Bacteria and Archaea
Although a relatively small proportion of pathogenic, or disease-causing, bacteria receives the most public attention, many bacteria perform critical ecological functions. Arguably the most important bacterial contribution to life on Earth was the transformation of the atmosphere by ancient photosynthetic cyanobacteria that made life possible for those organisms that require oxygen. However, bacteria still provide important ecological services.
All organisms require nitrogen for the formation of proteins. However, the nitrogen found in the atmosphere is not in a form that most living organisms can use. Some bacteria, called nitrogen-fixing bacteria, can convert atmospheric nitrogen into ammonia and other nitrogen-containing compounds. These compounds can then be used by plants, and subsequently animals, to supply their nitrogen needs. In this way, bacterial species are responsible for modifying atmospheric nitrogen to a form that eukaryotic organisms can use.
Bacteria function in the cycling of other elements through their role as decomposers. A decomposer is an organism that breaks down dead materials and organic wastes, releasing energy within an ecosystem. Bacteria break down these materials to release carbon, nitrogen, phosphorus, and other elements back into the ecosystem. Without the decomposing actions of bacteria and other organisms such as fungi, the supply of chemical elements in the environment would diminish and the continuity of life would cease.
A growing body of evidence has shown the importance of the microbiome, the types of organisms living in the human digestive tract. The specific organisms present, or absent, in the intestinal microbiome can play a role in obesity, diabetes, cardiovascular disease, and diseases of the digestive system. In addition, differences in intestinal microbiome can explain some nutritional deficiencies. Understanding the role of intestinal bacteria in health is a major area of research.
Major Groups of Bacteria
- Proteobacteria are a diverse gram-negative group of bacteria that contain both oxygen-loving (aerobic) and oxygen-avoiding (anaerobic) species. Salmonella and E. coli are members of this group, as is the bacteria behind stomach ulcers.
- Chlamydias are parasitic, requiring an animal host for survival. Their cell walls lack peptidoglycan, and one species from this group causes the human illness chlamydia.
- Spirochetes move in a rotating spiral fashion with the help of a motile cell structure, the flagella. Not all are pathogenic, but Lyme disease is caused by a bacterium in this group.
- Cyanobacteria are photoautotrophs that play an important role in the carbon cycle, converting carbon dioxide into organic compounds using energy from the sun. Ancient cyanobacteria were the evolutionary precursors of modern chloroplasts, the organelle involved in photosynthesis, and also helped to oxygenate the early Earth.
- Gram-positive bacteria are responsible for strep infections, botulism, and anthrax.