Classification of Bacteria | Prokaryotes |
Proteobacteria
This large and diverse clade of gram-negative bacteria includes photoautotrophs, chemoautotrophs, and heterotrophs. Some proteobacteria are anaerobic, while others are aerobic.
Subgroup: Alpha Proteobacteria, Beta Proteobacteria, Gamma Proteobacteria, Delta Proteobacteria, Epsilon Proteobacteria,
Chlamydias:
These parasites can survive only within animal cells, depending on their hosts for resources as basic as ATP. The gram-negative walls of chlamydias are unusual in that they lack peptidoglycan. One species, Chlamydia trachomatis, is the most common cause of blindness in the world and also causes nongonococcal urethritis, the most common sexually transmitted disease in the United States.
Spirochetes
These gram-negative heterotrophs spiral through their environment by means of rotating, internal, flagellum-like filaments. Many spirochetes are free-living, but others are notorious pathogenic parasites.
Cyanobacteria
These gram-negative photoautotrophs are the only prokaryotes with plantlike, oxygen-generating photosynthesis. (In fact, chloroplasts likely evolved from an endosymbiotic cyanobacterium.)
Gram-Positive Bacteria
Gram-positive bacteria rival the proteobacteria in diversity. Species in one subgroup, the actinomycetes (from the Greek mykes, fungus, for which these bacteria were once mistaken), form colonies containing branched chains of cells. Two species of actinomycetes cause tuberculosis and leprosy. However, most actinomycetes are free-living species that help decompose
the organic matter in soil; their secretions are partly responsible for the “earthy” odor of rich soil. Soil-dwelling species in the genus Streptomyces (top) are cultured by pharmaceutical companies as a source of many antibiotics, including streptomycin.
Mycoplasmas are the only bacteria known to lack cell walls. They are also the tiniest known cells, with diameters as small as 0.1 μm, only about five times as large as a ribosome.
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