https://orcid.org/0000-0002-3582-5120
ABSTRACT
Introduction
The gut microbiota affects the development of the gut immune system in early life. Perturbations to microbiota structure and composition during this period can have long-term consequences on the health of the individual, through its effects on the immune system. Research in the last few decades has shown that probiotic administration can reverse these effects in strain- and environment-specific ways. Bacillus clausii (B. clausii) has been in use for many decades as a safe and efficacious probiotic, but its mode of action has not yet been completely elucidated.
Areas covered
In this review, we discuss how the gut immune system works, the factors that affect its functioning, and the plethora of research highlighting its role in various diseases. We also discuss the known modes of action of Bacillus probiotics, and highlight the preclinical and clinical evidence that reveal how B. clausii acts to bolster gut defense.
Expert opinion
We anticipate that the treatment and/or prevention of dysbiosis will be central to managing human health and disease in the future. Discovering the pathophysiology of autoimmune diseases, infections, allergies, and some cancers will aid our understanding of the key role played by microbial communities in these diseases.
1. Introduction
The human gut microbiota plays a central role in immune regulation, with long-term consequences for health. The use of probiotics to restore dysbiosis is an alternative to pharmacological interventions; however, for some probiotics, such as B. clausii, the mode of their action remains unknown despite decades of safe and efficacious use in humans. In this review, we summarize the workings of the gut immune system, the current research linking the gut microbiota with early life immune imprinting, and the role of probiotics in maintaining the normal gut flora and in preventing diseases. We also include a synthesis of preclinical and clinical studies that demonstrate the beneficial effects of the probiotic B. clausii on the immune system.
2. Body
2.1. Gut immune system
The composition and function of the gut microbiota are key to gut homeostasis. Gut-associated lymphoid tissue (GALT) is the largest lymphoid tissue in the body, and is a major site where immune cells come into contact with antigens [Citation1]. The gut barrier consists of the outer mucus layer, the central single layer of intestinal epithelial cells, and the inner lamina propria [Citation2]. The mucus layer and the intestinal epithelium together constitute the physical barrier to gut microbes, whereas the immune cells of the lamina propria act as the immunological barrier [Citation3].
2.1.1. The physiological barrier
The mucus layer acts as the first line of defense in the gut and prevents bacteria from directly interacting with the underlying intestinal epithelium [Citation3]. This layer contains glycosylated mucin proteins, which form a gel-like sieve structure, and the antimicrobial peptides, secretory immunoglobulins and other proteins secreted by the epithelial cells [Citation3]. The epithelial layer is made up of enterocytes, goblet cells, and Paneth cells. The permeability of this barrier is influenced by tight junction proteins, which hold adjacent epithelial cells together [Citation2]. The loss of epithelial integrity and increase in permeability may lead to gastrointestinal (GI) disorders, such as inflammatory bowel disease (IBD), inflammatory bowel syndrome (IBS), obesity, metabolic syndrome, and necrotizing enterocolitis [Citation4].
2.1.2. The immunological barrier
The lamina propria is a connective tissue, which contains blood vessels, lymphatic vessels and different types of immune cells, such as macrophages, dendritic cells, plasma cells, and B- and T-lymphocytes [Citation5].
Macrophages in the intestinal lamina propria specialize in phagocytic and bactericidal activity; they destroy potentially harmful microbes by generating superoxide and nitric oxide, without simultaneously releasing pro-inflammatory cytokines [Citation5]. Dendritic cells sample the intestinal lumen, and phagocytose commensal and pathogenic bacteria () which they subsequently present as antigens to T-cells at mesenteric lymph nodes, allowing the host to develop tolerance to gut antigens [Citation5,Citation6].
Figure 1. Proiotics modulate the gut immune system in a variety of ways, including modifying the balance between Th1/Th2/Th17 and Treg cells, inducing the production of sIgA that inhibits adhesion and motility of pathogens, producing antimicrobial peptides such as bacteriocins and defensins, producing proteases that counter enterotoxins, competitive exclusion from mucosal adhesion sites, interaction with gut epithelial cells via cell-surface structures and increased synthesis of tight junction proteins, mucins, short-chain fatty acids, digestive enzymes, and chemicals with systemic effects, such as cortisol and tryptophan (reviewed in [Citation7,Citation8].
