-Hydroxyindole

Indole is an extracellular biofilm signal for Escherichia coli, and many bacterial oxygenases readily convert indole to various oxidized compounds including 7‐hydroxyindole (7HI). Here we investigate the impact of indole and 7HI on Pseudomonas aeruginosa PAO1 virulence and quorum sensing (QS)‐regulated phenotypes; this strain does not synthesize these compounds but degrades them rapidly. Indole and 7HI both altered extensively gene expression in a manner opposite that of acylhomoserine lactones; the most repressed genes encode the mexGHI‐opmD multidrug efflux pump and genes involved in the synthesis of QS‐regulated virulence factors including pyocyanin (phz operon), 2‐heptyl‐3‐hydroxy‐4(1H)‐quinolone (PQS) signal (pqs operon), pyochelin (pch operon) and pyoverdine (pvd operon). Corroborating these microarray results, indole and 7HI decreased production of pyocyanin, rhamnolipid, PQS and pyoverdine and enhanced antibiotic resistance. In addition, indole affected the utilization of carbon, nitrogen and phosphorus, and 7HI abolished swarming motility. Furthermore, 7HI reduced pulmonary colonization of P. aeruginosa in guinea pigs and increased clearance in lungs. Hence, indole‐related compounds have potential as a novel antivirulence approach for the recalcitrant pathogen P. aeruginosa.

Since indole is present at up to 500 μM in the stationary phase and is an interspecies biofilm signal (J. Lee, A. Jayaraman, and T. K. Wood, BMC Microbiol. 7:42, 2007), we investigated hydroxyindoles as biofilm signals and found them also to be nontoxic interspecies biofilm signals for enterohemorrhagicEscherichia coliO157:H7 (EHEC),E. coliK-12, andPseudomonas aeruginosa. The genetic basis of EHEC biofilm formation was also explored, and notably, virulence genes in biofilm cells were repressed compared to those in planktonic cells. In Luria-Bertani medium (LB) on polystyrene with quiescent conditions, 7-hydroxyindole decreased EHEC biofilm formation 27-fold and decreased K-12 biofilm formation 8-fold without affecting the growth of planktonic cells. 5-Hydroxyindole also decreased biofilm formation 11-fold for EHEC and 6-fold for K-12. In contrast, isatin (indole-2,3-dione) increased biofilm formation fourfold for EHEC, while it had no effect for K-12. When continuous-flow chambers were used, confocal microscopy revealed that EHEC biofilm formation was reduced 6-fold by indole and 10-fold by 7-hydroxyindole in LB. Whole-transcriptome analysis revealed that isatin represses indole synthesis by repressingtnaABC7- to 37-fold in EHEC, and extracellular indole levels were found to be 20-fold lower. Furthermore, isatin repressed the AI-2 transporterslsrABCDFGKR, while significantly inducing the flagellar genesflgABCDEFGHIJKandfliAEFGILMNOPQ(which led to a 50% increase in motility). 7-Hydroxyindole induces the biofilm inhibitor/stress regulatorycfRand repressescysADIJPU/fliC(which led to a 50% reduction in motility) andpurBCDEFHKLMNRT. Isogenic mutants showed that 7-hydroxyindole inhibitsE. colibiofilm through cysteine metabolism. 7-Hydroxyindole (500 μM) also stimulatesP. aeruginosaPAO1 biofilm formation twofold; therefore, hydroxyindoles are interspecies bacterial signals, and 7-hydroxyindole is a potent EHEC biofilm inhibitor.