In nature, bacteria, scientifically a major group of organisms called prokaryotes, are the most basic forms of life on Earth. They are also some of the most numerous forms of life essentially occupying almost every environmental niche on the Earth and possibly even on other planets in the solar system. 

It is estimated that there are around 40 million bacterial cells in a gram of soil  and approximately five nonillion (5×1030) bacteria on Earth.

A diagram showing the structure of a typical bacteria. image: wikipedia

Bacteria, E.Coli as viewed with a scanning electron microscope. image: Rocky Mountain Laboratories, NIAID, NIH

Most bacteria are around  0.5–5.0 micrometres though some species can grow to almost a millimetre ( 1mm = 1000micrometers) and are visible to the human eye eg Thiomargarita namibiensis grows to around 750 micrometres.