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Bugs in Our Guts—Not All Bacteria Are Bad
How Probiotics Keep Us Healthy

(Released September 2006)

  by Leila Kiani  


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Microflora in the Intestinal Tract


Considering the high number of microbes in the intestinal tract, what are their effects?

Intestinal microflora are important for maturation of the immune system, the development of normal intestinal morphology, and maintenance of a continued and immunologically balanced inflammatory response. The microflora reinforce the barrier function of the intestinal mucosa, helping it to prevent attachment of pathogenic microorganisms and the entry of allergens. Some members of the microflora may contribute to the body's requirements for certain vitamins, including biotin, B-complex vitamin and vitamin B12. Alteration of the microbial flora of the intestine, such as may occur with antibiotic use, disease, and aging, can reduce its beneficial role (5).

A close up photo of some of these species (14):

close-up of lactobacillus Located throughout the digestive tract, Lactobacillae are especially abundant in the small intestine. Some of the main human varieties are:
Lactobacillus acidophilus, Lactobacillus rhamnosus, Lactobacillus plantarum, Lactobacillus brevis and Lactobacillus salivarius.

close-up of bifidobacteria Located in the entire digestive tract, Bifidobacteria are especially abundant in the large intestine. The system's first inhabitants, these species evolve according to age, diminishing progressively towards the end of life. Some of the main varieties are: Bifidobacterium breve, Bifidobacterium infantis and Bifidobacterium longum in children, and Bifidobacterium bifidum and Bifidobacterium longum in adults.

It is known that microbes in the large intestine complete the digestion process on any food components that were not digested in the small intestine, such as lactose in lactose intolerant people or fibers resistant to the enzymes they encounter in the small intestine. There is evidence of non-digestive microbial activities as well. Certain intestinal microbes are known to produce vitamins. Also, in studies done with special microbe-free laboratory animals, evidence is strong that without normal microbial populations, the immune system functions poorly, and resistance to pathogenic bacteria is greatly reduced. Other evidence suggests that intestinal microbes might act on pre-carcinogenic or mutagenic (capable of inducing genetic mutation) compounds. Depending on the specific microbe, mutagenic or carcinogenic activity can be either increased or decreased (5).

There are obvious advantages in skewing the balance of bacteria toward beneficial ones. Both lactobacilli and bifidobacteria are normal inhabitants of the healthy intestine. Although they are not the dominant genera in either the small or large intestine of adults (bifidobacteria are generally the dominant flora of breast-fed infants), they are non-pathogenic and their presence is correlated with a healthy intestinal flora. The metabolic end products of their growth are organic acids (lactic and acetic acids) that tend to lower the pH of the intestinal contents, creating conditions less desirable for harmful bacteria (5).

The probiotics that are marketed as nutritional supplements and in functional foods, such as yogurts, are principally the Bifidobacterium species and the Lactobacillus species. Probiotics are sometimes called colonic foods. Most of the presently available probiotics are bacteria. Saccharomyces boulardii is an example of a probiotic yeast (5).

Some beneficial effects of lactic acid bacteria consumption include: (i) improving intestinal tract health; (ii) enhancing the immune system, (iii) synthesizing and enhancing the bioavailability of nutrients; (iv) reducing symptoms of lactose intolerance; (v) decreasing the prevalence of allergy; and (vi) reducing risk of certain cancers (2).

The following describe the various bacteria and yeasts used as probiotics (5):

  • Bifidobacterium
    Bifidobacteria are normal inhabitants of the human and animal colon. Newborns, especially those that are breast-fed, are colonized with bifidobacteria within days after birth. Bifidobacteria were first isolated from the feces of breast-fed infants. The population of these bacteria in the colon appears relatively stable until advanced age, when it seems to decline. They are saccharolytic organisms that produce acetic and lactic acids without generation of CO2, except during degradation of gluconate. They are also classified as lactic acid bacteria (LAB).
  • Lactobacillus, Lactococcus and Streptococcus thermophilus
    Lactobacilli are normal inhabitants of the human intestine and vagina. Lactococcus lactis (formerly known as Streptococcus lactis) is found in dairy products and is commonly responsible for the souring of milk. Steptococcus thermophilus is also found in milk and milk products. It is a probiotic and used in the production of yogurt. Streptococcus salivarus subspecies thermophilus type 1131 is another probiotic strain. These are all classified as LAB.
  • Enterococcus
    Enterococci are part of the intestinal microflora of humans and animals. Enterococcus faecium SF68 is a probiotic strain that has been used in the management of diarrhea illnesses.
  • Saccharomyces
    Saccharomyces belongs to the yeast family, which include Saccharomyces cerevisiae, also known as bakers' yeast. The principal probiotic yeast is Saccharomyces boulardii. S. boulardii is normally a nonpathogenic yeast, which has been used to treat diarrhea associated with antibiotic use.
Thanks to Deborah Whitman for all of her help, without which this Discovery Guide would never have been written

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