![Figure 1. Proiotics modulate the gut immune system in a variety of ways, including modifying the balance between Th1/Th2/Th17 and Treg cells, inducing the production of sIgA that inhibits adhesion and motility of pathogens, producing antimicrobial peptides such as bacteriocins and defensins, producing proteases that counter enterotoxins, competitive exclusion from mucosal adhesion sites, interaction with gut epithelial cells via cell-surface structures and increased synthesis of tight junction proteins, mucins, short-chain fatty acids, digestive enzymes, and chemicals with systemic effects, such as cortisol and tryptophan (reviewed in [Citation7,Citation8].](/cms/asset/6f7e995b-3022-456d-bb18-6d7adf34b7a7/ierm_a_2085559_f0001_oc.jpg)
Plasma cells secrete IgA, which is transported into the lumen, where it binds to microbes and reduces their motility and adhesion to the epithelium [Citation5,Citation9]. The predominant T-cell subset in the intestinal lamina propria comprises T cells expressing the cell surface marker CD4 [Citation5]. When naïve CD4+ T cells are activated by antigen-presenting cells, they differentiate into Th1, Th2 or Th17 cells, which secrete pro-inflammatory cytokines, which protect against certain infections, but are also linked to autoimmune diseases [Citation10,Citation11]. T-cell subsets with regulatory functions, known as Tregs, suppress immune responses by producing anti-inflammatory cytokines [Citation12]; (). The balance between the Th1, Th2, Th17 subsets and Tregs depends on interactions between the immune system and the gut microbiota, and is key to maintaining gut homeostasis [Citation13]. Because most infections, some autoimmune/chronic inflammatory diseases and allergies and some malignancies [Citation14] share similar immune pathways, an imbalance in the gut microbiota could have wide-ranging effects on host health. Changes in the gut microbiota have been associated with celiac disease, inflammatory bowel disease, inflammatory bowel syndrome, necrotizing enterocolitis, atopy, obesity, and cystic fibrosis [Citation15].
2.2. Immune imprinting in early life
Early life immune imprinting is believed to influence the long-term health of the individual. Connected to this idea is the hygiene hypothesis, which was formulated to explain the decrease in infectious diseases and the simultaneous increase in the last century in the incidence of atopy – the aberrant immune response that leads to asthma, eczema, and allergic rhinitis [Citation16]. A growing body of literature supports the hygiene hypothesis and the notion of a crucial period in the first 1000 days of life for the establishment of a healthy microbiota [Citation17,Citation18].
Gut microbiota modulate the developing immune system () [Citation17,Citation19]. The GI tract of the fetus is colonized during gestation [Citation18]. Maternal exposure to infections during pregnancy and early childhood exposure to infections of the central nervous system can alter neurodevelopment, and are associated with schizophrenia and autism in later life [Citation20]. The meconium (first bowel movement) of full-term infants contains microbes that are normally found in the amniotic, vaginal, and oral microbiota, suggesting that these populations contribute to the early gut microbiota of the newborn [Citation21,Citation22]. Infants born vaginally have gut microbiota that match the mothers’ vaginal microbiota, whereas those born by Cesarean section have gut microbiota that match the mothers’ skin microbiota [Citation23]. The gut microbiota of formula-fed babies differ significantly from those of breast-fed babies [Citation24–26]. After weaning from breastfeeding and when solid foods are introduced, the gut microbiota composition changes further [Citation27,Citation28]. Breast milk consumption, birth mode, presence of siblings and household pets, probiotic supplementation, antibiotic exposure, and geographical location are all important covariates for microbiota structure between 3–40 months of age [Citation27].
Figure 2. Exposure to different subsets of microbes due to different environments in the first 1000 days of life can lead to normal or pathological imprinting with long-term consequences on health. Caesarean section and formula milk contribute to dysbiosis in early life. Other factors such as diet and consumption of antibiotics can also lead to imbalances in the gut microbiota, ultimately contributing to chronic inflammatory disease later in life [19]. Adapted from [17].
