The increase in available sequence data has uncovered genome complements that include from one to several chromosomes, some of which even endowed with a linear structure, and has paved the way for the debate on the elusive difference between chromosome and megaplasmid. While providing a number of clues on the evolution of bacteria and on the distribution of genes and operons, the knowledge derived from genomic sequencing still leaves open some fascinating questions: Why do some bacteria have a single chromosome, others more than one, and still others maintain relevant genes on mobile genetic elements? Why are virulence genes in some bacteria always confined on a plasmid?
Shigelal and enteroinvasive E.coli (EIEC) are intracellular pathogens causing bacillary dysentery, a severe enteric syndrome in humans. The pathogenicity of Shigella and EIEC depends on the presence of a large plasmid (pINV) carrying the genes required for the invasion of macrophages and epithelial cells.
Previously our research group have shown that that the pINV plasmid of EIEC is able to integrate into the host chromosome and that integration results in the silencing of all the pINV virulence genes. While these observations have led to the intriguing hypothesis that pINV integration might constitute a further strategy to ensure plasmid maintenance, relevant steps of the integration process and of the mechanisms responsible for the modification of the regulatory pathways are still undefined.
This project aims to understand whether the presence of virulence genes on a large plasmid in Shigella and EIEC is the result of an evolutionary pathway towards the optimization of gene expression, and to assess to what extent this peculiar genetic organization is required by the physiology of Shigella and EIEC strains.
Each year Shigella is responsible for 125 million cases of illness, mainly in low income countries. Despite the enormous clinical relevance of Shigella infections and the emergence of multiresistance strains, no vaccine has been as yet released for public use. Several recent studies have focused on the development of novel treatment strategies targeting virulence instead of bacterial viability, since this is regarded a highly effective approach to combat bacterial infections while minimizing the emergence of antibiotic resistances. The expression of virulence factors is not required for cell viability and therefore bacterial pathogens should be subject to less selective pressure to develop resistance to inhibitors of virulence determinants.
The understanding of the genetic and molecular mechanisms governing the stability of virulence plasmids in Shigella and EIEC has the potential to decipher the strategies adopted by these life-threatening human pathogens to fully express their invasive capacity. This project is focused on the analysis of the genome architecture in Shigella and EIEC. This is a relevant issue not only for basic but also for applied microbiology. In particular, the project tackles several questions concerning the genetic and evolutionary relationships between the virulent phenotype and the genome arrangement of Shigella and EIEC: how stringent is the need for virulence genes to reside on a plasmid? Are these genes always silenced when they become part of a chromosome? Can an avirulent phenotype be established by affecting plasmid stability? Considering the crucial role played by the virulence genes encoded by the pINV plasmid for interaction of Shigella and EIEC with the host cells, gaining new insight into bacterial genome organization and defining how genomes are arranged will contribute in the long term to the design of new therapeutic strategies targeting the stability of genome elements in a life threating human pathogen.