By comparing two strains of Escherichia coli (K12 and O157:H7) with an outgroup of Salmonella and Klebsiella species and analyzing the sets of genes which are present or absent in either of the three groups, we study the gene history of K12, in particular, since the respective divergences of these bacteria. Furthermore, by using a compositional method based on context bias, we evaluate not only recently imported genes but also deleted genes. In addition, we examine recent gene duplications in the two E. coli strains. It is found that turnover of DNA is high in E. coli and, more importantly, that turnover is highest for genes of low GC content. Although levels of import are high, most of the imported genes seem to be "junk" or have poorly understood functions. Nevertheless, selected genes do persist, and may even define some E. coli strains as pathogenic. Our results support the conclusion that some of the pathogenic islands in O157:H7 are likely to have been imported in recent time.