- Literature Search: To retrieve relevant studies for the systematic review, a comprehensive search was conducted using the following databases: PubMed, SCOPUS, The Egyptian Knowledge Bank (EKB), and Google Scholarly. These databases were selected due to their extensive coverage of biomedical literature. The search was conducted by employing appropriate keywords and search terms related to the topic of interest. The search strategy aimed to capture all relevant articles on oral probiotic supplementation in healthy adults.
- Inclusion Criteria: The following inclusion criteria were applied to identify eligible studies:
- Studies focused on oral probiotic supplementation in healthy adults.
- Studies that examined the health effects or outcomes of probiotic supplementation.
- Studies published in peer-reviewed journals.
- Studies available in English language.
- Exclusion Criteria: The following exclusion criteria were applied to filter out irrelevant studies:
- Studies conducted on animal models or in vitro settings.
- Studies focusing on specific medical conditions or diseases.
- Studies with a sample size less than the minimum threshold for statistical significance.
- Review articles, editorials, and conference abstracts.
- Study Selection Process: The retrieved articles from the databases were imported into reference management software for efficient handling. The titles and abstracts of the articles were initially screened to exclude irrelevant studies. Subsequently, full-text articles were obtained for the remaining studies that met the inclusion criteria. Two independent reviewers assessed the full-text articles for final inclusion in the systematic review. Any discrepancies in study selection were resolved through discussion and consensus.
- Data Extraction and Analysis: Relevant data from the included studies were extracted using a predefined data extraction form. The extracted information typically included study characteristics (e.g., author, year of publication), participant characteristics, intervention details (probiotic strains, dosage), outcomes measured, and key findings. The extracted data were synthesized and presented in a structured manner, allowing for a comprehensive analysis of the studies' results.
- Quality Assessment: The quality and risk of bias of the included studies were evaluated using appropriate assessment tools, such as the Cochrane Risk of Bias tool for randomized controlled trials or the Newcastle-Ottawa Scale for observational studies. This assessment helped in determining the overall strength of evidence and the potential for bias in the included studies.
- Synthesis of Findings: The findings from the included studies were synthesized and analyzed to identify common trends, patterns, or inconsistencies across the literature. This synthesis provided a comprehensive overview of the current knowledge on oral probiotic supplementation in healthy adults.
- Limitations: Any limitations or potential sources of bias in the included studies were acknowledged and discussed within the systematic review.
Mechanism of Gut microbiota in the pathogenesis of IBD and the immune system
IBD patients exhibit increased paracellular permeability [4, 14] and abnormalities in the expression and distribution of tight junction (TJ) proteins [14]. Additionally, the excessive production of pro-inflammatory TNFα has been linked to the regulation of TJs transcription, and individuals with ulcerative colitis often have impaired mucosal barrier structure [20]. The heightened intestinal permeability observed in IBD patients facilitates bacterial translocation across the intestinal mucosa [12]. The interaction between commensal microbes and specific epithelial receptors triggers a chronically active inflammation in susceptible individuals, perpetuating the disease [11]. Dendritic cells and macrophages recognize microbes through pattern recognition receptors [17], leading to their activation and subsequent initiation of pro-inflammatory signaling cascades. This activation results in the production of various cytokines [6] (such as IL6, IL12, and TGFβ) and the initiation of an adaptive immune response, characterized by the proliferation of Th1/Th17 cells and the secretion of IFNγ, TNFα, and interleukin-2 (IL2) [6, 14]. Patients with IBD have been found to have a reduced number of peripheral T regulatory cells, which exhibit decreased suppressive function in the mucosa [12].
