| Group | Description |
| Archaeobacteria | The strangest and probably the most ancient evolutionary group of bacteria, living in the most extreme environments, where life is not expected to exist. |
| Eubacteria | The second evolutionary group of bacteria, including photosynthetic and chemosynthetic autotrophs. Most of them are harmless saprobes (using nonliving organic matter for food), some are normal residents of humans or other eukaryotes, and still others cause diseases such as botulism, diphtheria, gangrene, leprosy, plague, tetanus, tuberculosis, etc. |