Enteropathogenic Escherichia coli (EPEC) causes severe diarrhea in young children. Essential for colonization of the host intestine is the LEE pathogenicity island, which comprises a cluster of operons encoding a type III secretion system and related proteins. The LEE1 operon encodes Ler, which positively regulates many EPEC virulence genes in the LEE region and elsewhere in the chromosome. We found that Ler acts as a specific autorepressor of LEE1 transcription. We further show that Ler specifically binds upstream of the LEE1 operon in vivo and in vitro. A comparison of the Ler affinities to different DNA regions suggests that the autoregulation mechanism limits the steady-state level of Ler to concentrations that are just sufficient for activation of the LEE2 and LEE3 promoters and probably other LEE promoters. This mechanism may reflect the need of EPEC to balance maximizing the colonization efficiency by increasing the expression of the virulence genes and minimizing the immune response of the host by limiting their expression. In addition, we found that the autoregulation mechanism reduces the cell-to-cell variability in the levels of LEE1 expression. Our findings point to a new negative regulatory circuit that suppresses the noise and optimizes the expression levels of ler and other LEE1 genes.Colonizing enteropathogens compete with the gut flora to gain a foothold in the host tissue by expressing powerful colonization factors. However, to reduce the immune response of the host, the pathogen should minimize the expression of the colonization factors. To resolve this dilemma, pathogens evolved regulatory mechanisms that optimize the expression levels and timing, thus maintaining expression of just enough colonization factors and only when needed. Another layer of complexity is added when the colonization is dependent on the assembly of organelles like the type III secretion systems (TTSS), which are composed of ϳ30 different proteins of various relative amounts and encoded by several operons. In these cases, an orderly expression program is required for efficient assembly of the organelle.Enteropathogenic Escherichia coli (EPEC) causes severe diarrhea in young children. It employs the TTSS as a molecular syringe to inject a battery of toxic or colonization proteins into the membrane and cytoplasm of infected host cells (4). The TTSS and some of the effectors are encoded by a 35.6-kbp pathogenicity island, termed the locus for enterocyte effacement (LEE). The LEE consists of 41 genes, organized in five major operons (LEE1 to LEE5) and several additional transcriptional units (10,19). Ler, an H-NS paralog, encoded by the first gene of the LEE1 operon, is a key regulator of the LEE regulon, positively regulating expression of LEE2, LEE3, LEE4, LEE5, espG, and map (11,19,24,30). The regulation of ler (LEE1 operon) is complex and involves many factors, including H-NS, integration host factor (IHF), Fis, PerC, BipA, GrlA, GrlR, GadX, and quorum sensing (2,7,11,13,14,17,19,26,28,30,32). Most of these factors appe...

Enterohemorrhagic and enteropathogenic Escherichia coli (EHEC and EPEC, respectively) strains represent a major global health problem. Their virulence is mediated by the concerted activity of an array of virulence factors including toxins, a type III protein secretion system (TTSS), pili, and others. We previously showed that EPEC O127 forms a group 4 capsule (G4C), and in this report we show that EHEC O157 also produces a G4C, whose assembly is dependent on the etp, etk, and wzy genes. We further show that at early time points postinfection, these G4Cs appear to mask surface structures including intimin and the TTSS. This masking inhibited the attachment of EPEC and EHEC to tissue-cultured epithelial cells, diminished their capacity to induce the formation of actin pedestals, and attenuated TTSS-mediated protein translocation into host cells. Importantly, we found that Ler, a positive regulator of intimin and TTSS genes, represses the expression of the capsule-related genes, including etp and etk. Thus, the expression of TTSS and G4C is conversely regulated and capsule production is diminished upon TTSS expression. Indeed, at later time points postinfection, the diminishing capsule no longer interferes with the activities of intimin and the TTSS. Notably, by using the rabbit infant model, we found that the EHEC G4C is required for efficient colonization of the rabbit large intestine. Taken together, our results suggest that temporal expression of the capsule, which is coordinated with that of the TTSS, is required for optimal EHEC colonization of the host intestine.Enterohemorrhagic Escherichia coli (EHEC) is an emerging pathogen causing outbreaks of food-borne gastroenteritis manifested by bloody diarrhea, which may progress to the potentially fatal hemolytic-uremic syndrome. The latter involves severe complications, such as renal impairment, hypertension, and central nervous system manifestations mainly caused by SLT toxins (3,22). EHEC belongs to the family of the attaching and effacing (AE)-inducing pathogens, which includes the closely related species enteropathogenic E. coli (EPEC), Citrobacter rodentium, and rabbit EPEC. When colonizing the gut, these pathogens form AE lesions on the intestinal epithelial cell surface. AE lesions are characterized by localized destruction of the brush border microvilli, intimate bacterial attachment to host cells, and the formation of actin structures, termed pedestals, beneath the attached bacteria (24). This histopathology is dependent upon a type III protein secretion system (TTSS), which functions as a molecular syringe to translocate effector proteins from the bacterial cytoplasm directly into the cytoplasm of host epithelial cells (15). These effectors subvert normal host cell functions and are required for efficient host colonization (15,34,35). One of these effectors, Tir, is inserted into the host cell membrane to form a binding site for an outer membrane adhesin, intimin. Interaction of intimin with translocated Tir promotes tight bacterial attachment to the ho...