<P> 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 odour . </P> <P> Some species of bacteria kill and then consume other microorganisms, these species are called predatory bacteria . These include organisms such as Myxococcus 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 Vampirovibrio chlorellavorus, 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 . </P> <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>

Write an essay on the morphology of bacteria