Once considered a part of the plant kingdom, bacteria was the first organisms to be put into a separate kingdom called the Protista (in 1866). In 1938, the kingdom of Protista was divided into Monera and Protista.

The monera are single-celled organisms having no nuclear membrane (bacteria), and the protista are single-celled (or single-celled colonial) organisms which have a nuclear membrane (archaea), but do not have tissue.

Finally, in 2015 and after much dispute (often without consensus between biologist), the Kingdom of Monera was separated into two kingdoms, the Kingdom of Archaea and the Kingdom of Bacteria.

Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep portions of Earth's crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals. They are also known to have flourished in manned spacecraft.

There are approximately ten times as many bacterial cells in the human flora as there are human cells in the body, most of which are found in the gut flora, and on the skin. Although some are beneficial, the vast majority of these are rendered harmless by the immune system. However, several species of bacteria are pathogenic and cause infectious diseases, including cholera, syphilis, anthrax, leprosy, and bubonic plague. The most common fatal bacterial diseases are respiratory infections, with tuberculosis alone killing about 2 million people per year.

In industry, bacteria are important in sewage treatment and the breakdown of oil spills, the production of cheese and yogurt through fermentation, and the recovery of gold, palladium, copper and other metals in the mining sector, as well as in biotechnology, and the manufacture of antibiotics and other chemicals.

The possibility of more than a thousand bacterial phyla exists, however, currently, as of about the middle of the year 2020, 41 bacteria phyla have been accepted, another 89 are recognized, dozens more proposed and hundreds expected to be discovered. Some of the current major phyla are listed below.

Aquifice, 12 Genera, 27 Species

Bacteroids, 20 Genera, 130 Species

Chrysogenetes, 1 Genera, 1 Species

Canogacteria, 62 Genera, 78 Species

Chloribia, 6 Genera, 17 Species

Fibrobacter, 3 Genera, 5 Species

Firmicutes, 255 Genera, 2474 Species

Flavobacteris, 15 Genera,72 Species

Fusobacteria, 6 Genera, 29 Species

Planctomycetes, 5 Genera, 13 Species

Proteobacteria, 366 Genera, 1644 Species

Sphingobacteria, 22 Genera, 76 Species

Spirochaetes, 13 Genera, 92 Species

Thermomicrobia, 63 Genera, 78 Species

Verrucomicrobia, 2 Genera, 5 Species

Xenobacteria, 11 Genera, 29 Species