ABSTRACT
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.
Subject(s)
Adhesins, Bacterial/metabolism , Bacterial Capsules/metabolism , Enteropathogenic Escherichia coli/pathogenicity , Escherichia coli O157/pathogenicity , Escherichia coli Proteins/metabolism , Membrane Transport Proteins/metabolism , Virulence Factors/metabolism , Animals , Bacterial Adhesion , Bacterial Capsules/ultrastructure , Cell Line , Enteropathogenic Escherichia coli/metabolism , Enteropathogenic Escherichia coli/ultrastructure , Epithelial Cells/microbiology , Erythrocytes/microbiology , Escherichia coli Infections , Escherichia coli O157/metabolism , Escherichia coli O157/ultrastructure , Escherichia coli Proteins/genetics , Gene Deletion , Gene Expression Profiling , Gene Expression Regulation, Bacterial , Glycosyltransferases/genetics , Glycosyltransferases/metabolism , Humans , Intestine, Large/microbiology , Membrane Glycoproteins/genetics , Membrane Glycoproteins/metabolism , Membrane Proteins/genetics , Membrane Proteins/metabolism , Microscopy, Electron, Transmission , Mutagenesis, Insertional , Protein-Tyrosine Kinases/genetics , Protein-Tyrosine Kinases/metabolism , Rabbits , Trans-Activators/metabolismABSTRACT
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.
