<P> 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 . 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 . </P> <P> 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 . 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 . 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 . </P> <P> 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 . </P> <P> 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 . </P>

What do bacteria plants and animals have in common