Bacteria That Inhabit The Human Gut
The estimated number of bacteria carried by each healthy human mainly in the
gut is 1014, a figure outnumbering body cells by ten to one. Such
population sizes remain gigantic even after accounting for the fact that these
bacteria belong to multiple species. Several hundred species have been found on
the skin, and there have been estimates of up to 1000 species in the human
gut.[2]
The bacterial inhabitants of the human gastrointestinal tract constitute a
complex ecosystem. Anaerobic bacteria predominate. The upper gastrointestinal
tract (the stomach and upper intestine) normally contains a sparse microflora;
most of these organisms are derived from the mouth/throat and pass through the
gut with each meal. Colonization of the upper intestine by coliform organisms is
an abnormal event and is characteristic of certain infectious pathogens such as
Vibrio cholerae and E. coli.
In contrast, the large intestine normally contains a luxuriant microflora
with total concentrations of 1011 bacteria/g of stool. Anaerobes such as Bacteroides, anaerobic streptococci, and Clostridia outnumber facultative anaerobes such as E. coli by a factor of 1,000.
Penetration of bacteria through the mucosal surface of the intestine is an
abnormal event; pathogens such as Shigella, Salmonella, and Campylobacter invade
in this way.
The same mechanisms that control the normal flora also protect the bowel from
invasion by pathogens. Gastric acid in the stomach kills most organisms that are
swallowed. Individuals with reduced or absent gastric acid have a high incidence
of bacterial colonization in the upper small bowel and are more susceptible to
bacterial diarrheal disease. Bile has antibacterial properties and thus may be
another factor in controlling the flora. Forward propulsive motility
(peristalsis) is a key element in suppressing the flora of the upper bowel.
Finally, the microflora itself, by producing its own antibacterial substances
(e.g., bacteriocins and fatty acids), stabilizes the normal populations and
prevents implantation of pathogens.
Human Gut Bacteria Linked To Obesity
There are two predominant bacterial groups in the human GI tract,
Bacteroidetes and Firmicutes. Recent research has discovered a relationship
between the balance of these groups and obesity.
Firmicutes bacteria, which include Bacillus, Clostridium, and Lactobacillus
species, are very efficient in metabolizing plant polysaccharides into
monosaccharides and short-chain fatty acids. These can then be absorbed and
converted to more complex lipids (fats) in the liver. The Bacteroidetes group,
which includes Bacteroides and Prevotella species, are not as efficient and can
contribute to a significant accumulation of fat stores in the body.[3]
References
1. Medical Microbiology. The University of Texas Medical Branch at Galveston
2. Bacterial Population Genetics in Infectious Disease By D. Ashley Robinson, Daniel Falush, Edward J. Feil
3. Laboratory Evaluations for Integrative and Functional Medicine By Richard S Lord, Richard S. Lord and J. Alexander Bralley
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