The human intestinal microflora is complex, with total counts of about 1,000 bacteria per gram of stool.1 Among this vast number of organisms are at least 500 species of anaerobes and many facultative organisms, within which are several species of lactobacilli and bifidobacteria.1 This intestinal microflora is a highly active society of organisms, possessing a diverse complex of enzymes that perform extremely varied functions, both beneficial and detrimental. The delicate, yet critical, balance maintained among this enormous bacterial population plays an important role in maintaining not only intestinal health, but the overall health of the patient.

Probiotics are viable organisms and supportive substances that improve intestinal microbial balance, such as lactobacillus acidophilus (L. acidophilus) and bioactive proteins.2 The empirical evidence that, for many years, linked the use of fermented dairy products such as yogurt and milk with the promotion of intestinal health is today well supported by modern science. The ability of the probiotic L. acidophilus to help prevent pathogenic bacteria from proliferating and healthy bacteria from becoming toxic is well documented.3-8 When the proper strain is chosen, it may help to maintain a population equilibrium, or balance, between the different forms of microorganisms, curtailing their potential overgrowth and pathogenicity.8-13 and characteristics essential to establishment and activity of L. acidophilus in the intestinal environment, such as bile resistance and intestinal adherence. The L. acidophilus NCFM strain has been thoroughly researched and is recognized for its ability to survive in the presence of bile or stomach acid and adhere to the intestinal mucosa. In addition, studies have demonstrated many beneficial effects of the L. acidophilus NCFM strain, such as cholesterol assimilation and reduction of pro-carcinogenic fecal bacterial enzyme activity.

Bifidobacteria is another probiotic naturally occurring in the human intestine, with bifidobacterium infantis being the first flora to colonize the intestines of newborns. Research studies have documented several beneficial effects of bifidobacteria when given to infants, such as its effectiveness against a specific strain of enteropathogenic escherichia coli,14 in prevention of enteric infections,15 and in decreasing the growth of candida albicans.16,17

What is it that enables L. acidophilus and bifidobacteria to help maintain the proper balance between the different forms of microorganisms in the intestine? They produce organic acids that reduce intestinal pH and thereby inhibit the growth of acid-sensitive bacteria, including many pathogenic species. Lactobacilli, which are frequently more acid tolerant than other organisms, produce lactic acid, hydrogen peroxide, and possibly acetic and benzoic acids.18 Acids produced by bifidobacteria include short-chain fatty acids (SCFAs) such as acetic, propionic and butyric acids, as well as lactic and formic acids.19,20 At optimal pH values they exert several inhibitory influences on bacterial cell growth.21,22 The most plentiful SCFA produced by bifidobacteria is acetic acid, which exerts a wide range of antimicrobial activity against yeasts and molds as well as bacteria.20,21

In addition to lactic and other acids, lactobacilli have the capacity to secrete numerous metabolites, or endotoxins, that kill pathogenic bacteria.11,23-27 A variety of antibacterial/anti-yeast substances have been isolated such as lactocidin, lactobicillin, lactobreven and acidolin.10,11,23,28 Because these substances are difficult to isolate and stabilize, their value can best be obtained through the administration of those strains known to secrete these agents as a part of their life cycle.28

A great deal has been learned in the last few decades through intense study on many different strains of L. acidophilus, bifidobacteria and other forms of healthful microorganisms. Because probiotic strains vary greatly, their impact hinges upon the specificity of the strains that are used and the method of culturing, packaging and handling of the product.18 The L. acidophilus NCFM strain, developed at North Carolina State University, is perhaps the most extensively researched L. acidophilus strain available. A multitude of research studies conducted at leading universities have been published documenting the many beneficial properties of the NCFM strain.1,29,31-52

A number of factors should be considered when selecting a culture of L. acidophilus. Because cultures exhibit host specificity, it is desirable to select an organism originating from the human intestinal tract.29 The NCFM strain is one that has been isolated from the human fecal flora. In addition, DNA studies have revealed great genetic variability within strains that are designated as L. acidophilus.30 Out of several groups of the organism, researchers determined that only strains from the A1 homology group, which includes the NCFM strain, should be classified as true L. acidophilus.

