Definition

Bacteria are tiny living things made of a single cell with no nucleus, living almost everywhere, in soil, water, on skin, and inside the body, and while a few kinds like the ones causing strep throat make you sick, your body holds roughly as many bacterial cells as human cells, most of them helping rather than harming you. More precisely, bacteria are unicellular prokaryotic microorganisms with a peptidoglycan cell wall, a single circular chromosome, and no membrane-bound organelles, reproducing asexually by binary fission in shapes such as spheres, rods, and spirals; they act as decomposers, nitrogen fixers, and food producers, cause diseases including tuberculosis and food poisoning, and are killed by antibiotics, which have no effect on viruses. As one of the three domains of life, bacteria are distinguished from Archaea by peptidoglycan walls, ester-linked membrane lipids, and distinct ribosomal RNA and RNA polymerase, and they are the most metabolically diverse organisms known, including photoautotrophs, chemolithotrophs, and anaerobes; gene exchange by transformation, transduction, and conjugation, plus mobile plasmids and transposons, spreads traits including antibiotic resistance rapidly across species, and bacteria drive global biogeochemical cycles while forming biofilms that resist antibiotics and host defenses.