In the newborn infant, lifelong intestinal flora are typically acquired during the neonatal period. Consequently, the infant tends to acquire the flora swallowed from the vaginal fluid at the time of delivery (1,2). There may be some additional acquisition of flora early in life, but once it is established and immune tolerance to the organisms is achieved, intestinal flora remains relatively constant throughout life. Because vaginal flora and intestinal flora are similar, an infant's flora may closely mimic the intestinal flora of the mother. Infants born by Caesarean section commonly acquire and are colonized with flora from the hospital environment and, therefore, flora in these infants may differ from maternal flora (Table 1).
Attempts to modify intestinal flora originated through the feeding of live organisms in fermented food products (3). Since that time, a number of attempts have been made to alter or augment intestinal flora to achieve a positive clinical benefit or reduce the risk of potentially harmful conditions. The most obvious means would be the administration of live organisms or probiotics (4–7). These are micro-organisms that when ingested, may have a positive effect on the prevention or treatment of a specific pathologic condition. Traditionally, probiotics are organisms of human origin. Biotherapeutic agents are micro-organisms that are used for the treatment of a specific disease or illness (8). Saccharomyces boulardii is an example. Because it is not a normal human organism, it is often not considered a probiotic. Prebiotics are substances that, when ingested, promote growth and establish beneficial micro-organisms (9,10). Examples of prebiotics include fructose oligosaccharides and, in infants, lactose. They are often designed to promote the growth of bacteria, such as Bifidobacterium, because they thrive in a lower pH environment and may reduce colonization by more aggressive or invasive organisms. Any of these agents can be added to foods to form what is known as a “functional” food. Functional foods, therefore, alter human physiology in a meaningful way to produce some type of health benefit. A perfect example is yogurt, which may contain probiotic bacteria.
Most probiotics are lactic acid bacteria (5). This is a large group of bacteria so named because they produce lactic acid as an end product of metabolism. Because there is much genetic diversity among lactic acid bacteria, very few of them have probiotic effects, and those that do differ markedly. This accounts for much of the inconsistency in data regarding the efficacy of probiotics. It is also the probable reason for the efficacy of some species of bacteria that work in some conditions but not in others. Lactic acid bacteria are commonly used as probiotics because of their long history of safety in food and dairy products and the absence of untoward effects over many years of experience (11).
Mechanisms of probiotics include competition exclusion, colonization resistance, and immune modulation. There is considerable controversy and many gaps in the data in this regard. We know that bacteria that adhere to the intestinal epithelium are much more effective at colonizing the gastrointestinal tract, thereby giving them a distinct advantage as probiotics (3,5,6,12). Many bacteria, when ingested, are killed by gastric acid and bile; bacteria that survive exposure to gastric acid and bile have a distinct advantage as probiotics (13). Adhesion to the mucosal epithelium may competitively inhibit the attachment of more pathogenic organisms by a process known as competitive exclusion. Historically, this has been considered the main mechanism through which probiotics produce their therapeutic effect. However, there are probably many others. Some probiotic bacteria are known to produce chemical substances, or bacteriocins, that may kill or inactivate other bacterial species (14). Bacteria may also modulate the intestinal immune system or alter its targeting in a way that produces a positive clinical effect (15,16).
Lactobacillus GG is perhaps the most studied probiotic bacterium. It is known to grow in bile and is relatively acid resistant, growing well in pH down to 3.0 (13,17). It has been shown to survive and grow in the intestinal tract based on fecal excretion data. It is a nonpathogenic organism in humans. It adheres tightly to intestinal mucosal cells in an in vitro model system and produces an antimicrobial substance that has been shown to inhibit the growth of micro-organisms.
Probiotics have been shown to be useful in the treatment of a variety of disorders (Table 2). A number of these disorders have a significant inflammatory component in the small intestine or colon. Consequently, there is ample evidence to suggest that probiotic bacteria may be useful in the treatment of a number of conditions in which intestinal inflammation is present. Perhaps the greatest area of study thus far has been in diarrheal disease. Probiotics have been shown to reduce the duration and severity of viral diarrheas (18–23). They have also been shown to reduce the risk of traveler's diarrhea (24–26). It is likely that the incidence of the transmission of diarrhea in daycare centers can be reduced by prophylactic administration of probiotic bacteria, although more work is needed in this area (27). Certain probiotic organisms are capable of reducing the risk of relapse of Clostridium difficile diarrhea after antibiotic administration (28–31). They may also be capable of reducing the incidence on non–C. difficile antibiotic-associated diarrhea, especially in children (32).
