Probiotics and Microflora

Max Sherman, RPh

Disclosures

US Pharmacist. 2009;34(12):42-44. 

In This Article

Inhabitants

The adult human gastrointestinal (GI) tract contains all three domains of life—bacteria, archaea, and eukaryotes.[11] Archaea are a group of prokaryotic and single-celled microorganisms, and while similar to bacteria, have evolved differently. Archaea were originally described in extreme environments but have since been found in all habitats including the digestive tracts of animals such as ruminants, termites, and humans.[13] Eukaryotes are organisms whose cells contain a limiting membrane around the nuclear material (the nucleus). Bacteria living in the human gut achieve the highest cell densities recorded for any ecosystem.[14] The vast majority belong to two divisions, the Bacteroidetes (48%) and the Firmicutes (51%). Bacteroidetes include a number of Bacteroides genera, which have yet to be encountered in any environment other than animal GI tracts. Firmicutes include the genera Clostridium, Lactobacillus, Eubacterium, Ruminococcus, and several others. In the first comprehensive molecular survey of the gut microbiota (normal microflora), 395 bacterial and one archaeal phylotype (bacteria defined by their ribosomal RNA gene sequence) were identified.[14] Thus, the gut microbiota is a tremendously diverse bioreactor. Eight divisions with divergent lineages are represented. This diversity is desirable for ecosystem stability. There appears to be a strong host selection for specific bacteria whose behavior is beneficial to the host. Cooperative activity by bacteria is required to break down nutrients and provide the host with energy. Populations of bacteria are remarkably stable within the human gut, which implies that mechanisms exist to suppress undesirable bacteria and promote the abundance of those that are needed.[11]

Bacteroides thetaiotaomicron is the prominent and remarkable bacterial species in the distal intestinal tract of adult humans. It is a very successful anaerobic glycophile ("sugarloving" microbe) whose prodigious capacity for digesting otherwise indigestible dietary polysaccharides is reflected in its genome. It encodes 241 glycoside hydrolases and polysaccharide lyases. This means that the organism has the ability to break down xylan-, pectin- and arabinosecontaining polysaccharides that are common components of dietary fiber.[15] When dietary polysaccharides are scarce, B thetaiotaomicron turns to host mucus by deploying a different set of polysaccharide-binding proteins and glycoside hydrolases. Other Bacteroides species include B vulgatus, B distasonis, and B fragilis. All play a role in the digestive process.

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