Few microorganisms are as versatile as Escherichia coli. An important member of the normal intestinal microflora of humans and other mammals, E. coli has also been widely exploited as a cloning host in recombinant DNA technology. But E. coli is more than just a laboratory workhorse or harmless intestinal inhabitant; it can also be a highly versatile, and frequently deadly, pathogen. Several different E. coli strains cause diverse intestinal and extraintestinal diseases by means of virulence factors that affect a wide range of cellular processes.

Enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic E. col 0157:H7 are intestinal pathogens that profoundly damage the microvilli and subapical cytoskeleton of epithelial cells. Here we report finding in EPEC a 35-kbp locus containing several regions implicated in formation of these lesions. DNA probes throughout this locus hybridize to E. coli 0157:H7 and other pathogens of three genera that cause similar lesions but do not hybridize to avirulent members of the same species. The EPEC locus and a different virulence locus of uropathogenic E. coli insert into the E. coli chromosome at the identical site and share highly similar sequences near the point of insertion.

Virulence factors of pathogenic bacteria (adhesins, toxins, invasins, protein secretion systems, iron uptake systems, and others) may be encoded by particular regions of the prokaryotic genome termed pathogenicity islands. Pathogenicity islands were first described in human pathogens of the species Escherichia coli, but have recently been found in the genomes of various pathogens of humans, animals, and plants. Pathogenicity islands comprise large genomic regions [10-200 kilobases (kb) in size] that are present on the genomes of pathogenic strains but absent from the genomes of nonpathogenic members of the same or related species. The finding that the G+C content of pathogenicity islands often differs from that of the rest of the genome, the presence of direct repeats at their ends, the association of pathogenicity islands with transfer RNA genes, the presence of integrase determinants and other mobility loci, and their genetic instability argue for the generation of pathogenicity islands by horizontal gene transfer, a process that is well known to contribute to microbial evolution. In this article we review these and other aspects of pathogenicity islands and discuss the concept that they represent a subclass of genomic islands. Genomic islands are present in the majority of genomes of pathogenic as well as nonpathogenic bacteria and may encode accessory functions which have been previously spread among bacterial populations.

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 LuxS͞ 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 ␤-and ␣-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 luxS͞AI-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.enterohemorrhagic Escherichia coli ͉ quorum sensing ͉ type III secretion ͉ epinephrine E HEC O157:H7 is responsible for major outbreaks of bloody diarrhea and hemolytic uremic syndrome (HUS) throughout the world. Enterohemorrhagic Escherichia coli (EHEC) colonizes the large bowel and causes a lesion on intestinal epithelial cells termed attaching and effacing (AE), characterized by the destruction of the microvilli and rearrangement of the cytoskeleton to form pedestal-like structures that cup the bacteria individually (1). The genes involved in the formation of the AE lesion are contained on the locus of enterocyte effacement (LEE) pathogenicity island (2), which is present in EHEC but absent in commensal and K-12 E. coli. The LEE contains genes encoding a type III secretion system, an adhesin (intimin), and a receptor (Tir) for this adhesin (3). The majority of the LEE genes are organized in five operons (LEE1-5). The first gene of the LEE1 operon encodes a transcriptional activator (Ler) essential for the expression of the LEE genes (4, 5). EHEC also produces a potent Shiga toxin (Stx) responsible for the major symptoms of hemorrhagic colitis and HUS (1). We recently reported that several virulence-associated genes in EHEC such as the LEE genes, Stx genes, and the flagella regulon are activated through the bacterial cell-to-cell signaling mechanism known as quorum sensing (QS) (6, 7). This QS signaling is us...

SummaryEnteropathogenic (EPEC) and enterohaemorrhagic Escherichia coli (EHEC) constitute a significant risk to human health worldwide. Both pathogens colonize the intestinal mucosa and, by subverting intestinal epithelial cell function, produce a characteristic histopathological feature known as the 'attaching and effacing' (A/E) lesion. Although EPEC was the first E. coli to be associated with human disease in the 1940s and 1950s, it was not until the late 1980s and early 1990s that the mechanisms and bacterial gene products used to induce this complex brush border membrane lesion and diarrhoeal disease started to be unravelled. During the past few months, there has been a burst of new data that have revolutionized some basic concepts of the molecular basis of bacterial pathogenesis in general and EPEC pathogenesis in particular. Major breakthroughs and developments in the genetic basis of A/E lesion formation, signal transduction, protein translocation, host cell receptors and intestinal colonization are highlighted in this review.