![Figure 2. Exposure to different subsets of microbes due to different environments in the first 1000 days of life can lead to normal or pathological imprinting with long-term consequences on health. Caesarean section and formula milk contribute to dysbiosis in early life. Other factors such as diet and consumption of antibiotics can also lead to imbalances in the gut microbiota, ultimately contributing to chronic inflammatory disease later in life [19]. Adapted from [17].](/cms/asset/0b59a52d-3da3-4ac9-8424-668712a1fc65/ierm_a_2085559_f0002_oc.jpg)
The memory formed by the early, appropriate interaction between the immune system and a ‘healthy’ microbiota helps the development and maturation of the immune system and prevents inflammatory pathologies. Conversely, Cesarean sections, formula feeding in early life, and illness or medication in later life contribute to a dysbiotic microbiome, which may lead to pathological imprinting. These individuals may develop an overly reactive immune system and suffer from chronic inflammatory disorders later in life [Citation19]. Therefore, identifying the components of the microbiota and the diet that allows a healthy microbiome to thrive may be key to the prevention of pathological imprinting.
2.3. Effects of probiotics
Probiotics may be consumed to restore the composition of the gut microbiota and introduce beneficial functions to gut microbial communities [Citation29]. Probiotics are defined as live microorganisms that confer a health benefit on the host when administered in adequate amounts [Citation30]. Species belonging to the genera Lactobacillus, Bifidobacterium, Saccharomyces, Streptococcus, Escherichia and Bacillus are commonly used as probiotics [Citation31]. These beneficial microorganisms may act through a variety of ways (), such as producing organic acids [Citation32] and antimicrobials [Citation33,Citation34], resisting colonization by pathogens [Citation35–38], improving barrier function [Citation39–42], and modulating the pro- and anti-inflammatory responses of the immune system [Citation43–53]. Other modes of action include producing enzymes that aid in digestion, manufacturing small chemicals that have systemic effects (such as cortisol, tryptophan, and others), and mediating interactions with host cells by means of cell surface structures [reviewed in Citation7,Citation8].
Specific species and strains of probiotics that manipulate the gut microbiota have shown promise in the treatment or prevention of specific diseases (). Immunomodulation by probiotics is strain-specific and their influence on immunity depends on the physiological environment in which they act [Citation54]. Therefore, their use needs to be personalized to the individual’s intestinal milieu [Citation55].
Figure 3. Probiotics have shown promise in a variety of conditions affected by an imbalanced gut microbiota. Apart from gut-related diseases such as diarrhea, enterocolitis, IBS, IBD, and food allergies, the gut microbiota also influence diseases of the central nervous system, skin, liver, and lungs. Vaccine response and metabolic diseases are also affected by gut microbiota composition [Citation1,Citation4,Citation56–75].
![Figure 3. Probiotics have shown promise in a variety of conditions affected by an imbalanced gut microbiota. Apart from gut-related diseases such as diarrhea, enterocolitis, IBS, IBD, and food allergies, the gut microbiota also influence diseases of the central nervous system, skin, liver, and lungs. Vaccine response and metabolic diseases are also affected by gut microbiota composition [Citation1,Citation4,Citation56–75].](/cms/asset/7a773a13-126e-4d76-9940-122e1e2ea1f8/ierm_a_2085559_f0003_oc.jpg)
2.4. Bacillus species as probiotics
Bacillus spp. are a genus of rod-shaped, Gram-positive, spore-forming bacteria that are found in the soil, air, water, food, and inside the human gut [Citation76]. Bacillus species form endospores, which can survive the hostile acidic environment of the stomach and the GI tract, tolerate bile salts, and have higher stability than non-spore-forming probiotics during production and storage of pharmaceutical or food-based formulations [Citation77].
Probiotics that can naturally be isolated from the gut of the target population are preferred, as this indicates their ability to thrive in the gut. Spores from Bacillus spp. can be readily isolated from human feces in the range of 103 to 108 CFU/g of feces [Citation78]. In a study examining the diversity of Bacillus spp. that could be recovered from healthy human adult feces, 32 and 28 isolates out of a total of 124 isolates were identified to be B. clausii and Bacillus licheniformis, respectively, based on 16S rRNA gene sequence analyses [Citation79].