In contrast, the human immune system produces cytokines that can either drive inflammation to protect itself or dampen the immune response to maintain homeostasis and facilitate healing after insult or injury [17]. Different bacterial species present in the gut microbiota have been shown to selectively influence the immune system to produce specific cytokines. For example, Bacteroides fragilis and certain Clostridia species promote an anti-inflammatory response [14], while segmented filamentous bacteria stimulate the production of inflammatory cytokines [18]. The gut microbiota can also regulate antibody production by the immune system. One mechanism involves the induction of B cells to class switch to IgA [18]. Typically, B cells require activation from T helper cells to undergo class switching. However, in an alternative pathway, the gut microbiota induces NF-kB signaling in intestinal epithelial cells, leading to the secretion of additional signaling molecules. These molecules interact with B cells, promoting class switching to IgA. IgA is an important antibody type utilized in mucosal environments like the gut [16]. Research has shown that IgA helps diversify the gut microbial community and aids in the elimination of bacteria that trigger inflammatory responses [20]. Ultimately, IgA maintains a healthy balance between the host and gut bacteria, contributing to a harmonious environment [20]. These cytokines and antibodies can also have effects beyond the gut, impacting other tissues such as the lungs [20].
IBD patients often experience increased paracellular permeability [4, 14] and abnormalities in the expression and distribution of tight junction (TJ) proteins [14]. Furthermore, the overproduction of pro-inflammatory TNFα has been associated with the regulation of TJs transcription, and individuals with ulcerative colitis commonly have impaired mucosal barrier structure [20]. This increased intestinal permeability observed in IBD patients facilitates bacterial translocation across the intestinal mucosa [12]. The interaction between commensal microbes and specific epithelial receptors triggers a chronically active inflammation in susceptible individuals, perpetuating the disease [11]. Dendritic cells and macrophages recognize microbes through pattern recognition receptors [17], leading to their activation and subsequent initiation of pro-inflammatory signaling cascades. This activation results in the production of various cytokines [6] such as IL6, IL12, and TGFβ, and the initiation of an adaptive immune response characterized by the proliferation of Th1/Th17 cells and the secretion of IFNγ, TNFα, and interleukin-2 (IL2) [6, 14]. Patients with IBD have been found to have a reduced number of peripheral T regulatory cells, which exhibit decreased suppressive function in the mucosa [12].
On the other hand, gut bacteria can produce metabolites that affect cells in the immune system, altering its function [19]. For instance, certain gut bacteria can generate short-chain fatty acids (SCFAs) through fermentation. SCFAs stimulate a rapid increase in the production of innate immune cells like neutrophils, basophils, and eosinophils [14]. These cells are part of the innate immune system and play a role in limiting the spread of infection.
Currently, the prevailing theory revolves around a cycle involving high-risk factors in a "chain reaction" type process. Genetically predisposed individuals eventually develop an immune response to the gut microbiota due to a loss of tolerance, leading to a triggered chronic inflammatory cascade [12, 14]. An inflammatory process ensues against the commensal microorganisms in the gut, perpetuated by the gut microbiota [14]. This dysbiosis is associated with bacterial translocation resulting from alterations in the mucosal barrier [14], ultimately affecting intestinal permeability and, as mentioned earlier, impacting both gut and circulating immunity. The exposure of the host's immune system to unusual antigens, along with an altered biofilm formed by commensal bacteria, provokes an inflammatory response [11]. Table 1 demonstrates that several bacterial species are implicated in the pathogenesis of IBD [11]. In addition to dysbiosis, the dysbiosis is associated with specific changes in the microbiota composition in relation to the affected part of the intestine, leading to local alterations in microbiota diversity [12]. Recent studies have also shown a correlation between microbiome composition and disease activity, revealing reduced microbiota diversity in different stages of IBD. Biopsy analysis has shown decreased diversity of Bacteroidetes during the acute phase of the disease compared to remission patients [12]. Furthermore, the location of the inflammatory disease has been found to influence microbiota composition, with higher Firmicutes diversity in subjects with colonic disease and reduced diversity in patients with affected ileum [12]. It is worth noting that the site of sampling can influence microbiome analysis data in IBD studies. Mucosal samples from patients with Crohn's disease show increased abundance of Enterobacteriaceae and reduced diversity of Fecalibacteria, while analysis of fecal samples yields opposite results [12].