It is interesting to note that Lactobacillus GG, perhaps the most studied probiotic in diarrheal disease, is efficacious in both the reduction in severity and in duration of rotavirus diarrhea and other viral diarrheas (20). However, it appears to have little or no effect on bacterial diarrheas. These findings have recently been confirmed in a large multisite study in Europe (23).
In one recent study, Lactobacillus plantarum 299v was shown to reduce the risk of acute diarrhea in daycare centers when introduced into a daycare center where diarrhea was endemic (27). When L. plantarum 299v was given to half the children in daycare and placebo to the other half, incidence of diarrhea in all children in the daycare center dropped by approximately 70%. Unfortunately, there was no difference between treatment groups. For 2 months, all children received three meals a day at the center. The center closed for a month during holiday season, and during this month, the placebo group demonstrated a marked increased in reported diarrheal days; however, the treatment group did not. It is difficult to say in this situation whether L. plantarum 299v actually resulted in a reduction of diarrhea in this group of children, but the results suggest administration to half the children was adequate to reduce dissemination of diarrhea.
Lactobacillus GG has been shown to reduce the risk of traveler's diarrhea by 40% to 50% in people traveling from a developed country to a developing country with a much higher incidence of infectious diarrhea. However, the protective effect appears to be somewhat related to the destination, and the beneficial effect of the bacteria is greater after more prolonged exposure (24–26,33,34). Probiotics have also been shown to reduce the dissemination of viral diarrhea in hospitals (35).
Not all probiotic bacteria are effective in diarrheal disease. Studies comparing different probiotics have shown that certain well-studied organisms such as Lactobacillus GG are substantially more efficacious than other strains, particularly Lactobacillus acidophilus.
The mechanism by which probiotic bacteria prevents or ameliorates viral diarrhea is uncertain. It has been postulated that viral diarrheas, especially rotavirus diarrhea, are associated with a secondary bacterial invasion that results in prolongation of the illness. Therefore, probiotic bacteria might act by preventing such bacterial invasion through competitive exclusion. However, both Lactobacillus GG and L. plantarum 299v have been shown to upregulate the MUC-3 gene, which produces mucin in the gastrointestinal tract. Upregulation of mucin secretion could enhance protection from diarrheal disease in the gastrointestinal tract. This particular phenomenon is specific for certain bacteria, because it does not occur with L. acidophilus(36). In addition, Lactobacillus GG has been shown to stimulate the antibody response to a variety of stimuli including rotavirus disease, rotavirus vaccine, and typhoid vaccine (21,37). Immune enhancement could explain why Lactobacillus GG may be efficacious in acute diarrhea as well as explain its preventative role in traveler's diarrhea. Recently, studies in Italy have demonstrated that Lactobacillus GG significantly reduces the incidence of severe lung infections in children with cystic fibrosis when compared with placebo (38). This again suggests the likelihood that this particular probiotic organism may work through immune enhancement.
Lactobacillus GG has also been shown to be efficacious in the prevention of antibiotic-associated diarrhea. The incidence of loose stools was reduced from 49% to 17% during a 10-day course of antibiotic therapy with Lactobacillus GG when compared with placebo (32). Stools significantly loosened in the placebo group while on broad spectrum antibiotics for a variety of upper respiratory infections, although no loosening of stools was observed in the Lactobacillus GG group. The same was true for stool frequency. In a comparable study, there was no evidence of L. plantarum 299v having a comparable effect (39). Lactobacillus GG has also been shown to be efficacious in preventing the relapse of C. difficile diarrhea. Small numbers of patients in both adult and pediatrics in open-label trials have exhibited fewer relapses when treated with Lactobacillus GG. A recent double-blind, placebo-controlled study in adults has shown a marked reduction in the relapse in patients with C. difficile diarrhea after their first episode of C. difficile induced diarrhea (31). However, comparable results were not found in patients with multiple prior relapses, although further relapse could be achieved by increasing the treatment with Lactobacillus GG from 3 to 12 weeks.