The probiotic, Enterogermina® (Sanofi), is the most prevalent commercially available form of B. clausii and contains four strains: O/C, N/R, SIN, and T [Citation80,Citation81]. These strains are resistant to different classes of antibiotics to different degrees [Citation82]. Although the presence of antibiotic-resistance genes may be considered a risk with regard to their transfer to other microorganisms, the genes that confer some of these antibiotic resistance traits are chromosomally located indicating a low risk [Citation83–86]. This property allows for this probiotic to be used concomitantly with antibiotics, to reduce the gastrointestinal side effects of antibiotic treatment. It has been used safely as an adjunct with oral rehydration therapy in pediatric viral diarrhea and Helicobacter pylori treatment in adults for several decades [Citation81,Citation87].
Other Bacillus species commonly used as probiotics include B. subtilis, B. licheniformis, B. coagulans, B. toyoi (cereus), B. natto (subtilis), B. polyfermentans, and B. cereus [Citation88]. Some of these have shown promise in modulating immunity in clinical trials and in preclinical studies ().
Table 1. Immunomodulatory effects of Bacillus probiotics
2.5. Preclinical evidence for immunomodulation and antimicrobial activity by B. clausii
Although probiotics are generally regarded as safe, there have been rare cases of bacteremia and fungemia, particularly in immunocompromised patients [Citation107–110]. B. clausii has a limited number of reported cases of bacteremia following administration under specific conditions [Citation111,Citation112], but has been well tolerated as a probiotic for several decades. However, its mode of action is not fully understood. Several lines of evidence have emerged in recent years that show its diverse pathways in immunomodulation ().
Table 2. Modes of action identified for B. clausii from preclinical studies
2.5.1. Protection from Salmonella infection
Once ingested, B. clausii UBBC07 spores can survive and germinate in the human GI tract and also in in vitro simulations of the GI tract [Citation114]. The B. clausii strain MTCC-8326 modulates innate immune responses in RAW 264.7 murine macrophage cells and protects them from cytotoxicity caused by Salmonella typhimurium infection [Citation113]. Relative to untreated macrophages, those treated with B. clausii for six hours display a modified gene expression of IL-6, IL-10, IL-12, IFN-α, and TNF-α. Also, macrophages primed with B. clausii or a mix of B. clausii and L. acidophilus are more resistant to S. typhimurium-induced cytotoxicity. Thus, the induction of cytokines after exposure to the probiotic strains aids macrophages in clearing the Salmonella infection in vitro. In a mouse model of S. typhimurium-induced diarrhea, the B. clausii strain MTCC-8326 colonizes the ileum and colon of the mouse gut and protects Th2-biased BALB/c mice, but not Th1-biased C57BL/6 mice, from S. typhimurium infections [Citation115]. Thus, the host environment plays a key role in the efficacy of the probiotic.
2.5.2. Lantibiotic-mediated inhibition of bacteria
B. clausii, along with some other bacterial species, produces bacteriocins called lantibiotics, which act against other bacterial species by targeting the peptidoglycan lipid intermediates in the cytoplasmic membrane, resulting in the formation of pores and causing cell lysis. The B. clausii lantibiotic, clausin, may be a potential route through which B. clausii inhibits the viability and growth of other bacteria [Citation116].
2.5.3. Antimicrobial activity
The four B. clausii strains (O/C, N/R, SIN, and T) exhibit in vitro antimicrobial activity against S. aureus. The cell-free supernatant of the O/C strain inhibits Gram-positive species such as C. difficile 514, Micrococcus sp. LMBA 26, S. aureus CIP35053156, E. faecium LMBA 27323, Lactococcus lactis ATCC 11454, and L. lactic LMBA 374. Among the different growth phases of the O/C strain, anti-staphylococcal activity begins at the same time as the stationary phase, indicating that the spores of this strain are responsible for the antimicrobial activity. Conversely, vegetative cells of B. clausii strains also show immunomodulation; they stimulate nitrite production in Swiss murine peritoneal cells. All four strains induce IFN-γ production and CD4+ T-cell proliferation in murine C57BL/6j spleen cells [Citation88].
