The two kingdoms of bacteria are Archaebacteria and Eubacteria. The two kingdoms were formerly classified into one kingdom, Monera, but the two types of bacteria have shown enough variation to warrant reclassification into two separate kingdoms.
Although both Archaebacteria and Eubacteria are prokaryotes, meaning they do not have membrane-bound structures called organelles, the two kingdoms have so many differences between them that each belongs to a separate domain. The other four kingdoms, protists, fungi, plants and animals, belong to a single domain.
The Archaebacteria comprise organisms that live in hostile environments such as hot springs, salt water and deep sea vents. The Eubacteria include the bacteria that are more familiar to humans. The cyanobacteria, or blue-green algae, belong to the Eubacteria. Other Eubacteria cause human diseases such as strep throat and botulism.