In this review, we focus on a group of mobile genetic elements designated pathogenicity islands (PAI). These elements play a pivotal role in the virulence of bacterial pathogens of humans and are also essential for virulence in pathogens of animals and plants. Characteristic molecular features of PAI of important human pathogens and their role in pathogenesis are described. The availability of a large number of genome sequences of pathogenic bacteria and their benign relatives currently offers a unique opportunity for the identification of novel pathogen-specific genomic islands. However, this knowledge has to be complemented by improved model systems for the analysis of virulence functions of bacterial pathogens. PAI apparently have been acquired during the speciation of pathogens from their nonpathogenic or environmental ancestors. The acquisition of PAI not only is an ancient evolutionary event that led to the appearance of bacterial pathogens on a timescale of millions of years but also may represent a mechanism that contributes to the appearance of new pathogens within a human life span. The acquisition of knowledge about PAI, their structure, their mobility, and the pathogenicity factors they encode not only is helpful in gaining a better understanding of bacterial evolution and interactions of pathogens with eukaryotic host cells but also may have important practical implications such as providing delivery systems for vaccination, tools for cell biology, and tools for the development of new strategies for therapy of bacterial infections

Enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic E. coli (EHEC) produce the characteristic "attaching and effacing" (A/E) lesion of the brush border. Intimin, an outer membrane protein encoded by eae, is responsible for the tight association of both pathogens with the host cell. Several eae have been cloned from different EPEC and EHEC strains isolated from humans and animals. These sequences are conserved in the N-terminal region but highly variable in the last C-terminal 280 amino acids (aa), where the cell binding activity is localized. Based on these considerations, we developed a panel of specific primers to investigate the eae heterogeneity of the variable 3 region by using PCR amplification. We then investigated the distribution of the known intimin types in a large collection of EPEC and EHEC strains isolated from humans and different animal species. The existence of a yet-unknown family of intimin was suspected because several EHEC strains, isolated from human and cattle, did not react with any of the specific primer pairs, although these strains were eae positive when primers amplifying the conserved 5 end were used. We then cloned and sequenced the eae present in one of these strains (EHEC of serotype O103:H2) and subsequently designed a PCR primer that recognizes in a specific manner the variable 3 region of this new intimin type. This intimin, referred to as "," was present in human and bovine EHEC strains of serogroups O8, O11, O45, O103, O121, and O165. Intimin is the largest intimin cloned to date (948 aa) and shares the greatest overall sequence identity with intimin ␤, although analysis of the last C-terminal 280 aa suggests a greater similarity with intimins ␣ and ␥.

In this study, we determined the nucleotide sequence of the 5.4-kb SalI restriction fragment of the recombinant plasmid pEO40-1, cloned from the large plasmid of enterohemorrhagic Escherichia coli (EHEC) O157:H7 strain EDL 933. This revealed two open reading frames which shared approximately 60% homology to the hlyC and hlyA genes of the E. coli ␣-hemolysin (␣-hly) operon. We termed these genes EHEC-hlyA and EHEC-hlyC to distinguish them from the ␣-hly genes. Preliminary sequence analysis indicated that another open reading frame homolog to the hlyB gene is located close to the 3 end of EHEC-hlyA. The predicted molecular masses of the EHEC-hlyA and EHEC-hlyC gene products were 107 and 19.9 kDa, respectively. The EHEC hemolysin protein (EHEC-Hly) was not secreted into the culture supernatant by the strain EDL 933. However, hemolytic activity was found in the broth culture supernatant after transforming EDL 933 with the recombinant plasmid pRSC6 carrying the hlyB and hlyD genes from the E. coli ␣-hemolysin operon. The EHEC hemolysin was precipitated and used as an antigen for immunoblot analysis. This demonstrated that 19 of 20 reconvalescent-phase serum samples from patients with hemolytic uremic syndrome reacted specifically with the antigen; conversely, only 1 of 20 control serum samples demonstrated reactivity. To investigate the prevalence of EHEC hemolysin genes in diarrheagenic E. coli, a PCR was developed to specifically detect EHEC-hlyA. All Shiga-like toxin-producing O157 strains and 12 of 25 Shiga-like toxin-producing non-O157 strains were PCR positive; strains of other categories of diarrheagenic E. coli were PCR negative. All PCRpositive strains hybridized with the CVD 419 probe. We found the CVD 419 probe to be identical to the 3.4-kb HindIII fragment of plasmid pEO40 carrying most of the EHEC-hlyA gene and a part of the putative EHEC-hlyB gene. In this study, the newly discovered EHEC hemolysin was shown to be responsible for the enterohemolytic phenotype and demonstrated to be related but not identical to ␣-hemolysin. The EHEC hemolysin appears to have clinical importance because it occurs in all O157 strains tested and is reactive to sera of patients with hemolytic uremic syndrome.

SummaryIn this study, we identified and characterized a novel secreted protein, the extracellular serine protease EspP, which is encoded by the large plasmid of enterohaemorrhagic Escherichia coli (EHEC) O157:H7. The corresponding espP gene consists of a 3900 bp open reading frame that is able to encode a 1300-aminoacid protein. EspP is synthesized as a large precursor which is then processed at the N-and C-termini during secretion. It can be grouped into the autotransporter protein family. The deduced amino acid sequence of EspP showed homology to several secreted or surface-exposed proteins of pathogenic bacteria, in particular EspC of enteropathogenic E. coli and IgA1 proteases from Neisseria spp. and Haemophilus influenzae. Hybridization experiments and immunoblot analysis of clinical EHEC isolates showed that EspP is widespread among EHEC of the serogroup O157 and that it also exists in serogroup O26. A specific immune response against EspP was detected in sera from patients suffering from EHEC infections. Functional analysis showed that EspP is a protease capable of cleaving pepsin A and human coagulation factor V. Degradation of factor V could contribute to the mucosal haemorrhage observed in patients with haemorrhagic colitis.