According to Portland Community College, bacteria actually belong to two kingdoms: Archaebacteria and Eubacteria. Archaebacteria comprise the bacteria that live in the most hostile environments on Earth, such as thermal vents. Eubacteria are the more common organisms, such as those found in the human digestive system.
These two kingdoms contain the only prokaryotes among the six kingdoms of life. Prokaryotes are single-celled organisms that do not have structures called organelles within their cells. All bacteria have cell walls and are the smallest cells among living things. Some bacteria make their own food, whereas others must consume substances in their environment for survival. The bacteria differ greatly from one another in structure and function.