| Class | Description |
| Actinomycetes | They produce branching, multicellular filaments that resemble fungi. We obtain many valuable antibiotics from them. |
| Mycoplasmas | They have lost their cell walls and are the smallest living cells (0.1-0.25 µm); live as parasites in plant or animal cells; cause one kind of human pleumonia and many animal diseases. |
| Spirochetes | They have spiral shape, with a distinctive set of flagella, called axial filaments; parasitic ones include those that cause syphilis, yaws, and Lyme disease. |
| Rickettsiae | Tiny parasitic bacteria that usually live inside other cells; carried by ticks or insects and trasmitted to mammals by bites; cause diseases such as typhus (from lice) and Rocky Mountain spotted fever (from ticks). |
| Cyanobacteria | Blue-green bacteria; photosynthetic; may form filamentous or clustered colonies; some colonies show division of labor: spore-producing, attaching, etc. |