Genetic maps serve as a foundation for more detailed information provided by DNA sequencing. Geneticists have now determined the complete nucleotide sequences of more than two thousand bacterial genomes, and many additional microbial sequencing projects are under way. The size and content of bacterial genomes are discussed in Chapter 14.

The availability of genome sequences has provided evidence that many bacteria have acquired genetic information from other species of bacteria—and sometimes even from eukaryotic organisms—in a process called horizontal gene transfer. In most eukaryotes, genes are passed only among members of the same species through reproduction (a process called vertical transmission); in horizontal gene transfer, genes can be passed between individual members of different species by nonreproductive mechanisms, such as conjugation, transformation, and transduction. Evidence suggests that horizontal gene transfer has taken place repeatedly among bacteria. For example, as much as 17% of E. coli’s genome has been acquired from other bacteria through horizontal gene transfer. Of medical significance, some pathogenic bacteria have acquired the genes necessary for infection, whereas others have acquired genes that confer resistance to antibiotics.