Helper and satellite pathogenicity islands: the discovery of two novel subcellular elements with a huge impact on bacterial pathogenesis and evolution

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In recent decades, most pathogens have become progressively more contagious, more virulent and more resistant to antibiotics. This implies a rather dynamic evolutionary capability, representing a remarkable level of genomic plasticity, most probably maintained by horizontal gene transfer (HGT). HGT can, in a single step, transform a benign bacterium into a virulent pathogen. This is especially true for several notorious pathogens, including Staphylococcus aureus, used in this project as model, for which phage-mediated HGT enables relatively benign strains to cause lethal infections. Consequently, understanding bacterial horizontal gene transfer is vital to establishing how virulent clones emerge and disseminate, especially in the context of the global antibiotic crisis. Phages and pathogenicity islands make up a key component of the wider horizontal transfer map. Our results in support of this proposal have demonstrated that our current understanding of how the clinically relevant S. aureus pathogenicity islands (SaPIs) can be mobilised are extremely narrow in scope, and that we have underestimated the power of SaPIs as subcellular organisms. We describe here that a subset of SaPIs has evolved an intriguing strategy that promotes their high transfer by pirating other SaPIs. This elegant strategy allows intra- and inter-generic SaPI transfer. In this project, we will establish the molecular bases of this unprecedented strategy. Achieving this objective we will establish novel paradigms involving pathogenicity islands in bacterial evolution and virulence.