Escherichia coli Host Strains

To successfully perform molecular genetic techniques it is essential to have a full understanding of the properties of the various Escherichia coli host strains commonly used for the propagation and manipulation of recombinant DNA. E. coli is an enteric rod-shaped Gram-negative bacterium with a circular genome of 4.6 Mb (1 ). It was originally chosen as a model system because of its ability to grow on chemically defined media and its rapid growth rate. In rich media, during the exponential phase of its growth, E. coli doubles every 20–30 min; thus, during an overnight incubation period, a single selected organism will double enough times to yield a colony on an agar plate, or 1–2 billion cells per milliliter of liquid media. The ease of its trans-formability and genetic manipulation has subsequently solidified the role of E. coli as the host of choice for the propagation, manipulation, and characterization of recombinant DNA. In the past 60 yr E. coli has been the subject of intensive research and more is now known about these bacilli than any other organisms on earth.