Probiotics are live microorganisms that have been considered to enhance health when administered in adequate amounts to the host. Probiotics have special characteristics that allow them to withstand the harsh conditions of the GI tract, including gastric acid and bile secretion, and can survive competition with other enteric microorganisms.
Most probiotics were originally isolated from healthy humans and expected to be safe for humans. Clinical efficacy of probiotics has been shown in the treatment or prevention of some GI inflammation-associated disorders including traveler’s diarrhea, antibiotics-associated diarrhea, necrotizing enterocolitis and pouchitis in a restorative ileal pouch. However, probiotics have not been shown as an effective treatment to maintain remission in patients with inflammatory bowel disease (IBD) and some probiotics (Lactobacillus species) caused adverse effects in some patients with Crohn’s disease. Therefore, the administration of probiotics is not always indicated for the treatment of GI inflammation-associated disorders.
Probiotics can stimulate the intestinal immune system. In recent studies, the immunostimulatory effects of some Lactobacillus species have been explored as live vectors for the delivery of other bioactive drugs. Lactobacillus gasseri and Lactobacillus johnsonii change the enteric mucosal cytokine profile by inducing the activation of human dendritic cells (DCs). Probiotics are used in treating many diseases but studies on the molecular mechanism are still being conducted.
Irritable bowel syndrome (IBS) is a functional gastrointestinal (GI) disorder that is characterized by chronic abdominal discomfort or pain associated with bowel habit change such as diarrhea and constipation, without obvious organic abnormalities. IBS is not a serious illness, but it has a high prevalence with about 10%-15% of the adult population being afflicted with the disorder. The high prevalence of IBS is accompanied by large societal economic burdens and negative effects on the quality of life in affected patients.
Although there have been many studies attempting to divulge the pathophysiology of IBS, the disease is still not clearly understood. Traditionally, an altered brain-gut axis has been accepted as a main pathogenetic mechanism of IBS, which is associated with a dysfunction of the GI autonomic nervous system. These alterations may lead to abnormal visceral hypersensitivity and aberrations of gut motility. Recently, additional potential mechanisms of IBS have emerged including alteration of gut microbiota and low-grade inflammation/immune activation. These factors might lead to abnormal motility and visceral hypersensitivity and contribute to the symptoms. Naïve gut microbiota plays important roles in the maintenance of gut homeostasis by direct bacteriocidal effects and the evolution of both innate and adaptive immune systems.
Gut microbiota is thought to play important roles in the pathogenesis of IBS. This is evident from the fact that IBS occurs more frequently after intestinal infection or antibiotics treatment. Studies have shown that the alterations of the intestinal microbiota are observed in IBS patients. Considering the relationship between alteration of gut microbiota and inflammation of gut, manipulation of gut microbiota by probiotics appears to be an ideal treatment modality for IBS. However, the beneficial effects and efficacy of altering gut microbiota by probiotics to improve the symptoms of IBS have not been consistent in clinical trials and therefore it remains uncertain as an effective treatment.
This review summarizes the effects of altered gut microbiota on the pathogenesis of IBS, the action mechanisms of probiotics in gut inflammation/immune system and the results of clinical trials of probiotics in IBS.
Conclusion and Future Directions
In recent years, numerous studies have been conducted to understand the pathogenesis and treatment of IBS. However, IBS remains one of the most difficult GI disorders to manage regardless of its benign nature. Recommending probiotics has long been an alternative to the conventional medicine for the treatment of many diseases and is an attractive treatment modality for IBS considering its low costs and favorable safety profiles. In the context of dysbiosis as the pathogenesis of IBS, probiotic treatment seems reasonable and possibly ideal as it restores the intestinal microbiota. Many studies have indicated that probiotics might play important roles in maintaining gut homeostasis by the modulation of immunity and increasing epithelial barrier function. Clinical studies and systemic meta-analyses have shown that some strains of probiotics have beneficial effects in selected patients.
However, a number of controversial issues regarding the roles of probiotics in pathogenesis and treatment of IBS remain to be clarified. To be accepted as a definite treatment option, it should be clarified that in which subset of patients probiotics are effective, which species are effective, which is preferred in terms of a single or mixed species and which dosage and duration of treatment are optimum. To solve this problem, future well-designed, large-scale clinical trials should be performed using proper diagnostic criteria. Researches to widen and deepen our understanding of the effects of probiotics on a basic physiological level are also required.