It is worth noticing that most of the selected genes that are absent in K12 strains are involved in the pathogenicity of the other strains (52) . E.coli 0157-H7 is the strain exhibiting the greatest number of genes absent in K12 strains [about 1400 (54) ]. It has the greatest number of genes for which no homolog can be found (Table 5) . Moreover, we are unable to propose a donor for a great part of these genes (Table 5) . Many selected genes for E.coli 0157-H7 lie in the Ter region of the genome (between positions 2 000 000 and 2 500 000) in agreement with the published results (56). 