Interactions with other organisms::Bacteria


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Interactions with other organisms Despite their apparent simplicity, bacteria can form complex associations with other organisms. 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 will grow on any other surface. However, their growth can be increased by warmth and sweat, and large populations of these organisms in humans are the cause of body odor.


Some species of bacteria kill and then consume other microorganisms, these species are called predatory bacteria.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> These include organisms such as Myxococcus xanthus, which forms swarms of cells that kill and digest any bacteria they encounter.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> Other bacterial predators either attach to their prey in order to digest them and absorb nutrients, such as Vampirovibrio chlorellavorus,<ref name="GromovBV">Gromov, BV. "Electron Microscope Study of Parasitism by Bdellovibrio Chorellavorus Bacteria on Cells of the Green Alga Chorella Vulgaris." Tsitologiya 14.2 (1972): 256-60. Print.</ref> or invade another cell and multiply inside the cytosol, such as Daptobacter.<ref name="pmid11542073">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> 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.<ref name="pmid19174136">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>


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 that consume hydrogen.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> The bacteria in this association are unable to consume the organic acids as this reaction produces hydrogen that accumulates in their 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 rhizosphere (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 compounds.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> This serves to provide an easily absorbable form of nitrogen for many plants, which cannot fix nitrogen themselves. Many other bacteria are found as symbionts in humans and other organisms. For example, the presence of over 1,000 bacterial species in the normal human gut flora of the intestines can contribute to gut immunity, synthesise vitamins, such as folic acid, vitamin K and biotin, convert sugars to lactic acid (see Lactobacillus), as well as fermenting complex undigestible carbohydrates.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref><ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref><ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> 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 probiotic dietary supplements.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

Color-enhanced scanning electron micrograph showing Salmonella typhimurium (red) invading cultured human cells



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.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> 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.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> 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.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref><ref name="Saiman">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

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Bacterial infections may be treated with antibiotics, which are classified as bacteriocidal if they kill bacteria, or bacteriostatic if they just prevent bacterial growth. There are many types of antibiotics and each class inhibits a process that is different in the pathogen from that found in the host. An example of how antibiotics produce selective toxicity are chloramphenicol and puromycin, which inhibit the bacterial ribosome, but not the structurally different eukaryotic ribosome.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> Antibiotics are used both in treating human disease and in intensive farming to promote animal growth, where they may be contributing to the rapid development of antibiotic resistance in bacterial populations.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> Infections can be prevented by antiseptic measures such as sterilizing the skin prior to piercing it with the needle of a syringe, and by proper care of indwelling catheters. Surgical and dental instruments are also sterilized to prevent contamination by bacteria. Disinfectants such as bleach are used to kill bacteria or other pathogens on surfaces to prevent contamination and further reduce the risk of infection.

Bacteria sections
Intro  Etymology  Origin and early evolution  Morphology  Cellular structure  Metabolism  Growth and reproduction  Genetics  Behavior  Classification and identification  Interactions with other organisms  Significance in technology and industry  History of bacteriology  See also  References  Further reading  External links  

Interactions with other organisms
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