bacteria

Bacteria are single living cells so small that millions could fit on the head of a pin, yet they are everywhere — in soil, in deep ocean vents, on your skin, and by the trillions inside your gut. Most are completely harmless or even helpful; only a small minority cause disease. When people imagine 'germs,' they are usually picturing bacteria, but bacteria are far older and far more varied than that scary word suggests, having thrived on Earth for over three billion years.

A bacterium is a prokaryotic cell, meaning it has no nucleus and no membrane-bound organelles. Its single, usually circular chromosome floats freely in a region called the nucleoid, and the whole cell is wrapped in a cell membrane and, in most species, a tough cell wall. Bacteria reproduce by simply copying their DNA and splitting in two, a process called binary fission, which can happen every twenty minutes under good conditions. They come in shapes biologists name like a vocabulary list: spheres (cocci), rods (bacilli), and spirals.

Bacteria run much of the planet's chemistry. They recycle dead matter, pull nitrogen from the air into forms plants can use, ferment our food, and live in vast communities called the microbiome on and inside larger organisms. A useful correction to a common idea: bacteria are not just 'simple' germs to be wiped out. They are sophisticated chemical factories, the most numerous organisms on Earth, and life as we know it could not function without them. They are also one of the three great domains of life, distinct from archaea and from eukaryotes like us.