Species of bacteria from two genera, Lactobacillus and Bifidobacterium, represent the vast majority of probiotic microorganisms. They are members of a larger group of bacteria known as Lactic Acid Bacteria (L.A.B.) because they produce lactic acid as a primary byproduct. It is important to note that not all L.A.B. are probiotics, and one important feature that distinguishes probiotic L.A.B. from non-probiotic L.A.B. is their ability to colonize intestinal surfaces. Since these surfaces are covered with mucus, they are usually referred to as mucosal surfaces. Most Lactobacillus species tolerate low levels of oxygen and can colonize higher in the intestinal tract in the small intestine while Bifidobacteria, being obligate anaerobes, colonize lower in large intestine where conditions are anaerobic.
Such colonizing action requires sticky strains that possess adhesive compounds, usually complex carbohydrates, on the outside of their cell walls. These adhesive compounds are governed by specific genes and seem to function more effectively in the presence of certain dietary ingredients such as foods or supplements rich in calcium (Ca2+) and magnesium (Mg 2+). The trick is to get negatively charged probiotic cells to stick to negatively charged epithelial cells, certain positive ions such as Ca2+ and Mg2+ help bridge this charge barrier. Other dietary substances such as lignan rich fiber may also play a role in benefiting adhesion. Therefore, effective colonization seems to depend on a combination of proper strain genetics along with synergistic dietary components. A good reference text here is: Handbook of Probiotics and Prebiotics, 2nd edition, by Yuan Kun Lee and Seppo Salminen, 2009 John Wiley and Sons, Inc.
Once a probiotic strain is colonized, meaning it will grow as a microbial colony containing many thousands of cells per colony site, it will continue to reproduce on intestinal surfaces and remain functional for a period of time after oral consumption stops. This “residence time” varies from strain to strain. Some strains have longer residence times than others – these can vary from several days up to 8 weeks or more. In the specific case of the Theralac® probiotic product, the Lactobacillus paracasei F-19 strain can be isolated from feces up to 8 weeks after stopping its consumption. By comparison, the Lactobacillus acidophilus LA-1 (NCFM) strain in Theralac® can be isolated from feces up to 10-14 days after consumption stops.
Competitive Exclusion of Undesirable Microorganisms
Competitive Exclusion (C.E.) is one of the principle mechanisms by which probiotics benefit intestinal health. There are a finite number of microbial attachment sites on the mucosal surfaces, when probiotics occupy a significant percentage of these sites bad bugs can’t occupy the same sites. Thus, the probiotics maintain a positive balance by C.E. and all is happy and healthy down deep. Since there are approximately 100 trillion total microorganisms in the human intestinal tract, a probiotic must contain multibillions of CFU (colony forming units) to have any chance of maintaining such a positive balance and, of course, the probiotic strains employed must be able to colonize.
Laboratory Tests that Predict Colonization
Two human epithelial intestinal cell lines are used in vitro to determine the potential for colonization: HT-29 and Caco-2. Adhesion is rated on a scale of 0, +, ++, +++, ++++ where more pluses indicate greater adhesion. As referred to above, a second indicator is fecal presence after stopping consumption of the strain. Below are the HT-29 and Caco-2 results for seven probiotic strains found in the Theralac® and TruFlora® probiotic products:
Probiotic Strain HT-29 Caco-2
Lactobacillus acidophilus LA-1 (NCFM) +++ +++
Lactobacillus rhamnosus LR-44 ++++ ++++
Lactobacillus paracasei F-19 +++ ++++
Lactobacillus plantarum Lp-29 (Lp-115) ++ ++++
Lactobacillus salavarius Ls-33 ++++ ++++
Bifidobacterium lactis BL-34 +++ +++
Bifidobacterium lactis Bi-07 ++ ++++
All of these strains demonstrate good adhesion in vivo based on 10 years of commercial experience. Customers buying probiotics should ask their suppliers for this type of information.
Probiotics that adhere are only transient colonizers. This means they must be replenished on a regular basis according to the manufacturers directions. Some require daily consumption on a continuous basis, others with patented delivery technology can be taken less frequently once colonization is established (usually after taking the product daily for 2 weeks): Theralac® and TruFlora® are two examples of patented probiotics that can be taken twice weekly once colonization is established, Theralac and TruFlora for more information.