Another intestinal inflammatory disease that has been shown to be ameliorated through use of probiotic bacteria is intestinal allergy (40,41). Probiotic bacteria have been shown to reduce translocation of antigens subsequently tightening the mucosal barrier. In rats with enterocolitis, L. plantarum 299v has been shown to reduce bacterial translocation, reduce plasma enterotoxin levels, and reduce myeloperoxidase activity, an indicator of intestinal inflammation (42). Therefore, both Lactobacillus GG and L. plantarum 299v appear to be able to reduce the translocation of both bacteria and antigens (41,43). Lactobacillus GG, has been shown to reduce a number of indicators of inflammation in allergic disease, including a reduction in α1-antitrypsin and tumor necrosis factor-α, and Lactobacillus GG has been shown to improve the clinical score of children with atopic dermatitis. It is likely that some of the beneficial effects of probiotic bacteria in allergic disease rest with the ability of these organisms to alter mucosal permeability. However, it is also possible that intestinal bacteria can direct the intestinal cellular immune system to a T-helper (TH)-1 or TH-2 response. It has recently been shown that exposure to infectious diseases early in life through attendance at daycare centers significantly reduces the risk of allergy, suggesting that infectious organisms including bacteria can, through their exposure to the immune system, reduce the risk of allergic disease (44,45).
The effects of probiotics on inflammatory diseases have been shown most elegantly in animal models. L. plantarum 299v has been studied extensively in a methotrexate model of intestinal inflammation (42). It has been shown to reduce inflammation, reduce bacterial translocation, and enhance mucosal regeneration in rats after administration of methotrexate when compared with a comparable placebo. In the clinical setting of small intestinal inflammation, particularly small bowel bacterial overgrowth, probiotic bacteria have also been shown to be beneficial. Patients with small bowel bacterial overgrowth have increased concentrations of all bacteria within the small intestine, with the presence of some invasive strains. Consequently, mucosal inflammation occurs, and an immune response to the presence of absorbed bacterial antigens may exacerbate the inflammation as well.
We have had experience with both the use of Lactobacillus GG and L. plantarum 299v in treating small bowel bacterial overgrowth (32). One patient with severe bacteria overgrowth–induced arthritis that was unresponsive to anti-inflammatory medications responded dramatically to both L. plantarum 299v and Lactobacillus GG and has continued receiving Lactobacillus GG for several years with excellent results. We have demonstrated significant improvement in nutritional parameters including serum vitamin E levels, albumin levels, zinc levels, and magnesium levels in another patient with short bowel syndrome using Lactobacillus GG. Lactobacillus GG has also permitted us to discontinue parenteral nutrition in an otherwise recalcitrant patient with short bowel syndrome, small bowel bacterial overgrowth, and intestinal inflammation.
There are theoretical good reasons why probiotics ought to be effective in the treatment of inflammatory bowel diseases such as Crohn's disease and ulcerative colitis. Normally, the intestinal immune system appears to be tolerant to autologous flora; however, this tolerance appears to be lost in patients with Crohn's disease (46). L. plantarum 299v was shown to be beneficial in improving the inflammatory lesion in an interleukin (IL)-10 knockout mouse model of Crohn's disease. This histologic score, IL-12 levels, and interferon levels were all improved through the use of L. plantarum 299v (47). It appears that probiotics may be useful, not only in the treatment of the intestinal inflammation in bacterial overgrowth diseases, but also in some extraintestinal manifestations such as arthritis. Perhaps, the same is also true for inflammatory bowel disease. It is likely that lactobacilli could result in reduction of inflammation by preventing the overgrowth of potentially pathogenic bacteria, as well as by maintaining the integrity of the gut barrier.
Inflammatory diseases most likely to be candidates for use of probiotic therapy in infants include necrotizing enterocolitis, probably from a preventative standpoint, as well as allergic enterocolitis. Preliminary studies have demonstrated significant efficacy in allergic enterocolitis, but necrotizing enterocolitis has not yet been studied (48). Older children with inflammatory disorders including ulcerative colitis, Crohn's disease, and small bowel bacterial overgrowth also should be studied in a controlled fashion. Presently, such studies are under way.
Probiotics appear to be useful in the treatment of a variety of diarrheal diseases. The pathophysiology of many of these diseases includes inflammatory changes in the gastrointestinal tract. Probiotics have been shown to be useful in a number of such conditions and therefore may be useful in ameliorating inflammation in the gut. These effects are likely to be highly species specific. Clinical studies must therefore be performed with each species of bacteria in every condition for which the organism is thought to be potentially efficacious.
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