It is difficult to assess the relevancy of proposed donors, because genes detected as potential HT have generally undergone amelioration (8) . The comparison of recently diverged genomes (species or strains) provides the opportunity to find recent HTs, for which corresponding homologous genes in the donor species may be detected (52) . Such a study is performed for five E.coli strains (two K12 strains: E.coli MG1655, E.coli W3110, one uropathogenic strain: E.coli CFT073, two enterohaemorrhagic strains: E.coli O157-H7 RIMD 0509952, E.coli O157-H7_EDL933) and two Shigella flexneri strains (S.flexneri 2a 2457T, S.flexneri 2a 301). These seven strains/ species have recently diverged, genome sizes are different and the proportion of horizontally transferred genes varies from one strain/species to another (14, 52) . For instance, only $40% of the non-redundant set of proteins is common to E.coli strains CFT073, 0157-H7 EDL 9333 and MG1655 (53) . These strains/species can be clustered in four groups with respect to phylogeny (Table 5) .