SUMMARY: 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.

EnteropathogenicEscherichia coli(EPEC) and enterohemorrhagicE. coli(EHEC) produce the characteristic “attaching and effacing” (A/E) lesion of the brush border. Intimin, an outer membrane protein encoded byeae, is responsible for the tight association of both pathogens with the host cell. Severaleaehave 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 theeaeheterogeneity 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 wereeaepositive when primers amplifying the conserved 5′ end were used. We then cloned and sequenced theeaepresent 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 “ɛ,” was present in human and bovine EHEC strains of serogroups O8, O11, O45, O103, O121, and O165. Intimin ɛ is the largest intimin cloned to date (948 aa) and shares the greatest overall sequence identity with intimin β, although analysis of the last C-terminal 280 aa suggests a greater similarity with intimins α and γ.

We have isolated Shiga toxin (Stx)-producing Escherichia coli (STEC) strains from the feces of feral pigeons which contained a new Stx2 variant gene designated stx _2f . This gene is most similar to sltIIva of patient E. coli O128:B12 isolate H.I.8. Stx2f reacted only weakly with commercial immunoassays. The prevalence of STEC organisms carrying the stx _2f gene in pigeon droppings was 12.5%. The occurrence of a new Stx2 variant in STEC from pigeons enlarges the pool of Stx2 variants and raises the question whether horizontal gene transfer to E. coli pathogenic to humans may occur.

In this study, we determined the sequences of four intimin variant genes detected in attaching and effacingEscherichia coliisolates of human origin. Three of them were novel and were designatedeae-η (eta),eae-ι (iota), andeae-κ (kappa). The fourth was identical to the recently describedeae-ζ (zeta), isolated from a bovineE. coliO84:NM isolate. We compared these sequences with those of published intimin-α, intimin-β, intimin-γ1, intimin-γ2, intimin-ε, and intimin-θ alleles. Sequence analysis of these 10 intimin alleles confirmed extensive genetic diversity within the intimin gene family inE. coli.The genetic diversity was more prominent in the 3′ region (starting at bp 2112), which encodes the binding domain of intimin. Phylogenetic analyses revealed four groups of closely related intimin genes: α and ζ; β and κ; γ1 and γ2/θ; and ε and η. Calculation of homoplasy ratios of sequences of the 5′ region ofeae(positions 1 to 2111) revealed evidence for intragenic recombination. Split decomposition analysis also indicates that recombination events have played a role in the evolutionary history ofeae.In conclusion, we recommend aneaenomenclature system based on the Greek alphabet and provide an updated PCR scheme for amplification and typing ofE. coli eae.