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Bacteria are the world's most common organisms, with an estimated five nonillion (5 x 1030) bacteria on Earth. "Bacterium" means "small staff" in Greek. They come in a variety of shapes and sizes -- rods, spheres, spirals, helical, blogs, etc. The largest bacteria is half a millimeter long (though this is very atypical), and the smallest is just 0.3 microns across. Typical size is between 0.5 and 5.0 microns. The characteristics of bacteria are among the most varied in any domain of life -- the relatedness between two species of bacteria is often much less than the relation between any two given metazoans, say a human and a slug.
Though they infect everything and can kill off billions of other organisms, bacteria have a relatively simple structure. Though they were once thought of as simple cytoplasm bags, bacteria do actually have complexity, just not nearly as much as found in eukaryotic (large, complex, nucleated) cells. The basic structure is a capsule protected by a lipid membrane. Within the capsule is the bacterial "blood", cytoplasm; plasmids, semi-independent internal loops of DNA that can confer special abilities to bacteria in times of crisis; ribosomes, which manifest the "will" of the bacterial DNA by constructing protein complexes; and an irregularly shaped body containing the bacterial DNA, called the nucleoid. These structural characteristics of bacteria have been retained for billions of years.
Though relatively simple, what complexity there is to be had in the bacterial structure is fascinating, and accounts for the diverse characteristics of bacteria we see displayed in Nature. One of the key elements that accounts for the characteristics of bacteria are their internal plasmids. These short loops of DNA are exchanged like trading cards between bacteria, and like Magic cards, give them unique abilities. For instance, one plasmid may code for a protein that poisons all organisms in the area except for the bacteria and its immediate relatives. This can come in pretty handy for a bacteria trying to carve its own niche out of limited space and resources.
Other characteristics of bacteria come from the bacteria DNA, which is somewhat less modifiable than the plasmids. These include ultrastructural features such as the presence and number of pili (the organ used by bacteria to engage in conjugation with others of the same species), the number and size of flagella (important for movement in some species), the nature of the prokaryotic cytoskeleton (which determines the overall structure), and others. As bacteria are so small and diverse, we still have a lot to learn about how they work, and modern molecular genetics and advanced microscopy is showing the way.