Bacteria are small single-celled organisms. Bacteria are found almost everywhere on Earth and are vital to the planet's ecosystems. Some species can live under extreme conditions of temperature and pressure. The human body is full of bacteria, and in fact is estimated to contain more bacterial cells than human cells. Most bacteria in the body are harmless, and some are even helpful. A relatively small number of species cause disease.

Bacteria are microorganisms that come in various shapes. They can be spheres, they can be rods, or they can be spirals. There are bacteria that are bad, that we call pathogenic, and they will cause diseases, but there's also good bacteria. As an example, in our digestive system, in the gut, we have bacteria that are very necessary to help our bodies function in a normal way. What's interesting about bacteria is that in our bodies we have 10 times more bacterial cells than we have human cells. Bacteria are also important in biotechnology. They are also important in that they, again, will help the body maintain itself in a healthy manner.

Bacteria display a wide diversity of shapes and sizes, called morphologies. Bacterial cells are about one-tenth the size of eukaryotic cells and are typically 0.5–5.0 micrometres in length. However, a few species are visible to the unaided eye—for example, Thiomargarita namibiensis is up to half a millimetre long and Epulopiscium fishelsoni reaches 0.7 mm. Among the smallest bacteria are members of the genus Mycoplasma, which measure only 0.3 micrometres, as small as the largest viruses. Some bacteria may be even smaller, but these ultramicrobacteria are not well-studied.

Most bacterial species are either spherical, called cocci (singular coccus, from Greek kókkos, grain, seed), or rod-shaped, called bacilli (sing. bacillus, from Latin baculus, stick). Some bacteria, called vibrio, are shaped like slightly curved rods or comma-shaped; others can be spiral-shaped, called spirilla, or tightly coiled, called spirochaetes. A small number of other unusual shapes have been described, such as star-shaped bacteria. This wide variety of shapes is determined by the bacterial cell wall and cytoskeleton, and is important because it can influence the ability of bacteria to acquire nutrients, attach to surfaces, swim through liquids and escape predators.

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