Traveller’s diarrhea: Lactobacillus, Bifidobacterium, Entero-coccus and Streptococcus species have been used prophylac-tically to prevent traveller’s diarrhea with limited success. Hilton et al and Black et al both demonstrated a reduction in the risk of traveller’s diarrhea with prophylactic use of L rhamnosus or the combination of S thermophilus,
L bulgaricus, L acidophilus and B bifidum, respectively. Conversely, a well-controlled study on British soldiers failed to show any protective effect of Lactobacillus fermentum or L acidophilus. These mixed results may be due to the use of ineffective strains of microbes. Thus, while the concept of protection from traveller’s diarrhea using probiotics remains appealing, there are limited data to support this use. Further clinical trials are needed in this area. Antibiotic-induced diarrhea: The use of antibiotics results in a severe attack on the normal gastrointestinal flora. Probiotics can be used to help the indigenous gastrointestinal flora withstand this assault. Vanderhoof et al evaluated the use of concomitant ingestion of Lactobacillus GG in children who received antibiotics for various acute illnesses. They found that diarrhea occurred in 25% of children in the placebo group compared with only 8% in the group that received the probiotic. In addition, the probiotic shortened the duration and severity of the diarrhea. Prevention of antibiotic-associated diarrhea with S boulardii (1 g/day) has also been shown.
Gastrointestinal infection: The colonic microflora and epithelial cells normally present barriers to invading organisms, but pathogens can become established when the integrity of either becomes compromised through stress, illness, antibiotic treatment, changes in diet or physiological alterations in the gut. Both Lactobacillus and Bifidobacterium species have been widely used in treating diarrheal diseases such as pseudomembranous colitis, antibiotic-induced diarrhea, traveller’s diarrhea and infantile diarrhea, with mixed results. However, this may be due to the use of ineffectual strains of microbes. For instance, feeding freeze-dried powders of L acidophilus had no effect in patients with pseudomembranous colitis, but another study using Lactobacillus GG showed a successful eradication of Clostridium difficile in patients with relapsing colitis. In a double-blind, placebo controlled trial, Saavedra et al reported that a combination of B bifidum and S thermophilus was an effective prevention strategy to reduce the frequency and severity of acute diarrhea in children. The yeast S boulardii has also been used successfully in the prevention and treatment of diarrhea associated with C difficile infection. Over 50 published clinical studies have examined the impact of probiotics on diarrheal diseases. The most successful studies have involved the use of Lactobacillus GG at a dose of 1×1010 viable organisms per day and the yeast S boulardii at a dose of 1 g/day. Overall, the evidence supports the clinical efficacy of Lactobacillus GG and S boulardii in reducing the severity and duration of diarrhea in both viral and bacterial enteritis and enterocolitis.
One of the major problems that oral probiotics face is how to ensure survival of the microbe during the passage from the mouth to the colon. Indeed, microbial strains used as probiotics must be both acid- and bile-resistant. Whether it is necessary for probiotic bacteria to have the ability to colonize the colon for long term survival is not known. For example, common commercial strains such as Lactobacillus bulgaricus and Lactobacillus acidophilus are not adhesive in humans. However, convincing mucosal adhesiveness has been shown for Lactobacillus plantarum strains 299 and 299V, Lactobacillus rhamnosus strains GG and 271, and recently, L acidophilus strain LA1, Lactobacillus salivarius, and Bifidobacterium longum infantis. It must be remembered that when using preparations of microbes that are unable to adhere to the colonic mucosa, continuous consumption is necessary to maintain any beneficial effects. However, even strains such as L rhamnosus, which does adhere to mucosa, gradually disappear by approximately two weeks after the end of administration of the bacteria. However, it must also be noted that adherent probiotics can persist on colonic mucosa even after the strain is no longer detectable in fecal samples. Thus, testing for the presence of a probiotic by testing fecal samples alone may severely underestimate the levels present within the colon.
It is estimated that more than 400 bacterial species inhabit the human intestinal tract. Among these, only 30 to 40 species constitute some 99% of the mass of intestinal flora (Figure 1). Although environmental factors and physiological interactions can modulate the distribution of the microflora, diet appears to be the major factor that regulates the frequency and concentration of individual species of microorganisms that colonize the gut.
Figure 1) Distribution of bacterial species in the gastrointestinal tract
Oral probiotics are living microorganisms that, upon ingestion, affect the host in a beneficial manner by modulating mucosal and systemic immunity, as well as improving nutritional and microbial balance in the intestinal tract. The main probiotic preparations on the market belong to a large group of bacteria designated as lactic acid bacteria (Lactobacillus, Streptococcus, Bifidobacterium species) that are important and normal constituents of the human gastrointestinal microflora (Table 1). However, studies are also investigating the potential probiotic roles of other microbes, such as yeast (Saccharomyces boulardii), which are not normally found in the gastrointestinal tract.