Single-celled microorganisms that lack a nuclear membrane, are bounded by a cell wall, and reproduce by binary fission. They occur in nearly every environment, from soil and water to the bodies of other organisms.
Textbooks taught for decades that bacteria outnumber human cells 10 to 1 — a 1972 rough estimate cited without scrutiny. A 2016 Weizmann study found about 38 trillion gut microbes vs. 30 trillion human cells: 1.3 to 1, not 10 to 1. The number was inflated tenfold.