Bacteria reproduce by an asexual process called binary fission. In this sequence of events, the chromosome duplicates, the cell elongates, and the plasma membrane pinches inward at the center of the cell. When the nuclear material has been evenly distributed, the cell wall thickens and grows inward to separate the dividing cell. No mitotic structures (e.g., spindle, aster) are present as in eukaryotic cells.
Reproduction by binary fission lends a certain immortality to bacteria because there is never a moment at which the first bacterium has died. Bacteria mature, undergo binary fission, and are young again. In a sense, the original bacterium, though billions of years old, is still among us.
Once the division is complete, bacteria grow and develop the features that make each species unique. The interval of time until the completion of the next division is known as the generation time. In some bacteria, the generation time is very short; for others it is quite long.
For example, for Staphylococcus aureus, the generation time is about 30 minutes; for Mycobacterium tuberculosis, the agent of tuberculosis, it is approximately 18 hours; and for the syphilis spirochete, Treponema pallidum, it is a long 33 hours. The generation time is a determining factor in the amount of time that passes before disease symptoms appear in an infected individual.
One of the most remarkable generation times is the 20 minutes for Escherichia coli growing under optimal conditions. If you were to begin with a single rod at 8:00 A.M. this morning, two would be present by 8:20, four by 8:40, and eight by 9:00 A.M. Sixty-four rods would be present by 10:00 A.M. and 512 by 11:00 A.M. By 6:00 P.M. tonight, the culture would contain just over a billion rods.
One enterprising mathematician has calculated that if binary fission were to continue for 36 hours, or until 8:00 P.M. tomorrow night, there would be enough bacteria to cover the face of the earth!
Fortunately, the reproductive potential of a bacterium is never realized because of the limitations of the external environment. Thus we need never worry about being smothered with bacteria. Apparently bacteria are subject to the same controls as all other organisms on Earth.