In light of the reduced microbiome diversity, studies have demonstrated that a decrease in mucosa-associated Bifidobacteria and a concomitant increase in E. coli and Clostridia [15] can be associated with bacterial invasion (biopsy) [14]. Additionally, a decrease in Bacteroidetes and Firmicutes species is associated with a decrease in anti-inflammatory effects due to the deficiency of butyrate, a product of these bacteria known for its anti-inflammatory action [14]. It is worth mentioning that a peculiar event occurs in cases of IBD due to the nature of the disease, its treatment, and frequent hospital visits. Patients tend to develop "superimposed" recurrent Clostridioides (Clostridium) difficile infections [15]. It is important to note that in contrast to associated dysbiotic C. difficile proliferation, the patient acquires the infection on top of the preexisting disease (superimposed). The clinical significance here involves two main mechanisms:
I. Aggravation of an already dysbiotic gut flora with the introduction of different strains and increased bacterial volume, further shifting the dysbiosis toward disease progression and exacerbating symptoms [15].
II. Obscuring disease relapse, allowing practitioners to administer more therapeutic and remissive therapies (immunosuppressants), resulting in early symptomatic resolution but potentially leading to late complications and disease progression [15]. The immunotherapy can further contribute to dysbiotic events, particularly as C. difficile does not respond to traditional antibiotic therapy, which can create a more dysbiotic gut environment, aggravating or activating the chronically triggered cascade system [12].
Pouchitis is characterized by non-specific inflammation of the ileo-cecal reservoir. Symptoms associated with pouchitis include increased stool frequency and liquidity, abdominal cramping, urgency, tenesmus, and pelvic discomfort [10]. Rectal bleeding and extra-intestinal manifestations may also occur. The preferred procedure for pouchitis is Proctocolectomy with ileal pouch-anal anastomosis (IPAA) +/- ileostomy. In the majority of cases, the etiology and pathogenesis of pouchitis remain unclear, and patients are labeled as having idiopathic pouchitis [10]. Risk factors, genetic associations, and serological markers of pouchitis suggest a significant role played by the close interaction between the host immune response and the pouch microbiota in the etiology of this idiopathic inflammatory condition [10, 11]. It has been observed that dysbiosis, along with genetically mediated innate immune propagation against the microbiota, contributes to pouchitis [10]. While surgical intervention remains the mainstay treatment for pouchitis, systematic reviews have indicated that the use of VSL#3 (a probiotic species) was more effective than placebo in maintaining remission [10]. According to the European Crohn’s Colitis (ECCO) guidelines, VSL#3 is effective in maintaining antibiotic-induced remission and in preventing pouchitis onset.
Table 1. Pathogens involved in inflammatory bowel diseases. Though the IBD is not caused by a single bacterial species, evidence favor the implications of a few bacterial species in its pathogenesis [11].
|
Pathogens
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Mechanism involved in pathogenesis
|
|
Protective role against IBD
|
|
|
Intestinal parasites
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Modulation of innate and aquired hosts' immune response which keeps mucosal inflammation in check
|
|
H. pylori
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- Increase in IL-18 prduction.
- Enhanced immune tolerance.
- Accumulation of suppressive regulatory T-cells (Tregs).
- Reduces gastric secretions of leptin which has a pro-inflammatory effect.
|
|
Predisposes to IBD
|
|
|
Giardia duodenalis
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- Impairment of biofilm architecture.
- Destrcution of mucus coating of the epithelium.
- Damage of epthelial cell barier, physiology, and survival.
- Induction of bacterial dysbiosis
|
|
Enterohepatic helicobacter sp.
|
- Regulates the switchingf of a 'healthy' colonic microbiota to IBD predisposing dysbiosis.
- Chronic infection with these sp.
|
|
Campylobacter
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- in particular C. Concisus increase the risk for IBD.
- production of inflammatory cytokines.
- C. concisus has potential to invade Caco2 cells and secrete cytolethal distending toxins (CTD)-like toxin.
- Upregulation of otherwise low level of TLR-4 expression in intestinal epithelium, which keeps the gut mucosal system in a state to tolerate commensal intestinal bacteria.
|
|
Adherent-invasive E.coli
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- AIEC genes promote motility, capsule and LPS expression, serum resistance, iron uptake, adhesion to and invasion of epithelial cells, biofilm formation, degradation of mucins protease.
