Wikipedia:Today's featured article/April 27, 2023

Bacteria are ubiquitous, mostly free-living microorganisms often consisting of one biological cell. They constitute a domain of prokaryotic microorganisms. Among the first life forms to appear on Earth, bacteria are present in most habitats. They are vital in many stages of the nutrient cycle, where they recycle nutrients. The study of bacteria is known as bacteriology, a branch of microbiology. Humans and most other animals carry millions of bacteria; most bacteria in and on the body are harmless or rendered so by the immune system, and many are beneficial. Several species are pathogenic and cause infectious diseases. Antibiotics are used to treat bacterial infections and are also used in farming, making antibiotic resistance a growing problem. Bacterial cells do not contain a nucleus and rarely harbour membrane-bound organelles. Their scientific classification changed after the discovery in the 1990s that prokaryotes consist of two groups of organisms that evolved from a common ancestor.