Probiotics have the potential to improve human health, and to prevent and treat a wide variety of diseases. Results from human clinical trials and scientific studies have confirmed the preventive and therapeutic effects of selected strains of microbes in viral and bacterial intestinal infections, and in positively influencing immunological parameters. However, several of these documented results need more rigorous research to be confirmed. Furthermore, it must be remembered that not all Lactobacillus or Bifidobacterium species are equal, and not all over-the-counter products contain the bacterial species listed on the label or, indeed, any viable bacteria at all. Thus, until major improvements occur in the regulation of labelling and quality assurance procedures for probiotic compounds, it is difficult to recommend which products to purchase. In addition, more adequately designed and properly executed clinical trials are necessary to carefully explore and characterize the therapeutic applications of probiotics.
Oral administration of probiotic compounds has been demonstrated to be well tolerated and proven to be safe in 143 human clinical trials occurring between 1961 and 1999. No adverse effects or events were reported in any of the 7526 subjects participating in these trials. However, rare cases of local or systemic infections, including septicemia and endocarditis due to Lactobacillus, have been reported. These infections have occurred in immunocompromised patients with aplasia, organ transplantation and human immunodeficiency virus infection. In most of these cases, the source of the infection was the commensal Lactobacillus flora, rather than an ingested bacteria supplement, suggesting that these bacteria can act as opportunistic pathogens. With regard to Saccharomyces infections, there have been few reports of fungemia due to Saccharomyces species, again, usually in immunocompromised patients receiving high enteral doses of Ultra-Levure (Biocodex, Montrouge, France) containing S boulardii (1.5 g/day). Although rare, these reports suggest that caution and further studies are necessary to assess the safety of probiotic bacteria for immunodeficient hosts.
The probiotic approach to the treatment of gastrointestinal disease remains controversial and will remain so until the mechanisms through which probiotic bacterial strains antagonize pathogenic organisms or exert other beneficial effects in the host are fully understood through well-planned scientific study. Furthermore, there are significant differences between probiotic bacterial genera and species. It is crucial that each strain be tested on its own or in products designed for a specific function. Much research is directed toward understanding the mechanisms of action of oral probiotics. The main areas being examined are receptor competition, whereby probiotics compete with microbial pathogens for a limited number of receptors present on the surface epithelium; probiotic release of antimicrobial compounds; probiotic-induced increased levels of mucin secretion, which acts to block pathogen binding to epithelial receptors; probiotic bacterial ‘priming’ of gut-associated lymphoid tissue; and immunomodulation of gut-associated lymphoid and epithelial tissue response. Probiotics are able to enhance the activity of the intestinal immune system through the stimulation of macrophage and natural killer cells, the proliferation of lymphocytes and the increase of secretory immunoglobulin A production, although the specificity of the secretory immunoglobulin A production was unknown. Selected strains of probiotics are able to alter mucosal and systemic immune function at many levels, including stimulating mucosal production of interleukin-10 and producing systemic T helper 2 reponses. However, it remains to be proven which, if any, of these mechanisms have a clinical benefit or how they alter the pathophysiology of gastrointestinal diseases. With regard to the pathophysiology of inflammatory bowel disease, one of the most widely accepted theories is that the inflammation results from a dysregulation of the immune system to normal gut flora. Thus, common probiotic species may contribute to chronic inflammation. However, in several animal models, not all gut microflora cause the same degree of inflammation. Indeed, there are several reports demonstrating that probiotic species tend to down-regulate pro-inflammatory cytokine release rather than stimulate secretion. Thus, while it is possible that some probiotic strains may contribute to chronic inflammation, some of the strains may actively suppress inflammation.
While the above studies validate the clinical efficacy of the VSL#3 compound in maintenance therapy of some inflammatory bowel diseases, previous human trials of other probiotic compounds have produced less convincing results. For example, while Rembacken et al demonstrated that a nonpathogenic strain of Escherichia coli (serotype 06:D5:H1), two capsules twice daily (2.5×1010 viable bacteria per capsule), was as effective as mesalazine (1.4 to 2.4 g/day) in maintaining remission in patients with ulcerative colitis, this study had several flaws. The patient group was heterogeneous with regard to the severity of the illness (mild to severe), and patients were treated with several different corticosteroid formulations as well as the study medication. Also, the doses of mesalazine used were relatively low, and only a very small number of patients remained in remission at the end of the study.
Inflammatory bowel disease: Very recent reports have suggested that probiotics may be beneficial in the maintenance of remission of ulcerative colitis and pouchitis. In a preliminary study, 15 patients with ulcerative colitis who were intolerant to or allergic to 5-acetylsalicylic acid were treated with a new probiotic preparation (VSL#3 [VSL Pharmaceuticals, United States]) using a combination of three species of Bifidobacterium (B longum, Bifidobacterium breve and B infantis), four strains of Lactobacillus species (Lactobacillus casei, L plantarum, L acidophilus and Lactobacillus delbruekii subsp bulgaricus) and one strain of Streptococcus (Streptococcus salivarius subsp thermophilus) (5X1011 cells/g/day). In this study, 75% (12 of 15) of patients remained in remission after 12 months of treatment. This clinical response was associated with a significant increase in the fecal concentration of Lactobacillus species, Bifidobacterium species and S thermophilus from day 15 of treatment. This preparation has two main innovative characteristics compared with other probiotic compounds — a very high bacterial concentration and the presence of a mixture of different bacterial species that has the potential to have synergistic associations. Further to these studies, a double-blind, randomized trial was carried out to investigate the efficacy of the VSL#3 preparation in the maintenance treatment of chronic, relapsing pouchitis. Forty patients were randomly assigned to receive VSL#3 (6 g/day) or placebo for nine months. All patients had chronic pouchitis (defined as a history of the need for continuing medical suppressive therapy and recurrence within a few weeks of discontinuing suppression) and were in remission (Pouchitis Disease Activity Index [PDAI] score of zero after open induction of remission therapy with antibiotics). Relapse was defined as an increase of two or more points in the clinical portion of PDAI. Clinical assessment and stool culture were done monthly, while endoscopic and histological assessment were done every two months. At the end of the study period, 17 of the 20 patients treated with VSL#3 remained in remission compared with zero of 20 in the placebo arm. Fecal concentration of Lactobacillus, Bifidobacterium and S thermophilus increased only in the VSL#3 group and remained stable for the entire nine months of treatment. One month after stopping probiotic treatment, fecal concentrations of these bacterial species returned to baseline. Within four months of removing active therapy, 100% of the responding patients had relapsed. No toxicity or adverse effects of this treatment were observed. Whether all of these bacterial species are necessary for the VSL#3 effects is unknown and remains to be shown. Indeed, a small study examining the ability of Lactobacillus GG alone, albeit combined with FOS, to treat refractory pouchitis showed efficacy in reversing macroscopic endoscopic alterations.
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.