bacteria are single-celled prokaryotic microorganisms making up one of the three domains of life, found in nearly every environment on Earth; the word is the plural of bacterium.
Bacteria are prokaryotes, meaning they lack a membrane-bound nucleus and the other enclosed organelles that define eukaryotic cells, and they reproduce chiefly by binary fission. They are among the oldest known forms of life, with fossil evidence dating past 3.5 billion years, and they occupy soil, water, hydrothermal vents, polar ice, and the bodies of plants and animals. Their ecological work is foundational rather than incidental: decomposing organic material, cycling nitrogen and carbon, and forming 📝symbiotic relationships with hosts. Some species support human digestion or synthesize vitamins; others cause tuberculosis, cholera, and strep throat. Microbiology and genetics have since made them instruments as well as subjects, used in food production, antibiotics, and biofuels.
The word is often used loosely for microbes in general, but it names a specific domain and excludes several things people group with it. Viruses are not bacteria and not cells at all — they cannot reproduce without hijacking a host, which is why antibiotics do nothing against them. Archaea, once classified as bacteria and still superficially similar, form a separate domain with distinct biochemistry. Fungi and protozoa are eukaryotes. Germ is a lay umbrella term covering all of these and is not a taxonomic category.
The name is a nineteenth-century scientific coinage. The German naturalist Christian Gottfried Ehrenberg introduced bacterium in 1838, taking it from Greek bakterion, "little staff," the diminutive of baktron, "stick, rod," because the first organisms observed under the microscope were rod-shaped. The English plural is recorded from 1847. Because bacteria is a classical plural, the singular bacterium remains the correct form for one organism, though the plural is widely used as a singular in casual speech.