Bacteria are the largest group of single-celled organisms on Earth, with an incredible number of types, most of which are not yet classified. They can have very different characteristics and shapes, with some being harmful to humans while others are useful.
Bacteria are the largest group of single-celled organisms on Earth and make up most of the world’s known biomass. There are an incredible number of types of bacteria on the planet, most of which have not yet been classified. Some of what were once considered bacteria are, since the 1990s, now considered part of a separate domain, that of archaea.
Different types of bacteria can have very different characteristics, with the main common trait being the lack of a differentiated nucleus. The different phyla are generally classified into two different groups as having an outer membrane or lacking an outer membrane, with the number of phyla not yet determined. Some bacteria are incredibly small, such as those in the genus Mycoplasma, which are as small as the largest viruses, at about 0.3 micrometres. Others are relatively gigantic, such as Epulopiscium fishelsoni, which can actually be visible to the naked eye, though still very small at about 0.5 millimeters (0.019 inches) long.
There are two main types of bacteria in terms of their shape: bacilli and cocci. Cocci are spherical bacteria, while bacilli are rod-shaped. A third, smaller group is made up of bacteria that form in spiral formations, known as spirilla. In addition to these main groups, there are some more bizarre variations, such as what look like commas, plant leaves, or even long, elongated scales. Different types of bacteria tend to group together in different ways, with the same basic shape determining which larger group they belong to.
Bacilli, for example, can be singular or linked together in chains. The Bacillus itself is a single, slightly rounded, rod-shaped bacterium. Coccobacillus, on the other hand, is also a single knot, but is much more of an oval shape. Diplobacteria look like two Bacillus connected end to end. Streptobacteria are a long chain of bacilli, from end to end. There are countless types that have slight variations on this basic bacillus formation.
Cocci, likewise, have different configurations. Coccus itself looks like a single circle, while Diplococcus looks like two Coccus stuck together. Streptococcus, the bacterium responsible for strep throat, looks like a long chain of cocci together, while staphylococcus, the bacterium responsible for staph infection, looks like a bunch of grapes.
Of the many types of bacteria, only a few are harmful to humans. Most are relatively harmless, while some are actually useful. Many, for example, live in the digestive tract and help digest food. Other bacteria live on the skin and fight off various harmful fungi. Still others live around the mucous membranes, preventing other harmful membranes from establishing themselves. In fact, the average human body contains about ten times more bacterial cells than actual human cells, most of which do a good job for the body.
Some types of bacteria, however, are harmful to humans. The best known of these are probably Staphylococcus and Streptococcus. Others include Campylobacter, which is the leading cause of food poisoning among humans. Another well-known historical bacterium is Yersinia pestis, a bacterium found in fleas that transmits the bubonic plague to humans.
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