2.5.4. Gut barrier enhancement
B. clausii (O/C, SIN, N/R, T) is also used as an adjunct in the treatment of acute viral gastroenteritis in children; it reduces the duration and severity of diarrhea. In a human enterocyte model of rotavirus infection, B. clausii strains and their metabolites exert beneficial effects on cell cycle progression, epithelial integrity, enterocyte monolayer permeability, and innate immune responses [Citation117]. The strains, but not the metabolites, increase the synthesis of the antimicrobial peptides human beta-defensin 2 (HBD-2) and cathelicidin LL-37. The strains and their metabolites upregulate the mucin protein Mucin 5AC (MUC5AC) and the occludin and Zonula occludens-1 (ZO-1) tight junction proteins, allowing for effective mucosal barrier function [Citation117]. They also inhibit rotavirus-induced ROS production, the release of pro-inflammatory cytokines IL-8 and IFN-β, and pro-inflammatory Toll-like receptor 3 pathway gene expression [Citation117].
The composite genome of all four B. clausii strains (O/C, SIN, N/R, T) has been sequenced [Citation80]. Genome annotation indicates the presence of genes that confer resistance to rifampicin, chloramphenicol, streptomycin and neomycin, and tetracycline. The genome also contains genes that encode bacteriocins and gallidermin, which prevent S. aureus and S. epidermidis from forming biofilms [Citation80]. Analysis of predicted protein domains indicates the presence of proteins that bind to mucus, collagen, or fibronectin – which may be critical for mucosal adhesion in the intestine [Citation80,Citation118]. Proteomic analysis of the four strains suggests the expression and secretion of proteins key to binding human plasminogen/fibronectin/mucus, thus indicating their ability to interact with host cells and colonize the GI tract [Citation118].
2.5.5. Anti-inflammatory effect
Gram positive bacteria such as B. clausii can induce an anti-inflammatory effect. Their cell walls contain lipoteichoic acid, an amphiphilic polymer that stimulates macrophages in vitro and increases production of inflammatory molecules like nitric oxide (NO). Under normal physiological conditions, NO induces an anti-inflammatory effect by preventing platelet adherence and aggregation, and interaction between leukocytes and endothelial cells; however, when overproduced, it acts as a pro-inflammatory mediator [Citation119,Citation120]. Lipoteichoic acid from B. clausii strain O/C induces NO production in RAW 264.7 macrophages, albeit to a lower level than that induced by S. aureus, suggesting that it stimulates the immune system without leading to pathological inflammation [Citation119].
2.5.6. Protection from enteropathogens
Pathogens like C. difficile and B. cereus release toxins that target intestinal cells and induce severe inflammation in the host. C. difficile toxins alter gene expression of proteins such as ZO-1 (a tight junction protein key to epithelial integrity and barrier function). B. cereus produces enterotoxins such as cytotoxin K (CytK), nonhemolytic enterotoxin, and hemolysin BL, which can cause vomiting and diarrhea. The supernatant of the O/C strain of B. clausii contains a serine protease that inhibits the cytotoxic effects of C. difficile infection [Citation121]. Vero and Caco-2 cells exposed to C. difficile and B. cereus supernatants have low viability, reduced cell attachment and lowered activity of mitochondrial dehydrogenase, all indicative of a high rate of cell death. The supernatant of B. clausii strain O/C rescues these cell lines from these cytotoxic effects and inhibits the hemolytic effect of the B. cereus toxin, hemolysin. Supernatants from vegetative cells of O/C have 4-fold less protease activity than those from spores [Citation121].
In addition to these immunomodulatory effects, several B. clausii strains (O/C, SIN, N/R, T, DSM 8716, DSM 9783, and DSM 2512) exhibit anti-genotoxic activity and may protect the gut environment from the harmful effects of DNA damage due to mutagens present in food [Citation122].
2.6. Clinical efficacy of B. clausii on immunity
B. clausii strains have been shown to survive and persist in the human GI tract [Citation123], where they may exert their immunomodulatory effects to influence clinical outcomes in various diseases ().
Table 3. The effect of B. clausii on the clinical outcomes of different diseases
2.6.1. Diarrhea
A meta-analysis of randomized clinical trials (RCTs) conducted in four countries has shown that patients receiving B. clausii (O/C, SIN, N/R, T) experience a shortened duration of diarrhea, reduced stool frequency, and shortened duration of hospital stay when compared with control groups [Citation124]. B. clausii is also well-tolerated by these patients.
B. clausii has been recommended as an adjunct to oral rehydration solution and zinc in acute viral diarrhea, and for prevention of antibiotic-associated diarrhea and C. difficile-induced diarrhea [Citation125]. In children in the Philippines, B. clausii (O/C, SIN, N/R, T) administration without zinc significantly reduces the duration of diarrhea when compared with zinc treatment alone [Citation126]. In Indian children aged <5 years suffering from acute diarrhea, those receiving spores of B. clausii strain UBBC-07 experience significantly reduced duration and frequency of diarrhea than those receiving placebo [Citation127].
