Indole Can Act as an Extracellular Signal in Escherichia coli

Abstract

Previous work has shown that lacZ fusions to the cysK, astD, tnaB, and gabT genes in Escherichia coli are activated by self-produced extracellular signals. Using a combination of ethyl acetate extraction, reversedphase C 18 chromatography, and thin-layer chromatography, we have purified an extracellular activating signal from E. coli supernatants. Mass spectrometry revealed a molecule with an m/z peak of 117, consistent with indole. Nuclear magnetic resonance analysis of the purified E. coli factor and synthetic indole revealed identical profiles. Using synthetic indole, a dose-dependent activation was observed with lacZ fusions to the gabT, astD, and tnaB genes. However, cysK::lacZ and several control fusions were not significantly activated by indole. Conditioned medium prepared from a tnaA (tryptophanase) mutant, deficient in indole production, supported 26 to 41% lower activation of the gabT and astD fusions. The residual level of activation may be due to a second activating signal. Activation of the tnaB::lacZ fusion was reduced by greater than 70% in conditioned medium from a tnaA mutant.The use of chemical signals for bacterial communication is a widespread phenomenon (10,11,20,23,33). In gram-negative bacteria, these signals can be N-acyl derivatives of homoserine lactone, cyclic dipeptides, and quinolones (3,8,17,(28)(29)(30)43). In gram-positive bacteria, small peptides appear to be the predominant signal (7,15,16,18,25,36). In some cases, small proteins can mediate signaling (22,40). These signals regulate a variety of functions, including bioluminescence, differentiation, virulence, DNA transfer, and biofilm maturation (1,2,4,5,9,19,24,27,31,32).Indole production is a common diagnostic marker for the identification of Escherichia coli (37). Among the Enterobacteriaceae, indole is produced by E. coli and certain members of the Proteeae, such as Proteus vulgaris, Providencia spp., and Morganella spp. (37). Indole is formed from tryptophan by the tryptophanase enzyme, encoded by the tnaA gene (35). At very high concentrations (5 mM), indole is toxic to E. coli, possibly by causing membrane changes that result in the generation of superoxide (12). However, the concentration at which indole is toxic is approximately 15-fold higher than the physiological concentration seen in stationary-phase supernatants of E. coli (see below). The efflux of indole from E. coli is mediated by the AcrEF pump, and acrEF mutants exhibit enhanced indole sensitivity (21). The primary pathway for indole transport into the cell is via the Mtr permease (42).For E. coli, the role of cell-to-cell signaling in a variety of functions, including regulation of ftsQAZ, expression of type III secretion systems, inhibition of DNA replication, and activation of degradative pathways, has been described (1,13,34,38,39,41). However, the extracellular signals involved in these processes are poorly understood. Previous studies from our lab have identified the E. coli genes cysK, astD, tnaB, and gabT, which are activated by extracellular signals...

Indole induces the expression of multidrug exporter genes in Escherichia coli

“…Thus, the induction of E. coli drug exporter genes by indole is not likely to be a result of the envelope stress response. Recently, it was reported that indole acts as an extracellular signal like N-acyl homoserine lactone and AI-2 (Baca- DeLancey et al, 1999;Fuqua et al, 2001;Wang et al, 2001;Bassler, 2002;Chen et al, 2002). The indole concentration increases with cell growth as a byproduct of pyruvate production in E. coli cells (Wang et al, 2001).…”

“…The © 2004Blackwell Publishing Ltd, Molecular Microbiology , 55 , 1113-1126 indole concentration in the culture medium increases with cell growth. Wang et al . reported that indole regulates the expression of some genes such as astD , tnaB and gabT (Wang et al ., 2001). DeMoss reported that several Gramnegative species, including E. coli , Klebsiella oxytoca , Haemophilus influenzae and so on, showed indole production (DeMoss and Moser, 1969).…”

New tryptophanase inhibitors: Towards prevention of bacterial biofilm formation

“…Therefore, E. coli may have evolved to utilize alternative signals, such as the accumulation of certain metabolites. Signaling via metabolites may allow cells to fine-tune the regulation of target genes in response to changing environmental conditions [10].…”

BaeSR, Involved in Envelope Stress Response, Protects against Lysogenic Conversion by Shiga Toxin 2-Encoding Phages

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