The most widely studied prokaryotic organism, and one of the best genetically characterized of all species, is the bacterium Escherichia coli (Figure 7.18). Although some strains of E. coli are toxic and cause disease, most are benign and reside naturally in the intestinal tracts of humans and other warm-blooded animals.

Escherichia coli is one of the true workhorses of genetics: its twofold advantage is rapid reproduction and small size. Under optimal conditions, this organism can reproduce every 20 minutes; in a mere 7 hours, a single bacterial cell can give rise to more than 2 million descendants. One of the values of rapid reproduction is that enormous numbers of cells can be grown quickly, and so even very rare mutations will appear in a short period. Consequently, numerous mutations in E. coli, affecting everything from colony appearance to drug resistance, have been isolated and characterized.

Escherichia coli is easy to culture in the laboratory in liquid medium (see Figure 7.1a) or on solid medium in petri plates (see Figure 7.1b). In liquid culture, E. coli cells can grow to a concentration of a billion cells per milliliter, and trillions of bacterial cells can be easily grown in a single flask. When E. coli cells are diluted and spread on the solid medium of a petri dish, individual bacteria reproduce asexually, giving rise to a concentrated clump of 10 million to 100 million genetically identical cells, called a colony. This colony formation makes it easy to isolate genetically pure strains of the bacteria.

The E. coli genome is on a single chromosome and—­compared with those of humans, mice, plants, and other multicellular organisms—is relatively small; the common laboratory strain of E. coli has 4,638,858 base pairs of DNA in its genome. The information within the E. coli chromosome is compact, as there is little noncoding DNA between and within the genes and few sequences for which there is more than one copy. The E. coli genome contains an estimated 4300 genes, many of which have no known function. These “orphan genes” may play important roles in adapting to unusual environments, coordinating metabolic pathways, organizing the chromosome, or communicating with other bacterial cells. The haploid genome of E. coli makes it easy to isolate mutations because there are no dominant genes at the same locus to suppress and mask recessive mutations.

Wild-type E. coli is prototrophic and can grow on minimal medium that contains only glucose and some inorganic salts. Under most conditions, E. coli divides about once per hour, although, in a rich medium containing sugars and amino acids, it will divide every 20 minutes. It normally reproduces through simple binary fission, in which the single chromosome of a bacterium replicates and migrates to opposite sides of the cell, and the cell then divides, giving rise to two identical daughter cells. Mating between bacteria, called conjugation, is controlled by fertility genes normally located on the F plasmid. In conjugation, one bacterium donates genetic material to another bacterium, and genetic recombination integrates that material into the bacterial chromosome. Genetic material can also be exchanged between strains of E. coli through transformation and transduction (see Figure 7.7).