Enterohaemorrhagic Escherichia coli O157 and non-O157 serovars differ in their mechanisms for iron supply.

Clinical isolates of enterohaemorrhagic Escherichia coli, both O157 and non-O157 serotypes, were investigated for siderophore production, for growth promotion by haem and esculetin in iron-restricted conditions, for production of enterohaemolysin and esculin hydrolase, and for the presence of the chuA and ehx genes by PCR. As expected, all the strains produced enterobactin, but the prevalence of other factors varied among the serovars tested. None of the O157 and O26 strains produced aerobactin or "colibactin", whereas among other enterohaemorrhagic E. coli non-O157 serovars the frequencies of aerobactin and "colibactin" production were similar to those of commensal E. coli strains. The ability to use ferric esculetin for growth in iron-limited media was markedly more prevalent among non-O157 serovars and less prevalent among O157 strains compared with commensal E. coli strains. Almost all O157, O26 and O103 strains expressed enterohaemolysin, compared with only 50% of other non-O157 strains. Similarly, almost all O157 and O26 strains utilised haem as a host iron source; the frequency of haem use by other non-O157 strains was generally lower and variable among serovars, such that none of the O103:H2 isolates tested used haem as an iron source. The gene chuA, which encodes the haem transport protein ChuA and which is prevalent in O157:H7 strains, was only rarely noted among non-O157 serovars of enterohaemorrhagic E. coli, even among isolates that could use haem as an iron source. Overall our data demonstrate that O157:H7 and non-O157 serovars, in particular O26:H(-)/H11 and O103:H2, use distinctly different strategies for obtaining iron, and suggest two evolutionary distinct lines of enterhaemorrhagic E. coli.

ISPa20 advances the individual evolution of Pseudomonas aeruginosa clone C subclone C13 strains isolated from cystic fibrosis patients by insertional mutagenesis and genomic rearrangements.

Pseudomonas aeruginosa clone C strains, which chronically colonize the lungs of cystic fibrosis patients reorganize their genome structure. In this study, a novel member of the IS3 subfamily of IS elements, ISPa20, was detected which was specific for clone C subclone C13 strains. ISPa20, which was present in high copy number, mediated events of genomic reorganization. ISPa20 was inserted into P. aeruginosa backbone genes leading to adaptation to the cystic fibrosis lung habitat and into DNA acquired through horizontal gene transfer. Further on, large chromosomal inversions were mediated by ISPa20. In contrast to strains of other subclonal linages high rates of genomic rearrangements of subclone C13 strains were observed in vitro. The acquisition of mobile elements by P. aeruginosa clone C strains in the lungs of cystic fibrosis patients supports the chronic colonization by insertional mutagenesis and chromosome restructuring leading to microevolution within clone C that reflects macroevolution observed on the species level.

Impact of large chromosomal inversions on the adaptation and evolution of Pseudomonas aeruginosa chronically colonizing cystic fibrosis lungs.

Pseudomonas aeruginosa chronically colonizing the lungs of cystic fibrosis (CF) patients undergoes fast evolution leading to clonal divergence. More than half of the genotypes of P. aeruginosa clone C isolates exclusively from CF lung infection exhibit large chromosomal inversions (LCIs). To analyse the impact of LCIs, as a novel mechanism of bacterial adaptation, the underlying molecular mechanism was examined. Analysis of inversion breakpoints suggested an IS6100-induced coupled insertion-inversion mechanism. A selective advantage was created by insertion of IS6100 into wbpM, pilB and mutS which leads to common CF phenotypes such as O-antigen and type IV pili deficiency and hypermutability. Speciation in bacteria is accompanied by LCIs. Therefore adaptation by LCIs that allows persistence of P. aeruginosa in the CF lung and species diversification in that new ecological niche can serve as a model for bacterial genome evolution.