Mobile genetic elements (MGEs) such as bacteriophages, transposons, plasmids, genomic islands and integrative conjugative elements (ICEs) could represent about 20% of the bacterial genome and play central roles in the dissemination of toxins, virulent factors, environmental adaptation and transfer of genetic information among bacterial species (Gogarten et al, 2002). Staphylococcus aureus pathogenicity islands (SaPIs) are MGEs with extreme importance in virulence intimately related to certain temperate (helper) bacteriophages, whose life cycles they parasitise (Novick et al, 2010). Recently, SaPI-like elements with similar characteristics have been found in others Gram-positive bacteria (Martínez-Rubio, submitted), such as, lactococci and pneumococci/streptococci plus a few elements in Enterococcus faecalis conforming along with the well-characterised SaPIs a new family of mobile genetic elements: the Phage-Inducible Chromosomal Islands (PICIs) (Penades & Christie, 2015). In this project, we have identified by an in silico analysis a considerable collection of putative PICI-like elements in different Gram-negative bacteria, such as Escherichia coli, Klebsiella pneumoniae, Salmonella enterica, Shigella bordii, Yersinia aleksiciae, among others. This result suggests that this family is widely distributed in both genera. The comparison of the different PICI-like elements shows a conserved organization where genes are clustered in functional modules: integration, regulation, replication and packaging. After the identification of these MGE, we performed the characterization in vitro and in vivo of the different modules. We have tested the functionality of the integrase and the replicase genes using a thermosensitive plasmid. Also, we have performed deletions in the key genes of each module to test for the essentiality of them. Overall, based on these findings we propose that all these elements along with the elements found in Gram-positive bacteria represent the discovery of a new and unique class of mobile genetic elements, the phage-inducible chromosomal islands (PICIs), which have had a broad impact on lateral gene transfer in the bacterial world.