The interbacterial communication system known as quorum sensing (QS) utilizes hormone-like compounds referred to as autoinducers to regulate bacterial gene expression. Enterohemorrhagic Escherichia coli (EHEC) serotype O157:H7 is the agent responsible for outbreaks of bloody diarrhea in several countries. We previously proposed that EHEC uses a QS regulatory system to “sense” that it is within the intestine and activate genes essential for intestinal colonization. The QS system used by EHEC is the LuxS/autoinducer 2 (AI-2) system extensively involved in interspecies communication. The autoinducer AI-2 is a furanosyl borate diester whose synthesis depends on the enzyme LuxS. Here we show that an EHEC luxS mutant, unable to produce the bacterial autoinducer, still responds to a eukaryotic cell signal to activate expression of its virulence genes. We have identified this signal as the hormone epinephrine and show that β- and α-adrenergic antagonists can block the bacterial response to this hormone. Furthermore, using purified and in vitro synthesized AI-2 we showed that AI-2 is not the autoinducer involved in the bacterial signaling. EHEC produces another, previously undescribed autoinducer (AI-3) whose synthesis depends on the presence of LuxS. These results imply a potential cross-communication between the luxS /AI-3 bacterial QS system and the epinephrine host signaling system. Given that eukaryotic cell-to-cell signaling typically occurs through hormones, and that bacterial cell-to-cell signaling occurs through QS, we speculate that QS might be a “language” by which bacteria and host cells communicate.

Summary: Quorum sensing is a cell‐to‐cell signalling mechanism in which bacteria secrete hormone‐like compounds called autoinducers. When these auto‐inducers reach a certain threshold concentration, they interact with bacterial transcriptional regulators, thereby regulating gene expression. Enterohaemorrhagic Escherichia coli (EHEC) O157:H7 as well as E. coli K‐12 produces the autoinducer‐2 (AI‐2), which is synthesized by the product of the luxS gene, and previous work from our laboratory has shown that genes encoding the EHEC type III secretion system were activated by quorum sensing. Recently, by hybridizing an E. coli K‐12 gene array with cDNA synthesized from RNA extracted from EHEC strain 86‐24 and its isogenic luxS mutant, we observed that other potential virulence‐associated factors, such as genes encoding the expression and assembly of flagella, motility and chemotaxis, were also activated by quorum sensing. The array data also indicated that several genes encoding putative E. coli regulators were controlled by quorum sensing. In this report, we describe a two‐component system regulated by quorum sensing that shares homology with Salmonella typhimurium PmrAB, which we have named quorum sensing E. coli regulator B and C (QseBC). The qseBC genes, previously identified only as open reading frames b3025 and b3026, are organized in an operon in the E. coli chromosome, with qseB encoding the response regulator and qseC the sensor kinase. We confirmed the regulation of qseBC by quorum sensing using qseB::lacZ transcriptional fusions and characterized the phenotypes of an isogenic qseC mutation in EHEC. This mutant expressed less flagellin and had reduced motility compared with the wild‐type and complemented strains. Transcription of flhD, fliA, motA and fliC::lacZ fusions was decreased in the qseC mutant, suggesting that qseBC is a transcriptional regulator of flagella genes. A qseC mutant was also generated in E. coli K‐12 strain MC1000 that showed the same phenotypes as the EHEC mutant, indicating that qseBC regulates flagella and motility by quorum sensing in both EHEC and K‐12. QseBC activates transcription of flhDC, which is the master regulator for the flagella and motility genes and, in the absence of flhD, QseBC failed to activate the transcription of fliA. Motility of a luxS, but not of a qseC, mutant can be restored by providing AI‐2 exogenously as preconditioned media, suggesting that the qseC mutant is unable to respond to AI‐2. However, QseC has no effect on the expression of other quorum sensing‐controlled genes such as those encoding for the type III secretion system. These data indicate that QseBC is one component of the quorum‐sensing regulatory cascade in both EHEC and K‐12 that is involved in the regulation of flagella and motility genes, but that additional regulators in this cascade remain to be characterized.

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is responsible for outbreaks of bloody diarrhea and hemolytic-uremic syndrome in many countries. EHEC virulence mechanisms include the production of Shiga toxins (Stx) and formation of attaching and effacing (AE) lesions on intestinal epithelial cells. We recently reported that genes involved in the formation of the AE lesion were regulated by quorum sensing through autoinducer-2, which is synthesized by the product of the luxS gene. In this study we hybridized an E. coli gene array with cDNA synthesized from RNA that was extracted from EHEC strain 86-24 and its isogenic luxS mutant. We observed that 404 genes were regulated by luxS at least fivefold, which comprises approximately 10% of the array genes; 235 of these genes were up-regulated and 169 were down-regulated in the wild-type strain compared to in the luxS mutant. Down-regulated genes included several involved in cell division, as well as ribosomal and tRNA genes. Consistent with this pattern of gene expression, the luxS mutant grows faster than the wild-type strain (generation times of 37.5 and 60 min, respectively, in Dulbecco modified Eagle medium). Up-regulated genes included several involved in the expression and assembly of flagella, motility, and chemotaxis. Using operon:: lacZ fusions to class I, II, and III flagellar genes, we were able to confirm this transcriptional regulation. We also observed fewer flagella by Western blotting and electron microscopy and decreased motility halos in semisolid agar in the luxS mutant. The average swimming speeds for the wild-type strain and the luxS mutant are 12.5 and 6.6 μm/s, respectively. We also observed an increase in the production of Stx due to quorum sensing. Genes encoding Stx, which are transcribed along with λ-like phage genes, are induced by an SOS response, and genes involved in the SOS response were also regulated by quorum sensing. These results indicate that quorum sensing is a global regulatory mechanism for basic physiological functions of E. coli as well as for virulence factors.

The uropathogenicEscherichia colistrain CFT073 has multiple iron acquisition systems, including heme and siderophore transporters. AtonBmutant derivative of CFT073 failed to use heme as an iron source or to utilize the siderophores enterobactin and aerobactin, indicating that transport of these compounds in CFT073 is TonB dependent. The TonB⁻derivative showed reduced virulence in a mouse model of urinary tract infection. Virulence was restored when thetonBgene was introduced on a plasmid. To determine the importance of the individual TonB-dependent iron transport systems during urinary tract infections, mutants defective in each of the CFT073 high-affinity iron transport systems were constructed and tested in the mouse model. Mouse virulence assays indicated that mutants defective in a single iron transport system were able to infect the kidney when inoculated as a pure culture but were unable to efficiently compete with the wild-type strain in mixed infections. These results indicate a role for TonB-dependent systems in the virulence of uropathogenicE. colistrains.