bacteria Bacteria are distinguished from all other life forms by their prokaryotic (literally, ‘before the nucleus’) cell structure. These microscopic organisms, typically 1–5 micrometres (μm) long, are distinguished by the absence of sub-cellular organelles, such as a nucleus, mitochondria, and chloroplasts. These organelles occur in the eukaryotic cells of higher organisms. Bacteria are ancient organisms, being the first to evolve some 3.8 billion years (Ga) ago and they were the sole type of life for 70 per cent of the history of life on Earth. They are not, however, primitive. They are well adapted to their large range of habitats; they are the most numerous organisms on Earth, and their distribution defines the limits of the biosphere.
Although the variety of different bacterial cell types is limited (there are, for example, rods, cocci, spirilla, and filaments), this belies their vast metabolic diversity and their ability to grow under a remarkably wide range of conditions (for example, at −5 to 113 °C, at pH values ranging from 0 to 11, in near-vacuum or at pressures 1000 times greater than atmospheric, and in distilled water or in saturated salt solution).
Different types of bacteria obtain energy in different ways.(1) Some bacteria obtain energy from photosynthesis, by processes similar to those used by green plants. Blue-green algae, which are actually bacteria, are thought to have been the first to have evolved oxygen-forming photosynthesis; their activity was responsible for the formation of our oxygenated atmosphere and hence the evolution of complex metazoans (plants, animals), which require oxygen.(2) Some photosynthetic bacteria, however, neither produce nor require oxygen; these
anoxygenic photoautotrophs represent a more primitive form of photosynthesis. They use reduced compounds such as hydrogen sulphide and ferrous (Fe
2+) iron, oxidizing them respectively to sulphur and ferric iron. The anaerobic formation of ferric (Fe
3+) iron may have been important for the early formation of geological banded iron deposits.(3) Like animals, some bacteria can conduct aerobic respiration. These types are termed
aerobic heterotrophs.(4) Unlike animals, however, some
heterotrophic bacteria are not restricted to using oxygen for respiration and for many of these anaerobic bacteria oxygen is poisonous. Other bacteria can use instead the oxygen in other compounds such as nitrates (NO
3−), sulphate (SO
42−), carbon dioxide (CO
2), and even from metal oxides (for example, iron and manganese).(5) Other heterotrophic bacteria do not require any respiratory compound, but instead gain energy from splitting organic compounds into a reduced and an oxidized product, a processes called
fermentation. Fermentation products are of considerable commercial importance; for example, citric acid is used widely in foods and beverages for flavouring, and itaconic acid is used in the production of acrylic resins.
Heterotrophic bacteria (types (3)–(5) above) are the detrital specialists. Working together with other micro-organisms they degrade the organic compounds from dead plants and animals extremely efficiently and in this way return nutrients that are essential for further photosynthesis. They similarly drive the biogeochemical cycles of the major elements carbon, sulphur, and nitrogen. These microbial processes are optimized in sewage treatment plants, which enable humans to live at high population densities without contaminating each other and their local environment.(6) Some bacteria specialize in obtaining energy from inorganic rather than organic sources. This unique metabolism includes the oxidation of reduced metals and minerals, generally by using oxygen directly. Relatively little energy is obtained from these reactions, and these bacteria therefore have to process a large amount of material. During mining, minerals are exposed to oxygen. Bacterial oxidation can then be a major problem, especially in abandoned mines. In these circumstances, high concentrations of metals can be produced, together with inorganic acids (such as sulphuric acid), because bacteria oxidize sulphide minerals such as pyrite (FeS
2) to sulphate and ferrous iron. The acid waters are referred to as
acid mine drainage. Groundwater and local streams can be made very acidic (pH less than 2), which can kill most wildlife. These conditions are, however, optimal for the bacterial ‘miners’. Microbial mining reactions can, on the other hand, be turned to commercial advantage to extract metals from low-grade ores. Reduced metals and sulphides are also a major source of energy for bacterial communities at hydrothermal vents at ocean ridges. Reduced hydrothermal fluids are geothermal products, and these bacteria and the animal communities that feed on them are unique ecosystems.(7) An even stranger inorganic metabolism, inorganic fermentation, is conducted by some bacteria.
In the environment, bacteria tend to work as an interacting team. Although large bacterial populations are commonly present (about 2000 million bacteria per cubic centimetre in soil, for example), a much smaller number may be active. Small environmental changes can produce conditions that are more suitable to a portion of the non-active bacterial population. There can consequently be rapid changes in the bacterial community to maintain efficient processing of energy sources, and hence stable biogeochemical cycles. In addition, bacterial growth rates can be very rapid (the fastest are about one cell division every 20 minutes), providing further opportunity for bacterial populations to adapt to changing conditions. Associated with these high growth rates are high mutation rates, and hence the possibility for genetic modification and adaptation. Consideration of the extensive metabolic activity of bacteria and their capacity to adapt and evolve resulted in the concept of ‘microbial infallibility’. This concept implied that bacteria would adapt to degrade any chemicals that were artificially introduced into the environment, such as pesticides and herbicides (xenobiotics, strange to life), and hence few precautions would be required in their use. This proved to be a great oversimplification, because although bacteria can degrade many xenobiotics, some are directly toxic to bacteria and some produce toxic break-down products; others provide insufficient energy to support degradation.
Bacteria have adapted effectively to inhabit other organisms, both external and internal. They are commonly present in large numbers in the digestive systems of animals, particularly herbivores, where they assist in the breakdown of decay-resistant compounds such as cellulose from plant material. In ruminants, such as cows and sheep, this relationship has developed to such an extent that a separate stomach (the rumen) has evolved to provide space in which large microbial populations can develop and break down the cellulose in grass. The animal does not have the enzymes to break down cellulose, but survives by absorbing the microbial cellulose degradation products and digesting the bacterial cells that are produced. The importance of microbes to cows is demonstrated by the size of its rumen, which is between 100 and 150 litres. A similar symbiosis occurs in the roots of plants, such as peas and beans, where bacteria develop in small nodules. These bacteria are fed by the plant, and in return they supply the plant with ammonia, an essential nutrient. The bacteria obtain this ammonia from nitrogen in air by
nitrogen fixation, in a process unique to bacteria. Bacterial interactions with higher organisms are not, however, always benevolent, for some bacteria are major pathogens that cause a variety of diseases, some of which can be fatal. Fortunately, micro-organisms, including bacteria, have provided a source of antibotics with which to combat these diseases. Many bacterial pathogens have, however, developed resistance to antibiotics, and some bacterial diseases are now increasing in their prevalence. For example, tuberculosis, caused by
Mycobacterium tuberculosis, kills three million people every year.
Molecular genetic analysis has demonstrated two distinct types of prokaryotes: Bacteria (formerly Eubacteria) and Archaea (formerly Archaebacteria). Both of these represent the highest order of life, Domains. All eukaryotes, including plants and animals, exist in a single Domain, Eukarya. Two out of the three Domains of life thus contain exclusively prokaryotic organisms, and this underlines their great diversity. Interestingly, the organelles of eukaryotic cell, mitochondria and chloroplasts, belong to the Domain Bacteria, not Eukarya. This demonstrates that these organelles were originally free-living bacteria that have evolved a stable endosymbiotic relationship with eukaryotic cells. Bacteria are thus an integral component of all of us.
R. John Parkes
Bibliography
Madigan, M. T.,, Martinko, J. M.,, and and Parker, J. (1997) Brock biology of microorganisms (8th edn) Prentice Hall, London.