In children suffering from rotaviral diarrhea, B. clausii administration is significantly more effective than standard treatment in reducing duration of diarrhea, vomiting and fever and other clinical symptoms such as general weakness, duration of swelling and/or abdominal pain. During the onset of disease, IgA and secretory IgA levels reduce and IgM levels increase in children with rotavirus infection when compared with healthy children. B. clausii therapy normalizes immunoglobulin levels by the end of the convalescence period, whereas standard therapy does not bring immunoglobulins to pre-infection levels [Citation128]. Thus, B. clausii exerts a beneficial effect on the humoral immunity of children with rotavirus infection.
2.6.2. Adjunct to H. pylori treatment
Treatment to eradicate Helicobacter pylori infections is not successful for about 10% of patients due to antibiotic resistance and side effects from triple therapy [Citation87]. In RCTs comparing the effects of B. clausii (O/C, SIN, N/R, T) versus placebo in patients with H. pylori infections, those in the probiotic group experience fewer side effects (such as nausea, diarrhea, and epigastric pain) from triple therapy than those in the placebo group [Citation87,Citation129]. Although the mode of action for B. clausii in H. pylori adjunct treatment is unknown, other probiotics have been shown to exert similar effects by secreting antibacterial substances such as bacteriocins, enhancing the gut barrier, preventing colonization by pathogens by competing for binding sites, and reducing the expression of pro-inflammatory cytokines from H. pylori-infected cells [Citation130]. It is possible that B. clausii also employs similar modes of action when used alongside triple therapy.
2.6.3. Esophagitis
B. clausii administration along with esomeprazole has been shown to affect the expression of genes involved in immunity, inflammation, cell growth, differentiation, apoptosis, cell adhesion, and cell-cell signaling in a study of six male patients affected by esophagitis [Citation131]. Growth differentiation factor 3, IL-1β, CD79B antigen, and the Fc fragment of IgG were among the 92 upregulated genes, whereas IL-13, TNF superfamily member 17, chemokine-like receptor 1, and the IL-6 receptor, were among the 265 downregulated genes [Citation131]. This study could form the basis of future research into specific mechanisms by which B. clausii administration may help an aberrant immune response.
2.6.4. Necrotizing enterocolitis
Pre-term infants born before 34 weeks are at risk of late-onset sepsis due to developmentally immature immune systems, and exposure to antibiotics and colonizing bacteria from the intensive care environment. In a study comparing B. clausii (O/C, SIN, N/R, T) with placebo, pre-term infants on the probiotic attain full feeds (180 mL/kg/day) significantly faster without an increase in the risk of feed intolerance or necrotizing enterocolitis [Citation132]. Although the O/C, SIN, N/R, and T strains of B. clausii do not affect the risk of late-onset sepsis in pre-term infants in the study described, other studies have shown that certain probiotic strains reduce the risk of necrotizing enterocolitis and late-onset sepsis in pre-term infants [Citation133–135].
2.6.5. Respiratory infections and allergies
Apart from having direct effects on gut health, B. clausii modulates the immune system in a way that can affect respiratory health and allergies. In a pilot study of children aged three to six years, the administration of B. clausii (O/C, SIN, N/R, T) over a three-month period has been shown to significantly reduce the number and duration of respiratory infections in comparison to the control group, over the study and follow-up periods [Citation136]. In children with allergic rhinitis, administration of B. clausii (O/C, SIN, N/R, T) results in a significant decrease in IL-4 levels and a significant increase in IFN-γ, IL-12, TGF-β, and IL-10 levels [Citation137]. In adults with allergic rhinitis, B. clausii (O/C, SIN, N/R, T) administration leads to an increase in the levels of IFN-γ, TGF-β, and IL-10, and a decrease in the levels of IL-4 [Citation138]. High levels of IL-4, IL-5 and IL-13 are indicative of Th2 polarization, and high levels of IL-10 and TGF-β are characteristic of Th1 and Treg polarization. This indicates that B. clausii may modulate the gut immune system to a Th1/Treg-based profile, rather than a Th2-based profile in these patients. Since infection, allergy, and autoimmunity share a common basis for immunomodulation, B. clausii may exert a significant clinical outcome and improve the quality of life of these patients.
