Probiotics for Optimal Nutrition: from Efficacy to Guidelines^,

Manipulation of the enteric microbiota with the goal of improving gut health is not a new strategy. This has been attempted through the use of antibiotics since the 1940s. However, despite identification of the first probiotic in 1917, progress in the therapeutic application of probiotics (health-promoting bacteria) has lagged behind consumer awareness and demand. Indeed, only in the past 5–10 y have scientists begun to understand the mechanisms of probiotic action and their associated applicability to specific disease conditions. This has opened up debate on the regulatory implications for probiotics and the subsequent commercialization of specific health claims, influenced by indications that factors sourced from certain probiotics may themselves be therapeutically useful. The 4 speakers discussed the targeted development of new probiotics and probiotic factors, the latest techniques to assess their efficacy, the expanding range of conditions with probiotic applicability, and the associated underlying changes in regulatory requirements.

In the first presentation, Dr. Gordon Howarth summarized the known mechanisms of probiotic action described to date, which include competitive inhibition of epithelial attachment, modulation of the mucosal immune system, modulation of enterocyte kinetics (apoptosis and proliferation), production of bacteriocins, and improved mucosal barrier function (1). However, most if not all probiotics exert various combinations of these mechanisms of action and it is highly unlikely that a single probiotic will exert a single mechanism of action. Nevertheless, often one mechanism of action will predominate. For example, the capacity of Escherichia coli Nissle 1917 to maintain epithelial barrier function has been well documented, whereas many bifidobacteria and lactobacilli are able to stimulate mucosal immune responses.

The key to preclinical efficacy studies lies in aligning the specific components of pathogenesis of a specific condition with the known mechanisms of candidate probiotic action. At present, both in vitro and in vivo (animal model) strategies are being utilized to screen the tens of thousands of probiotic candidates for therapeutic efficacy. Perhaps the greatest challenge to researchers over the next decade will be the development of more robust assay systems to predict probiotic efficacy in vivo. Dr. Howarth’s research supports chemotherapy-induced mucositis as a useful model system, because its pathogenesis has been well described, incorporating aspects of decreased barrier function, oxygen radical production, decreased enterocyte proliferation, increased apoptosis, and, in the case of methotrexate-induced mucositis, reduced folate bioavailability. Consequently, probiotic candidates capable of combating specific aspects of these pathogenetic features could be short-listed for preclinical (and subsequently, clinical) investigations. For example, the folic acid-producing probiotic, Streptococcus thermophilus TH-4, has demonstrated efficacy against methotrexate-induced mucositis in a preclinical setting and the thiol-producing probiotic, Lactobacillus fermentum BR11, has been shown to reduce indicators of acute bowel inflammation.

Paradoxically, the recent therapeutic success of the probiotic preparation known as VSL#3 (a combination of 4 species of lactobacilli, 3 species of bifidobacteria, and Streptococcus thermophilus) could actually inhibit the targeted development of new probiotics. VSL#3 has now entered mainstream medicine, exhibiting superior therapeutic properties to conventional treatment regimens for the treatment of certain variants of inflammatory bowel disease. However, in some instances, its success has prompted a rather haphazard approach to the development of probiotic combinations that may not necessarily combat the specific features of a given disease condition. Indeed, such an approach has the potential to identify probiotics capable of exacerbating certain disorders. For example, the widely available probiotic Lactobacillus rhamnosus GG has been demonstrated to increase the severity of NSAID-enteropathy in the experimental setting.

Increasingly, factors released from probiotic bacteria are being described for their potential to duplicate many of the mechanisms manifest by their parent bacterium. Indeed, in predominantly in vitro studies, cell-free probiotic supernatants have been demonstrated to increase epithelial resistance, stimulate mucosal immune responses, and decrease enterocyte apoptosis (2). Moreover, specific proteins responsible for the physiological effects are beginning to be identified. To this end, a growing number of bacteriocins have been described that are capable of decreasing the viability of pathogens such as uropathogenic E. coli and Listeria monocytogenes. Although somewhat overdue compared with the therapeutic development and application of antibiotics, the pipeline from strategic preclinical development of new probiotics through to their clinical application is now becoming better defined. Perhaps the next challenge will be the identification of biomarkers capable of noninvasively identifying probiotic efficacy in vivo.