Additional factors that are important in selecting a strain that is able to function by surviving and growing in the intestinal tract include bile tolerance, bacteriocin production and ability to adhere to the intestinal wall. The NCFM strain has been shown to not only meet the strict criteria of purity and viability, but also to survive and implant in the gut and produce beneficial effects. 1,29,31-52

The primary objective of supplying L. acidophilus is to establish the bacterium in the intestinal tract. The stomach and intestinal tract represent a hostile environment that can easily discourage growth or survival of microorganisms. Although many lactobacilli survive selective pressures of this environment, flow rates of digesta through the small intestine would wash out any organism unable either to multiply rapidly enough to avoid dilution or to maintain their residence by physical attachment to the intestinal epithelium.31

According to one researcher, many lactobacilli products do not appear to live up to claims regarding human gastrointestinal adherence.18 Only a limited number of strains have actually been shown in vitro to adhere, including the NCFM strain. The ability of the NCFM strain to adhere to the intestinal wall has been demonstrated in several in vitro studies utilizing human intestinal cells.31-33 An in vivo study has shown that the NCFM strain, when consumed in milk, is capable of surviving and implanting in the intestinal tract of humans.34 The researchers determined this by analyzing the fecal flora of healthy males after consuming non-fermented milk containing the NCFM strain of L. acidophilus. Highly significant increases in bile-resistant facultative lactobacilli were seen in the group consuming L. acidophilus, indicating successful implantation. These numbers decreased after the milk feeding was stopped, although most subjects retained considerably higher counts than they had before feedings.

Bile tolerance is considered to be an important characteristic of L. acidophilus that enables it to survive, grow and exert its action in the small intestine.35 Strains that are able to grow and metabolize in the presence of physiological levels of bile should logically be more likely to survive intestinal transit.36 Although the degree of bile tolerance required for maximum growth of the organism in the intestinal tract is not known, it is important to select one having a high degree of bile resistance.29 There is a wide range in the ability of cultures to grow in the presence of bile.29 Studies have shown that the NCFM strain is capable of growing in bile concentrations of up to 3% 29,35-38 and an in vivo study demonstrated that the NCFM strain survives in the presence of human gastric juice.32

Suppression of undesirable intestinal bacteria through antimicrobial action continues to be a major benefit attributed to L. acidophilus. This intestinal antagonism is dependent on intrinsic properties of the lactobacilli that enable them to compete for, and maintain, their residence in the intestinal environment. Therefore, the ability to produce effective antimicrobial substances becomes an important consideration when selecting strains.39 Broad-spectrum inhibition has been clearly demonstrated for organic acids and hydrogen peroxide produced by L. acidophilus, including the NCFM strain.18,40

Bactericidal proteins with a more specific spectrum of antagonistic activity, termed bacteriocins, are produced by some strains of L. acidophilus. The bacteriocins of Lactobacillus species usually prove inhibitory to bacteria that are closely related to the producer strain or that compete for the same ecological niche.29,41,42 It is important to select a culture of L. acidophilus that will be able to compete and grow well in the presence of similar bacteria.29 Some studies have shown the NCFM strain exhibits significant bacteriocin activity against closely related Lactobacillus species, which provides it with an advantage in being able to establish and grow in the intestinal tract.29,40,43 Researchers have even isolated and studied a specific bacteriocin produced by the NCFM strain, called lactacin B, which is active against closely related lactobacilli such as L. bulgaricus and L. helveticus.40,44

Because many bacteriocins have been identified for lactobacilli, the practice of mixing L. acidophilus products together with other species groups (i.e., L. bulgaricus, L. leichmannii) may hinder therapeutic efforts due to significant bacteriogenic activity by these species groupings.18,41,42 By combining various strains together, as in a broad-spectrum probiotic, the beneficial strains may be rendered innocuous.18

Several studies have reported that ingestion of L. acidophilus can result in decreased serum cholesterol levels in humans and animals.35,45 Certain strains of L. acidophilus have the ability in vitro to assimilate cholesterol, as shown by the appearance of cholesterol in the cells during growth and decreases in the concentrations of cholesterol in the growth medium.29,45 This uptake of cholesterol occurred only when the culture was growing anaerobically in the presence of bile. These conditions required in the in vitro system for cholesterol uptake by L. acidophilus would also be expected to occur in the human intestinal tract. Such assimilation of cholesterol in the small intestine may be important in reducing the absorption of dietary cholesterol from the digestive system into the blood.35,45

If the purpose in using the dietary culture is to provide a beneficial influence on serum cholesterol levels, the culture should very actively assimilate cholesterol during growth under conditions existing in the intestinal tract.29 Data from one study showed wide variation in the ability to assimilate cholesterol among L. acidophilus cultures of human origin.29 Among 13 isolates of L. acidophilus, the NCFM strain ranked first in cholesterol assimilation over 16 hours of growth.