3. Conclusion
Understanding immune imprinting may be the key in the future to solving the increasingly common occurrence of chronic inflammatory disorders. Viral and bacterial infections, allergies, degenerative disorders, metabolic diseases, and autoimmune disorders share common immune pathways that can be modulated by the use of probiotics. Bacteria such as B. clausii and certain Bifidobacterium and Lactobacillus strains that have clinically-proven efficacy, are preferred for use as probiotics as they can naturally be isolated from the gut of the target population, indicating their ability to thrive in the gut. There are several strands of evidence demonstrating the effect of B. clausii on the gut immune system. However, clinical studies differ in the immunological and clinical parameters used, making it difficult to compare studies directly, or conduct meta-analyses. The use of standardized clinical scores (such as the Gastrointestinal Symptom Rating Scale, Scoring Atopic Dermatitis (SCORAD) index, and the Visual Analog Scale) alongside determinations of immunological markers (such as IgA/IgM levels and the expression of pro-inflammatory and anti-inflammatory cytokines) will allow for a better understanding of the efficacy of probiotics. In addition, clinical trials that stratify the target population by responders vs. non-responders may help in identifying population subgroups for whom B. clausii treatment can be personalized.
4 Expert opinion
Decoding the language that hosts and microbes use to communicate, and understanding the consequences of disrupting this communication, will help us understand the development of several widespread multifactorial diseases. The current consensus is that a small number of choices in early life exert long-term impact on health and disease; especially interesting are the differences in infants’ microbiota with birth mode, infant feeding practices, and exposure to antibiotics. To this end, we believe that governments around the world should be urged to create and implement policies that encourage vaginal delivery, breastfeeding, and prudent use of antibiotics; this may lead to better long-term health for people and lower healthcare costs. We are particularly excited about future research that will unravel the early role of the human gut microbiota in aspects of brain development and behavior, including outcomes related to the pathophysiology of psychological disorders. We expect that future studies will identify the specific microbiome composition and host-microbe interactions in early life that promote health and the potential role of probiotics in driving the gut microbiomes of newborns toward a healthier composition.
The prophylactic use of probiotics could reduce the risk of acute viral gastroenteritis and upper respiratory tract infections, for example, in children attending childcare centers, who may be exposed to infections more frequently. This would lead to a decrease in absenteeism and help to lower overall healthcare costs for communities. Although there have been pilot studies showing a substantial effect of B. clausii in preventing respiratory infections, these need to be followed by clinical trials so that clinicians can recommend the inclusion of B. clausii as a non-pharmacologic intervention to prevent recurrent respiratory infections. There is also a need for clinical trials that investigate the efficacy of probiotics in preventing other infectious diseases such as C. difficile-induced diarrhea and antibiotic-associated diarrhea.
The specific role of each microbial species should be more deeply researched to determine their beneficial or detrimental effect on immunity and pathogenesis of disease. This can be achieved through new experimental approaches such as next-generation probiotics with specific functions, genetically engineered microbes that produce clinically-relevant products, or synthetic microbiomes combined with mathematical modeling to help predict the dynamics and interactions between the different species that make up the microbiome. Correlation analyses of species abundance and functional potential may not be adequate to understand complex community interactions and we therefore believe that mathematical modeling approaches will be important to integrate the many layers of multi omics data (metagenomic, metabolomic, metatranscriptomic and metaproteomic) currently generated in some metagenomics studies. Multidisciplinary research that combines the outcomes from studies of the microbiota with those from studies of the microbiome will be critical to understanding the specific genes and biochemical pathways that confer specific functions to probiotics.
Large-scale studies in differing clinical populations with different genetic, socioeconomic backgrounds, and ages will highlight the role of specific probiotic strains in strengthening immunity. These studies should integrate clinical endpoints with biomarker-level investigations, so that the mode of action of the probiotic strain can be related to its clinical efficacy. Furthermore, stratifying large-scale trial populations into responders and non-responders will enable personalized therapies based on the specific disease risk of the individual (for instance, immunocompromised individuals or those with allergies). These studies will also examine the response of the microbiota to specific interventions, including drugs or dietary changes. We believe that metadata management tools, such as machine learning or deep learning algorithms that allow the prediction of key features, will help stratify patients with specific phenotypes or distinguish responders from non-responders; such advances in analytics will aid with interpreting the extensive data generated by metagenomics studies.
