Kingdom Bacteria comprises all organisms with prokaryotic cell structure: they have small ribosomes surrounding their nucleoids, but they lack membrane-bounded nuclei. In activity and potential for rapid unchecked growth, bacteria are unrivaled among living organisms. About 10,000 different forms have been described as "species;" most of them are cyanobacteria (Phylum Cyanobacteria). Their genes are easily passed from one to another; the set of genes, or genophore, is organized into thin (25 Å) fibrils that, when visible as a light region in electron micrographs, are called the nucleoid. The distinguishing traits of all the prokaryotes (and therefore of bacteria) are listed and compared with those of eukaryotes in Table I-2. Prokaryotes, unlike all members of the superkingdom Eukarya, lack pore-studded nuclei that contain chromosomes. Prokaryotes also lack membrane-bounded organelles such as mitochondria and plastids; they did not evolve by cell symbiosis.

Bacteria are the most hardy of living beings. Some can survive very low temperatures, well below freezing, for years; others thrive in boiling hot springs; and still others even grow in very hot acid or live by deriving hydrogen and carbon dioxide from rocks. By forming propagules such as spores - traveling particles of life that contain at least one copy of all the genes of an organism - many tolerate boiling water or total desiccation. Bacteria are the first to invade and populate new habitats: land that has been burned, volcanic soils, or newly emerged islands.