Thehuman body, consisting of about 100trillioncells, carries about ten times as many microorganisms in theintestines.[1][2][3][4] The metabolic activities performed by these bacteria resemble those of an organ, leading some to call gut flora a 'forgotten organ'.[5] It is estimated that these gut flora have around 100 times as manygenes in aggregate as there are in the humangenome.[6]
Bacteria make up most of the flora in thecolon[7] and up to 60% of the dry mass offaeces.[2] Somewhere between 300[2] and 1000 differentspecies live in the gut,[3] with most estimates at about 500.[4][5][8] However, it is probable that 99% of the bacteria come from about 30 or 40 species.[9]Fungi andprotozoa also make up a part of the gut flora, but little is known about their activities.
Research suggests that the relationship between gut flora and humans is not merelycommensal (a non-harmful coexistence), but rather asymbiotic relationship.[3] Though people can survive without gut flora,[4] the microorganisms perform a lot of useful functions, such asfermenting unused energy substrates, training theimmune system, preventing growth of harmful, pathogenic bacteria,[2] regulating the development of thegut, producingvitamins for the host (such as biotin andvitamin K), and producinghormones to direct the host to store fats. However, in some conditions, some species can causedisease by producing aninfection or increasingcancer risk for the host.[2][7]
The traditional view is that nometazoanphylum can break downcellulose by producing theenzyme cellulase. Instead,herbivores contain, in their gut, microorganisms which produce cellulase.[12][13] This is important because cellulose is the most common organic compound on Earth. About 33% of all plant matter is cellulose (the cellulose content ofcotton is 90% and that ofwood is 40–50%).[14] Recently, evidence has emerged that some animals do produce their own cellulase.[15] The question is not yet quite settled.[16]
↑Gibson RG (2004). "Fibre and effects on probiotics (the prebiotic concept)".Clinical Nutrition Supplements.1 (2):25–31.doi:10.1016/j.clnu.2004.09.005.
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