Studies that include probiotics as part of a normal diet will shed light on ways of promoting a healthy gut microbiota and reducing disease risk. Apart from the gut microbiome, we anticipate that research into site-specific microbiomes, such as the skin microbiome, the urogenital microbiome, and the respiratory microbiome, will uncover the roles of the specific microorganisms that provide critical functions to the corresponding organs. We foresee studies targeting strain-specific probiotic administration to specific local areas of the body to exert optimal effects on immunity, e.g. skin-specific applications for psoriasis or atopic dermatitis, or respiratory tract-specific applications for allergic rhinitis or upper respiratory tract infections. We also look forward to research that will investigate the modification of gut microbiomes to enhance immunological response to vaccines and improving patient response to cancer immunotherapies. In the areas of consumer wellness and over-the-counter therapies, we predict that labels that claim immunity-related effects for specific probiotic strains will be robustly supported by an evidence base that includes clinical trials, mechanistic studies using in vivo and in vitro models (e.g. Simulator of Human Intestinal Microbial Ecosystem, or SHIME®) and real-world evidence highlighting patient-reported outcomes.
Article highlights
The gut immune system consists of a physical and immunological barrier, which can be affected by the structure and composition of the gut microbiota.
There is a crucial period in the first 1000 days of life, when the immune system is exposed to healthy microbiota, which shape its development and maturation; an absence of this early interaction may lead to inflammatory pathologies.
Probiotics can restore the composition of a dysbiotic gut microbiota, and confer beneficial effects through several modes of action. They have shown promise in preventing or treating several diseases related to infection, allergy, autoimmunity and other inflammatory conditions.
B. clausii probiotics have been used safely for several decades and have been shown to improve outcomes in acute pediatric diarrhea, rotavirus infections, necrotizing enterocolitis and late-onset sepsis, H. pylori treatment, and respiratory tract infections and allergic rhinitis.
Various modes of action for B. clausii probiotics are beginning to be uncovered, including their antimicrobial and immunomodulatory properties.
This article highlights the preclinical and clinical evidence supporting the use of B. clausii.
Declaration of interest
R Wong-Chew has the following conflicts of interests: Medical writing support for current manuscript funded by Sanofi. Activities and relationships in the past 36 months include consulting fees and support for attending meetings/travel from Sanofi; honoraria for lectures from Becton Dickinson and Innova Salud Mexico; participation on a Data Safety Monitoring Board or Advisory Board for Innova Salud and Temporal COVID-19 Hospitalization Unit Mexico (unpaid); and, a leadership or fiduciary role in COFEPRIS (Federal Commission for Protection from Sanitary Risks) and CONACYT (National Council of Science and Technology).
J-A de Castro has the following conflicts of interests: Medical writing support for current manuscript funded by Sanofi. Activities and relationships in the past 36 months include honoraria for a speaker bureau from Sanofi.
L Morelli has the following conflicts of interests: Medical writing support for current manuscript funded by Sanofi. Activities and relationships in the past 36 months include consulting for and participation in an advisory board for Sanofi.
M Perez has the following conflicts of interests: Employee of Sanofi, issued vested shares by Sanofi as part of employee benefits.
M Ozen has the following conflicts of interests: Medical writing support for current manuscript funded by Sanofi. Activities and relationships in the past 36 months include advisory/consulting roles for Abdi-Ibrahim, Amgen, DMG Italia, Generica, GSK, Menarini, Montero, MSD, Pfizer, Sandoz, and Sanofi.
The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.
Reviewer disclosures
Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.
Geolocation information
The authors are located in Italy, Turkey, Philippines, Mexico, and Germany.
Acknowledgments
The authors would like to thank Dorothea Maren Greifenberg of Sanofi for support with development of this article. The authors would like to acknowledge Subhashini Muralidharan, PhD, and Ella Palmer, PhD CMPP, of inScience Communications, Springer Healthcare Ltd, UK, for medical writing support that was funded by Sanofi.
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There are no datasets associated with this article.
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References
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