SummaryEnteropathogenic Escherichia coli (EPEC) is the prototype organism of a group of pathogenic Gram-negative bacteria that cause attaching and effacing (AE) intestinal lesions. All EPEC genes necessary for the AE phenotype are encoded within a 35.6 kb pathogenicity island termed the locus of enterocyte effacement (LEE). The LEE encodes 41 predicted open reading frames (ORFs), including components of a type III secretion apparatus and secreted molecules involved in the disruption of the host cell cytoskeleton. To initiate our studies on regulation of genes within the LEE, we determined the genetic organization of the LEE, de®ning transcriptional units and mapping transcriptional start points. We found that components of the type III secretion system are transcribed from three polycistronic operons designated LEE1, LEE2 and LEE3. The secreted Esp molecules are part of a fourth polycistronic operon designated LEE4. Using reporter gene fusion assays, we found that the previously described plasmid-encoded regulator (Per) activated operons LEE1, LEE2 and LEE3, and modestly increased the expression of LEE4 in EPEC. Using single-copy lacZ fusions in K-12-derived strains, we determined that Per only directly activated the LEE1::lacZ fusion, and did not directly activate the other operons. Orf1 of the LEE1 operon activated the expression of single-copy LEE2::lacZ and LEE3::lacZ fusions in trans and modestly increased the expression of LEE4::lacZ in K-12 strains. Orf1 was therefore designated Ler, for LEE-encoded regulator. Thus, the four polycistronic operons of the LEE that encode type III secretion components and secreted molecules are now included in the Per regulon, where Ler participates in this novel regulatory cascade in EPEC.

Enteropathogenic Escherichia coli (EPEC) causes a characteristic histopathology in intestinal epithelial cells called the attaching and effacing lesion. Although the histopathological lesion is well described the bacterial factors responsible for it are poorly characterized. We have identified four EPEC chromosomal genes whose predicted protein sequences are similar to components of a recently described secretory pathway (type III) responsible for exporting proteins lacking a typical signal sequence. We have designated the genes sepA, sepB, sepC, and sepD (sep, for secretion of E. coli proteins). The predicted Sep polypeptides are similar to the Lcr (low calcium response) and Ysc (yersinia secretion) proteins of Yersinia species and the Mxi (membrane expression of invasion plasmid antigens) and Spa (surface presentation of antigens) regions of Shigella flexneri. Culture supernatants of EPEC strain E2348/69 contain several polypeptides ranging in size from 110 kDa to 19 kDa. Proteins of comparable size were recognized by human convalescent serum from a volunteer experimentally infected with strain E2348/69. A sepB mutant of EPEC secreted only the 110-kDa polypeptide and was defective in the formation of attaching and effacing lesions and protein-tyrosine phosphorylation in tissue culture cells. These phenotypes were restored upon complementation with a plasmid carrying an intact sepB gene. These data suggest that the EPEC Sep proteins are components of a type III secretory apparatus necessary for the export of virulence determinants.Enteropathogenic Escherichia coli (EPEC) causes infantile diarrhea throughout the world. EPEC infections result in the formation of attaching and effacing (AE) lesions which are characterized by effacement of intestinal microvilli, intimate adherence of bacteria to enterocytes, and accumulation of polymerized actin and other cytoskeletal components in the eukaryotic cell. Filamentous actin accumulates below the bacteria, resulting in the formation of cup-like pedestals (1, 2). Several signal transduction mechanisms have been associated with AE lesion formation, including tyrosine phosphorylation of a 90-kDa host cell protein (Hp9O) (3), fluxes in inositol phosphate levels (4), increased intracellular Ca2+ levels (5), and phosphorylation of myosin light chain (6). We recently described a large (35-kb) region in the EPEC chromosome, termed LEE (locus of enterocyte effacement), that encodes all of the virulence determinants for AE lesion formation so far identified (7). Two chromosomal loci within the LEE, eaeA and eaeB (eae, for E. coli attaching and effacing

The distribution of EDL 933 O island 122 (OI-122) was investigated in 70 strains of Verocytotoxin-producing