Unravelling Staphylococcus aureus pathogenicity islands role in lateral transduction

Abstract

Abstract Staphylococcus aureus pathogenicity islands (SaPIs) were the first mobile pathogenicity islands identified and they are the best-characterized members of the phage-inducible chromosomal island (PICI) family of MGEs. As they encode virulence and fitness factors, this dissemination contributes to bacterial evolution, host adaptation and pathogenesis. During the last two decades, the relationship between SaPIs and helper phages has been studied, representing one of the best characterized phage satellites.

Recently, SaPI-like elements have been characterised in other Gram-positive species, constituting a new family of mobile genetic elements (MGEs): the phage inducible chromosomal islands (PICIs), with SaPIs being the prototype member. PICIs are defined by the following features: exclusive attachment sites, unique genetic organisation, exclusive PICI genes, helper phage dependant induction, convergent mechanisms of phage interference and PICI DNA packaging into phage particles. Moreover, other cos PICI elements were shown to codify their own terSS, representing the first cos PICIs described that are not dependent on phage terS. Until now, this had only been described for pac PICIs. Likewise, the contribution of cos MGEs, including phages and PICIs, into the new mechanism of phage transfer, lateral transduction, was investigated. This intriguing contribution highlights the inadvertent importance that cos MGEs have in bacterial gene transfer. Likewise, this study shows for the first time that cos phages and, especially, cos PICI elements are involved in lateral transduction, representing a shift in the understanding of their influence in bacterial evolution. Also, this study shows for the first time that pac SaPIs that encode terS can be involved in lateral transduction, and transfer a large segment of bacterial DNA representing a shift in the understanding of their influence in bacterial evolution.The results presented in this thesis show how two examples of non-related members of the PICI family follow the same evolutionary convergent strategy to interfere with their helper phage. These findings could indicate that the described strategies might be widespread among PICIs and implicate a significant impact of PICIs mediated-virulence gene transfer in bacterial evolution and the emergence of pathogenic bacteria. Finally, our results demonstrated for the first time some proteins are involved in the phage replication cycle. Mutational studies revealed that deletion of the genes encoding these enzymes significantly reduced phage replication and the generation of infectious particle. Complementation studies demonstrated that the expression of these families of proteins, are required both in the donor and the recipient cells to allow normal phage reproduction.