Despite an apparent simplicity, bacteria can form complex associations with other organisms.
Contrast the ecological associations among microorganisms
- Due to their small size, commensal bacteria are ubiquitous and grow on animals and plants exactly as they would grow on any other surface. Their growth can be increased by warmth and sweat; in humans, large populations of these organisms are the cause of body odor.
- Pathogenic bacteria are a major cause of human death and disease, and cause infections such as tetanus, typhoid fever, diphtheria, syphilis, cholera, foodborne illness, leprosy and tuberculosis.
- Certain bacteria form mutualistic associations, such as close spatial associations that are essential for their survival; for example, interspecies hydrogen transfer.
- mutualism: Any interaction between two species that benefits both; typically involves the exchange of substances or services.
- parasitism: Interaction between two organisms, in which one organism (the parasite) benefits and the other (the host) is harmed.
- commensalism: Describes a relationship between two living organisms where one benefits and the other is not significantly harmed or helped.
Ecological Associations Among Microorganisms
Despite an apparent simplicity, bacteria can form complex associations with other organisms. This process is known as symbiosis. These symbiotic associations can be divided into parasitism, mutualism and commensalism. Due to their small size, commensal bacteria are ubiquitous and grow on animals and plants exactly as they would grow on any other surface. However, their growth can be increased by warmth and sweat; in humans, large populations of these organisms are the cause of body odor.
Some species of bacteria kill and then consume other microorganisms; these species are called predatory bacteria. These include organisms such asMyxococcus xanthus, which forms swarms of cells that kill and digest any bacteria they encounter. Other bacterial predators either attach to their prey in order to digest them and absorb nutrients, such as Vampirococcus, or invade another cell and multiply inside the cytosol, such as Daptobacter. These predatory bacteria are thought to have evolved from saprophages that consumed dead microorganisms through adaptations that allowed them to entrap and kill other organisms.
Certain bacteria form close spatial associations that are essential for their survival. One such mutualistic association, called interspecies hydrogen transfer, occurs between clusters of anaerobic bacteria that consume organic acids, such as butyric acid or propionic acid, and produce hydrogen; and methanogenic Archaea, which consume hydrogen. The bacteria in this association are unable to consume the organic acids since this reaction produces hydrogen, which accumulates in the bacteria’s surroundings. Only the intimate association with the hydrogen-consuming Archaea keeps the hydrogen concentration low enough to allow the bacteria to grow.
In soil, microorganisms that reside in the rhizospehere (a zone that includes the root surface and the soil that adheres to the root after gentle shaking) carry out nitrogen fixation, converting nitrogen gas to nitrogenous compound. This serves to provide an easily absorbable form of nitrogen for many plants that cannot fix nitrogen themselves. Many other bacteria are found as symbionts in humans and other organisms. For example, the presence of over one thousand bacterial species in the normal human gut flora of the intestines can contribute to gut immunity. Synthesis vitamins such as folic acid, vitamin K, and biotin convert sugars to lactic acid (see Lactobacillus), as well as fermenting complex undigestible carbohydrates. The presence of this gut flora also inhibits the growth of potentially pathogenic bacteria (usually through competitive exclusion), and these beneficial bacteria are consequently sold as probioticdietary supplements.
If bacteria form a parasitic association with other organisms, they are classed as pathogens. Pathogenic bacteria are a major cause of human death and disease and cause infections such as tetanus, typhoid fever, diphtheria, syphilis, cholera, foodborne illness, leprosy, and tuberculosis. A pathogenic cause for a known medical disease may only be discovered many years after, as was the case with Helicobacter pylori and peptic ulcer disease. Bacterial diseases are also important in agriculture, with bacteria causing leaf spot, fire blight, and wilts in plants; as well as Johne’s disease, mastitis, salmonella, and anthrax in farm animals.
Each species of pathogen has a characteristic spectrum of interactions with its human hosts. Some organisms, such as Staphylococcus or Streptococcus, can cause skin infections, pneumonia, meningitis and even overwhelming sepsis, a systemic inflammatory response producing shock, massive vasodilation, and death. Yet these organisms are also part of the normal human flora and usually exist on the skin or in the nose without causing any disease at all. Other organisms invariably cause disease in humans, such as the Rickettsia, which are obligate intracellular parasites able to grow and reproduce only within the cells of other organisms. One species of Rickettsia causes typhus, while another causes Rocky Mountain spotted fever. Chlamydia, another phylum of obligate intracellular parasites, contains species that can cause pneumonia, or urinary tract infection and may be involved in coronary heart disease. Finally, some species such as Pseudomonas aeruginosa, Burkholderia cenocepacia, and Mycobacterium avium are opportunistic pathogens and cause disease mainly in people suffering from immunosuppression or cystic fibrosis.