There is considerable interest in the metabolic activities of the intestinal microflora, especially in relation to the etiology of colon cancer.46 Epidemiological studies indicate a correlation between regular consumption of fermented dairy products and low incidence of colon cancer.1,46,47 To investigate this, several studies have involved measurements of fecal bacterial enzymes, such as B-glucuronidase, nitroreductase and azoreductase, which are known to catalyze reactions that convert procarcinogens to carcinogens.1,46-48 In one study, oral administration of L. acidophilus NCFM to meat-fed rats substantially reduced the activities of these fecal bacterial enzymes.47 Similarly, in a study with seven human subjects, it was found that supplementing the diet with L. acidophilus NCFM for one month significantly reduced fecal B-glucuronidase and nitroreductase activities.1 In a larger study with 21 human subjects, reductions of two- to four-fold in the activities of the three fecal enzymes were observed during a four-week period of L. acidophilus NCFM supplementation.46 Whether these changes in bacterial enzyme activity directly affect colon cancer risk is a matter of speculation.

To investigate the role of L. acidophilus NCFM in prevention of chemically induced colon tumors in rats, two groups of rats were challenged with a colon cancer inducing agent.49 The experimental group, which was fed a supplement of the L. acidophilus NCFM strain, showed a lower incidence of colon cancer after a 20-week induction period than the control group. This difference was not observed after 36 weeks, indicating that L. acidophilus may play a role in delaying the initiation of colon cancer.

Small bowel bacterial overgrowth (SBBO), well known to occur in end-stage kidney failure, is responsible for producing toxic amines such as dimethylamine (DMA).50 These toxic amines cause general chronic renal failure symptoms as well as target organ dysfunction, especially in the brain.51 In a recent study, L. acidophilus NCFM was administered to hemodialysis patients to determine if supplementation could modify SBBO, thereby reducing the toxic levels of amines in the blood.50 The patients treated with NCFM for at least one month showed significant reductions in serum DMA levels as well as levels of nitrosodimethylamine, a potent carcinogen. The researchers concluded that L. acidophilus NCFM changed small bowel pathobiology by modifying metabolic actions of SBBO, reducing generation of toxins and carcinogens with no adverse side effects.

For L. acidophilus to establish and react within the intestinal environment, the bacteria must be in a viable condition. The viability of the selected strain can be determined through third-party laboratory analysis of the number of colony forming units (cfu) per unit weight (grams) and bile resistance (oxgall bile test) for each batch produced. The laboratory assay results should be provided by the culture supplier, expressed as the ratio of bile resistant cfu per gram to total cfu per gram. A high ratio, above 90%, indicates high viability of the organism in the gastrointestinal tract.18 The importance of this analysis cannot be understated, as a study conducted in 1990 suggests that there are serious problems associated with some commercial probiotic preparations.53 Researchers analyzed 11 products claiming to contain L. acidophilus in powder, capsule and tablet form for number and type of bacteria present. Only two of the products were found to contain L. acidophilus, while the remainder contained L. casei. Problems with culture viability and contamination with enterococcus and clostridium were also found.

In addition to selecting viable strains, the method of packaging and storing of the product is important in maintaining viability.18 Temperature, moisture, light and air can all adversely impact viability. These variables can be controlled through the use of amber glass containers to prevent entry of oxygen, moisture and light. Most importantly, refrigeration of the product from the time of manufacture through delivery and storage is critical in ensuring the potency of the bacterial strains.

Bioactive proteins are another class of supportive substances that qualify as probiotics because they may beneficially affect intestinal microbial balance. Secretory immunoglobulin A (IgA) is the predominant antibody, or immune protein, the body manufactures and releases in external secretions such as saliva, tears and milk, and through the epithelial cells lining the intestine out into the lumen.54,55 It plays a major role in the defense mechanism on the surface of the intestine by preventing the absorption of, and by disposing of, microbial antigens.54,55 Mucosal IgA also neutralizes viruses and, in the case of bacterial infections, blocks the attachment of pathogens to mucosal tissues and cells.54,55 Other immune proteins, such as immunoglobulin M (IgM) and especially immunoglobulin G (IgG), may also be helpful because they are known to have remarkably similar specificities.54,56

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