- AIEC properties empower them ti escape oxidative reactive species, tumor necrosis factor-alpha and other proinflammatory cytokine which enhances dysbiosis.
- Exploitation of host mechanisms of spoptosis in favor of their own intracellular replication and prevention of antimicrobial response.
|
|
Pseudomonas aeruginosa
|
- Virulence-related attachment factor increases the paracellular permeability.
- Transform apical membrane of epithelial cells into basolateral membrane.
|
|
Listeria monocytogenes
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1. Weakens the defensive mucosal barrier, leading to invasive infection with L. monocytogenes
|
|
Fungal dysbiosis
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1. Anti-inflammatory effects in colitis models
|
Probiotic interventional therapy in IBD
The World Health Organization (WHO) defines probiotics as "live organisms that, when administered in adequate amounts, confer a health benefit on the host" [2]. However, even this generally accepted definition of probiotics lacks clarity regarding their function, mechanism, or classification. This lack of specificity is a recurring theme in most of the reviewed articles on the topic. To date, no clinical trial, meta-analysis, or retrospective study has definitively defined the role of probiotics in the management, treatment, and prevention of IBD [1, 2, 3, 4, 11]. While there is general agreement on the importance of dysbiosis in IBD pathogenesis, not all studies support the use of probiotics alone, and there are significant differences in their efficacy as therapeutic agents.
Before delving further into the topic, it is appropriate to address the general issue with isolated probiotic use. As we have seen, the gut microbiota plays a crucial role in normal gastrointestinal (GI) physiology and homeostasis. It exerts an immunomodulatory function by directly affecting the host's immune response, maintaining the gut barrier, and competitively excluding non-commensal bacteria [7, 8, 9, 14]. It has even been demonstrated that long-term administration of specific probiotics can enhance disease resistance in certain trout species during the "grow-out" phase [6]. These findings suggest that probiotics generally have a positive effect on organisms. However, most of these results are based on prophylactic measures performed on low-risk individuals, and the efficacy of probiotics is not effectively determined in many clinical trials [2]. Therefore, while probiotics may appear effective on paper, as we will demonstrate, this is not always the case.
Probiotic use in the context of Quantitative Risk-Benefit analysis
Due to the heterogeneous and complex nature of probiotics as a "drug," there is significant heterogeneity in study design as a result. Only a small number of trials meet high methodological standards [2, 3]. Consequently, the risks associated with probiotic use in IBD patients specifically are unknown [2]. Patients taking probiotics can be divided into two main groups: low risk (healthy individuals) and high risk (immunosuppressed, young, elderly, etc.) [2]. The parameters required for studies involving probiotic use are highly variable and challenging to measure. Therefore, in cases like this, a risk-benefit analysis is required to determine the drug's efficacy relative to the patient (patient-specific treatment or patient-tailored treatment) [2, 3].
An illustrative example of the methodology used in assessing clinical cases according to the searching method to the quantitative risk analysis method is provided below, extracted from the review article "Quantitative Risk-Benefit Analysis of Probiotic Use for Irritable Bowel Syndrome and Inflammatory Bowel Disease" by William E. Bennett Jr., published on 14 October 2015. The researchers conducted a search on the website http://www.pubmed.org to identify relevant studies on the use of probiotics in the treatment of irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD). Specific search terms were used, combining probiotic-related terms (e.g., "probiotic," "lactobacillus," "bifidobacteria," "VSL," or "saccharomyces") with "irritable bowel syndrome" for IBS, and with "inflammatory bowel disease," "Crohn disease," "Crohn's disease," or "ulcerative colitis" for IBD.
The researchers limited the results to studies conducted up to 30 June 2014, written in English, and classified as clinical trials. They also examined the bibliographies of relevant published meta-analyses to identify additional studies. The initial search yielded 77 studies for IBS and 109 studies for IBD. The details and results of their search strategy are displayed in Figure 1. Subsequently, the selected studies for further quantitative analysis were listed and described in more detail in Table 3. Ultimately, 15 studies were chosen for analysis in IBS, and ten studies in IBD [2].
