E. coli cells containing fragments of human DNA can be stored in freezers indefinitely. When researchers are ready to retrieve DNA from the library, they revive the cells by bringing them back up to 37 degrees Centigrade - gut temperature.
To make many copies of the human DNA, the E. coli cells act as copiers. A few related cells containing the same bit of human DNA inside them are released into a rich, warm broth. Machines shake the broth vigorously so the cells have plenty of air and divide rapidly - about once every half hour. After a single night, a third of a teaspoon of broth contains billions of copies of E. coli - and, so, billions of copies of the particular fragment of human DNA they contain.
Human Genome Research Institute