<P> As with bacterial classification, identification of bacteria is increasingly using molecular methods . Diagnostics using DNA - based tools, such as polymerase chain reaction, are increasingly popular due to their specificity and speed, compared to culture - based methods . These methods also allow the detection and identification of "viable but nonculturable" cells that are metabolically active but non-dividing . However, even using these improved methods, the total number of bacterial species is not known and cannot even be estimated with any certainty . Following present classification, there are a little less than 9,300 known species of prokaryotes, which includes bacteria and archaea; but attempts to estimate the true number of bacterial diversity have ranged from 10 to 10 total species--and even these diverse estimates may be off by many orders of magnitude . </P> <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>

Which type of media can be used to determine if a bacteria is motile