ESCHERICHIA COLI IS THE MODEL BACTERIUM
Although extreme bacteria are interesting and useful, more typical bacteria are the routine workhorses for research in molecular biology and biotechnology. The most widely used is Escherichia coli, a rod-shaped bacterium about 1 by 2.5 microns in size. E. coli normally inhabits the colon of mammals including humans (Fig. 1.6). E. coli is a gram-negative bacterium that has an outer membrane, a thin cell wall, and a cytoplasmic membrane surrounding the cellular components. Like all prokaryotes, E. coli does not have a nucleus or nuclear membrane, and its chromosome is free in the cytoplasm. The outer surface of E. coli carries about 10 flagella that
and these can be grown in plastic dishes or flasks using culture media containing growth factors and nutrients. Cell lines must be maintained at 37°C and require an atmosphere rich in carbon dioxide. Adherent cell lines stick to and divide on the plastic dishes, whereas suspension cells grow and divide in liquid culture. Most cell lines are one particular type of cell from a particular tissue, and many different cell lines have been grown from kidney, liver, heart, and so forth. The original cell lines cannot divide in culture forever. Primary cells, as they are called, can be maintained for only a short time. Using cancer cells overcomes this limitation since cancer cells do not stop dividing. These cell lines are immortal and can, in principle, be grown under the correct circumstances forever.
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