Comparing DNA sequencing data provides another method for estimating the rates of HGT between bacterial species. Escherischia coli and Salmonella spp. are very close relatives, having diverged from a common ancestor to form the two species approximately 100 million years ago, which is very recent in prokaryotic evolution. Since their speciation, the two bacteria have transferred a large number of genes back-and-forth. In fact, approximately 18% (about 750 genes) of the E. coli genome has been obtained through HGT (Lawrence and Ochman, 1998).

This percentage provides an estimate of the highest possible frequency of HGT because:

• HGT is much more common within the prokaryotes than either between prokaryotes and eukaryotes or within the eukaryotes, and
• the more closely related two species are, the higher the frequency of HGT between them.

In spite of these predisposition factors that encourage gene exchange between E. coli and Salmonella spp., the rate of HGT is about one gene every 50,000 years (Ochman et al., 2000)