It is well recognized that type IV secretion systems (T4SSs) play a key role in HGT, thus contributing to the generation of genomic diversity, the evolution of new pathogenic strains, the dissemination of antibiotic resistance genes and other virulence traits (Christie et al., 2005; Juhas et al., 2008) . Based on the organization of genetic determinants, shared homologies and evolutionary relationships, T4SSs can be classified into three major types: (i) type IVA systems resemble the archetypal VirB/ VirD4 system of Agrobacterium tumefaciens and are considered to be the paradigm of T4SS; (ii) type IVB systems resemble the Dot/Icm system found in Legionella pneumophila and Coxiella burnetii; and (iii) other T4SSs bear little or no homology to Type IVA or IVB systems, and their functions await further investigation. Representative of the third group are the recently described genomic island (GI)-type T4SSs that are found in a broad spectrum of Gram-negative pathogens (Juhas et al., 2008) . Ultimately, it is the identification of the GI-type T4SSs that brings insight into the mechanisms by which GIs transfer between bacteria (Juhas et al., 2009) .