Life and Growth of Bacteria
Though bacteria occur in a wide range of environments, they are most abundant in moist places of moderate temperature. Many bacteria die if exposed to drought or extreme temperatures. Some bacteria, however, are able to withstand these conditions—certain types survive by changing into endospores. An endospore is an inactive cell, containing a bacterium's DNA, surrounded by a thick, protective coating. Some endospores can survive for centuries. Endospores change back into active bacteria when environmental conditions improve.
Most bacteria are heterotrophic; that is, they obtain food from other organisms. Others are autotrophic; that is, they manufacture their own food. Heterotrophic bacteria usually feed on dead organic matter, although some species are parasitic. Most autotrophic bacteria use energy from sunlight to manufacture food by photosynthesis. Heterotrophic and autotrophic bacteria can be either aerobic or anaerobic. Aerobic bacteria live in environments in which free (molecular) oxygen is present; anaerobic bacteria live in environments in which free oxygen is absent.
Bacteria generally reproduce by simple, or binary, fission (splitting). Under favorable conditions, an adult organism splits apart to form two new organisms. Bacteria multiply rapidly, often doubling their population every 20 minutes. Some bacteria reproduce by budding, a process in which a small outgrowth from a single parent develops into a new, identical organism. A few bacteria can reproduce sexually under certain conditions. In this case two bacteria unite to form a zygote, which then splits into new cells. Some bacteria form colonies that may become large enough to be seen with the unaided eye.