Bacteria are the most common organisms on Earth, with a variety of shapes and sizes. They have a simple structure, but internal plasmids give them unique abilities. Bacterial DNA determines other characteristics such as pili, flagella, and cytoskeleton. Modern molecular genetics and advanced microscopy are helping us learn more about them.
Bacteria are the most common organisms in the world, with an estimated five non-illion (5 x 1030) bacteria on Earth. “Batterium” means “little stick” in Greek. They come in a variety of shapes and sizes: sticks, spheres, spirals, twists, blogs, etc. The largest bacterium is half a millimeter long (although this is highly atypical) and the smallest is only 0.3 microns in diameter. The typical size is between 0.5 and 5.0 microns. The characteristics of bacteria are among the most varied in every area of life: the relationship between two species of bacteria is often much less than the relationship between any two metazoans, say a human being and a snail.
Although they infect everything and can kill billions of other organisms, bacteria are relatively simple in structure. Though once thought of as simple pockets of cytoplasm, bacteria actually have complexity, but not as much as found in eukaryotic (large, complex, nucleated) cells. The basic structure is a capsule protected by a lipid membrane. Inside the capsule is the bacterial “blood”, the cytoplasm; plasmids, semi-independent inner loops of DNA that can bestow special abilities on bacteria in times of crisis; ribosomes, which manifest the “will” of bacterial DNA by building protein complexes; and an irregularly shaped body containing the bacterial DNA, called a nucleoid. These structural features of bacteria have been preserved for billions of years.
While relatively simple, the complexity of bacterial structure is fascinating and explains the different characteristics of bacteria that we see displayed in Nature. One of the key elements that explains the characteristics of bacteria are their internal plasmids. These short loops of DNA are traded like trading cards between bacteria and, like cards in Magic, give them unique abilities. For example, a plasmid may code for a protein that poisons all organisms in the area except bacteria and its immediate relatives. This can come in very handy for a bacterium trying to carve its own niche out of limited space and resources.
Other characteristics of bacteria derive from bacterial DNA, which is somewhat less modifiable than 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. Because bacteria are so small and diverse, we still have a lot to learn about how they work, and modern molecular genetics and advanced microscopy are showing the way.
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