Considering the limited information available on utilities for IBS and IBD in the literature, along with the abundance of efficacy and risk data derived from numerous clinical trials, the researchers opted to use the risk-benefit plane for their analysis. This method allows for the simultaneous visualization of both risks and benefits, providing a two-dimensional confidence region that is better suited for modeling data with diverse and heterogeneous distributions across both risk and benefit dimensions. Figure 2.0 depicts the risk-benefit plane for all included trials of probiotics in IBD, with the mean risk-benefit lying above the dotted line, indicating a higher benefit than risk for the majority of hypothetical patients. However, the confidence region is quite large, reflecting the wide heterogeneity in confidence intervals that formed the primary data source. Therefore, while probiotics generally appear to confer more benefit than risk for most patients, the risk-benefit ratio can vary significantly. It is important to note that the heterogeneity of the confidence region does not imply the existence of specific sub-populations. The method samples from all existing confidence intervals, and the shape of the confidence region takes into account this variation. Sub-group analyses for factors such as Crohn's disease vs. ulcerative colitis or active vs. inactive disease were not performed due to the relative lack of high-quality data available to construct a risk-benefit plane for sub-groups.
The main issue here is that the study of probiotic effects, particularly in high-risk individuals such as immunocompromised individuals, lacks sufficient data to support any arguments for or against its use. Therefore, this risk analysis remains a preliminary method to estimate risks relative to benefits for patients, and it is still subject to trial-and-error approaches.
The use of Probiotics in combination
Recent studies have shown promising results when using probiotics in combination with other therapies [3]. Probiotics can be used in conjunction with specific probiotic species, such as VSL #3 and L. Reuters (DSM 17938), or S. thermopilus, L. acidophilus, bifidobacterium breve, B. animals, and specific Lactis species [3].
A recent study conducted by the School of Biotechnology, Kalinga Institute of Industrial Technology, revealed that probiotics, specifically Lactobacillus GG, when combined with Mesalazine (5-ASA), prolonged the relapse-free time associated with ulcerative colitis (UC). However, the study did not find a significant impact on endoscopic remission rates [1]. Additionally, patients who received adjuvant probiotic therapy with 5-ASA for 2 years in moderate to severe UC showed greater improvements in disease score compared to those treated with 5-ASA alone [1]. Limited evidence is available for the use of probiotic therapy in Crohn's disease (CD), which may explain the lack of positive results in this context [1].
The use of probiotics in combination with antibiotics has been shown to reduce antibiotic-induced dysbiosis [7]. However, it is important to administer the probiotics at least three hours after antibiotic administration to prevent any potential interaction that may interfere with antibiotic function. The probiotics enhance competitive exclusion of other bacteria, leading to a synergistic effect [7]. Generally, the use of probiotics in addition to conventional therapy has been inversely associated with the need for systemic steroids, hospitalization, and surgery per person per year [3].
Nevertheless, it is crucial to acknowledge that the use of probiotics is subject to high variability, both in terms of the drugs used and the host's response [2, 3]. Factors such as patient demographics need to be taken into consideration [3], and assessing the patient on the risk-benefit graph can provide a rough estimate of where the patient may lie in terms of benefit and risk [2].
A retrospective cohort study on probiotic use in adults with IBD concluded that clinical effectiveness of combined therapy, which has previously shown efficacy, relies on patient compliance with the therapy [3]. The data suggests that more compliant patients, those taking the medication for more than 75% of the disease course, may experience a greater reduction in the number of adverse effects related to IBD per patient per year, regardless of IBD phenotype [3]. However, details such as the specific probiotic mixture, optimal dosage, and duration of treatment are still unknown.
Interestingly, in most of the current literature, authors consistently report that the effects of probiotic therapy, whether used in combination or in isolation, vary significantly between the two IBD phenotypes. Probiotics have a strong positive correlation with UC patients in remission, maintenance, and prevention, while their clinical significance in CD is lacking [1, 2, 3, 4, 5].