All bacteria are prokaryotic and lack membrane-bound organelles. Most are unicellular and reproduce by fission. Most species are heterotrophic, but some are photosynthetic or chemosynthetic. More than 4,000 living, described species of bacteria exist.


Alpha Proteobacteria many species, such as Rhizobium sp., are parasitic or mutualistic bacteria that live within eukaryotic hosts

Beta Proteobacteria diverse modes of nutrition; some species, such as Nitrosomonas sp., are important in the nitrogen cycle

Gamma Proteobacteria some species are photo-synthetic; some species derive energy by reducing ammonia and hydrogen sulfide; some species, such as Escherichia coli, are enteric

Delta Proteobacteria some species, such as Bdellovibrio sp., are predators of other bacteria

Epsilon Proteobacteria many species, such as Helicobacter pylori, are pathogenic

Gram-Positive Bacteria

Most, but not all, members of this diverse group are Gram-positive. One subgroup, the actinomycetes, is the source of many antibiotics. Many species, including actinomycetes, can cause disease.


These long, spiral cells have flagellated ends. Some cause serious diseases such as syphilis.


This group includes obligate internal parasities. Some cause diseases.


This group includes photosynthetic bacteria which are common on land and in water. The chloro-plasts of some protists probably evolved from cyanobacteria.

Lyngbya sp., a cyanobacterium

I Green algae and plants i

I Land plants i caryotes i Vascular plants i

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