Next, Dr. Ross Butler reviewed how noninvasive biomarkers can be used to assess gut function. The gastrointestinal mucosa can be stressed by a variety of environmental factors ranging from physiological, psychological, and environmental-, dietary-, and drug-related agents to a number of diseases, disorders, and syndromes. Currently, there are few functional biomarkers available for assessing gut function, particularly by noninvasive means. For example, small intestinal permeability can be determined in urine or blood using an estimate of the lactulose:rhamnose or lactulose:mannitol ratio, measured at a designated time after ingestion of known amounts of these sugars. This essentially provides a measure of loss of tight junction patency between epithelial cells. It also gives an indirect indication of a reduced surface area, implying damage to the mucosa. More recently, Dr. Butler’s laboratory developed a breath test to quantify the extent of damage to the small intestine using the brush border enzyme sucrase as a reporter enzyme (3). This provides an indicator of health status, degree of damage, and hence, absorptive function of the small intestinal villus.

Because the villus is the primary unit of absorption for macro- and micro-nutrients in the small intestine, its degree of compromise represents a useful functional biomarker that can be easily applied to probiotic intervention studies in disorders such as infectious diarrhea. This biomarker has demonstrated a correlation between depressed zinc absorption and degree of gut damage in celiac disease and environmental enteropathy. It has further been utilized to reveal the lack of efficacy of a probiotic in infectious diarrhea in indigenous Australian infants. It is recommended that biomarkers of the functional status of barrier function in the small intestine be incorporated into probiotic intervention studies. This will allow stratifications of populations with a preexisting enteropathy and the implementation of objective biomarkers of response to probiotic-based interventions.

The potential for specific probiotics to downregulate immune responses suggests applications for probiotics beyond that of gastrointestinal disease. The development of atopic disease is often associated with prematurity, mode of delivery, and family history of allergic diseases. The next speaker, Dr. Seppo Salminen, reviewed several studies demonstrating the capacity for specific probiotics to prevent allergic diseases, especially in cases with early food involvement. However, conflicting results have been reported, with a number of confounding factors, including the mechanistic properties of the probiotic under investigation, strain-specific properties, and appropriate study design and meta-analysis of data (4).

Several gut microbiota deviations have been linked to atopic disease development. The gut microbiota of the mother and breast milk microbiota can reveal detailed information about the interaction between the microbiota and host in allergic diseases. The gut microbiota is transferred from the mother to infant and breast milk microbiota continues the interaction, modulating microbiota development in the infant and thereby influencing infant health (5). Probiotic efficacy in the prevention of atopic disease is influenced by strain-specific properties, revealed by genomic studies, together with the impact of manufacturing on probiotic properties and food matrices as delivery vehicles.

Study design is also known to affect probiotic effectiveness. Studies focusing on perinatal intervention have generally been most successful. Indeed, the incorporation of nutrition counseling into such studies has been demonstrated to beneficially influence the outcome. Exposure to specific probiotics has an early impact on mother and infant, and factors such as duration of breastfeeding may also influence later health. Many meta-analyses and comprehensive reviews have been reported, but few, if any, take into consideration the strain-specific properties of probiotics and the impact of manufacturing processes or food matrices. Well-designed human intervention studies are a superior alternative to meta-analyses to better identify effective probiotic strains, probiotic strain combinations, or probiotic-derived factors capable of preventing the development of atopic conditions. Additionally, healthy, balanced nutrition should also be recommended to pregnant mothers with potential nutrition counseling to help achieve healthy goals for the mother and the infant.

The final speaker, Dr. Glenn Gibson, reviewed the promising evidence for the efficacy of probiotics for the prevention, treatment, and management of a range of infectious and noninfectious disorders. For example, in irritable bowel syndrome, Bifidobacterium infantis has been associated with normalization of the basal ratio of IL-10:IL-12 cytokines. The probiotic formulation VSL#3 was assessed in a study with 34 ambulatory ulcerative colitis patients and a remission/response rate of 77% was observed. In addition, a meta-analysis of acute diarrhea patients showed that probiotics significantly reduced antibiotic-associated diarrhea by 52%, travelers’ diarrhea by 8%, and diverse acute diarrhea of diverse causes by 34%. In the context of Clostridium difficile-associated diarrhea, 135 hospital patients were randomized to receive a probiotic drink containing Lactobacillus casei DN-114 001 or a placebo drink twice per day. Only 12% of the patients in the probiotic group developed diarrhea associated with antibiotic use compared with 34% in the placebo group. Importantly, none of the patients in the probiotic group had diarrhea caused by C. difficile compared with 17% of placebo patients. Increasingly, probiotic applications are being extended to nongastrointestinal disorders. For example, it has been reported that some of the gastrointestinal difficulties in autistic spectrum disorders could be ameliorated by ingestion of L. plantarum (6).

In summary, with the emergence of specific health claims for specific probiotics and probiotic combinations, regulatory guidelines will need to be substantially modified, with important implications for all clinicians and nutritionists. Moreover, the identification of specific, therapeutically beneficial factors in cell-free probiotic supernatants signals the requirement for a regulatory framework that goes far beyond the current “foodstuff” classification.