Escherichia coli is an excellent model for studying the evolution of pathogenicity since within one species various genes can be found in pathogenic islands and plasmids causing a wide spectrum of virulence. A collection of 122 strains from different human and wild mammal hosts were analysed by PCR and Southern hybridization for the presence of a subset of the genes included in the LEE (locus of enterocyte effacement). In the PCR analysis, two markers (cesT/eae and espB genes) were found together in more strains (254 %) than either were found alone. The cesT/eae gene was less frequently found alone (82 %) than was the espB gene (156 %). Four regions of the LEE were analysed in a subsample of 25 strains using Southern hybridization. The four regions were all present (44 %), all absent (12 %) or present in different combinations (44 %) in a given strain. The flanking regions of the LEE showed the highest rate of hybridization (in 72 % of the strains). The results indicate that the LEE is a dynamic genetic entity, both the complete gene cluster and the individual genes. The genes that comprise this locus seem to be horizontally acquired (or lost) in an independent way and may control other functions in non-pathogenic E. coli lineages. In this way, horizontal transfer may allow the gradual stepwise construction of gene cassettes facilitating coordinate regulation and expression of novel functions.

Background: Pseudomonas aeruginosa is an important cause of severe infections in immuno-compromised patients. It has been commonly associated to repeated bronchial infections in patients with CF and nosocomial infections. Its pathogenic ability derives from the presence of several cell-associated and secreted virulence factors. Various genomic islands (PAPI-1, PAPI-2, PAGI-1, PAGI-2, PAGI-3, PAGI-4, and pKLC102) have been characterized in different P. aeruginosa strains from diverse habitats and geographical locations.Methods: The aim of this study was to characterize phenotypically and genetically a set of 100 P. aeruginosa strains isolated from 85 unrelated patients with pneumonia hospitalized in an intensive care unit. To assess the occurrence of these genomic islands (GEIs) we built a chromosomal DNA macroarray of all P. aeruginosa strains and it was then hybridized with labeled probes derived from specific genes according to each GEI. Additionally, PFGE patterns with SpeI, the frequency of virulence genes and susceptibility patterns of the strains were studied.Results: Our results showed that almost all strains presented up to eight virulence genes. Only two strains were pilA gene negative. The dendrogram based on PFGE patterns showed that our strains based on >80% of homology identified among them presented 49 restriction patterns. The strains showed a variable number of islands or a different genes composition among them, besides of a specific antimicrobial pattern indicating that each one of them was an unrelated isolate. In terms of the number of genomic islands per strain, 7 GEIs were found in 34% of the strains, 6 in 18%, 5 in 12%, 4 in 14%, 3 in 10%, 2 in 7%, and 1 in 4%; only one isolate did not present any GEIs. Conclusion:In conclusion our data show that theP. aeruginosastrains exhibited an epidemic population structure with high recombination rate resulting in a high frequency of GEIs.