Probiotics: Shaping the gut immunological responses

Abstract

Probiotics are live microorganisms exerting beneficial effects on the host’s health when administered in adequate amounts. Among the most popular and adequately studied probiotics are bacteria from the families Lactobacillaceae, Bifidobacteriaceae and yeasts. Most of them have been shown, both in vitro and in vivo studies of intestinal inflammation models, to provide favorable results by means of improving the gut microbiota composition, promoting the wound healing process and shaping the immunological responses. Chronic intestinal conditions, such as inflammatory bowel diseases (IBD), are characterized by an imbalance in microbiota composition, with decreased diversity, and by relapsing and persisting inflammation, which may lead to mucosal damage. Although the results of the clinical studies investigating the effect of probiotics on patients with IBD are still controversial, it is without doubt that these microorganisms and their metabolites, now named postbiotics, have a positive influence on both the host’s microbiota and the immune system, and ultimately alter the topical tissue microenvironment. This influence is achieved through three axes: (1) By displacement of potential pathogens via competitive exclusion; (2) by offering protection to the host through the secretion of various defensive mediators; and (3) by supplying the host with essential nutrients. We will analyze and discuss almost all the in vitro and in vivo studies of the past 2 years dealing with the possible favorable effects of certain probiotic genus on gut immunological responses, highlighting which species are the most beneficial against intestinal inflammation.

Key Words: Probiotics, Lactobacillaceae, Bifidobacteriaceae, Saccharomyces, Intestinal inflammation, Immune responses

Core Tip: Probiotics, such as Lactiplantibacillus plantarum and Saccharomyces cerevisiae, exert remarkable anti-inflammatory properties on the gut's immune responses. These beneficial microorganisms not only restore immunity markers but also enrich the gut's microbiota, crucial for a healthy microbial balance. Incorporating probiotics or foods rich in these beneficial microorganisms, particularly in conditions such as inflammatory bowel disease, holds promise for restoring gut health, boosting the immune system, and alleviating inflammation.

INTRODUCTION

According to the current definition, “probiotics are live microorganisms that, when administered in adequate amounts, confer a health effect on the host”[1,2]. They can be found either as pure forms supplied by various pharmaceutical companies or as essential parts of everyday foods, mainly fermented, such as cheese, yogurt, beer and others[3]. Some of the most well-studied probiotics are bacteria, such as the Lactiplantibacillus plantarum, and yeasts, such as Saccharomyces, for which extensive research has shown that they possess anti-inflammatory and wound healing properties[4,5].

Probiotics are considered to exert their beneficial effects not only on the host’s cells, but also on its natural microbiota composition. Since the onset of the Microbiome Project, several species of bacteria and yeasts have been identified, which has led to the extensive identification/characterization, of the human microbiota composition, found on the cutaneous and mucus surfaces of the human body[6]. Microbiota has been proved to be essential for the host’s survival, not only acting as a defense mechanism against potential pathogenic microorganisms, but also providing viable nutritional supplementation[7]. Over time, it has been shown that the microbiota population is 10 times greater than the total number of cells composing the human body, and, as a result, it has been proposed that humans are “symbiotic” organisms, living in harmony with their microbiota[8].

One of the mucosal surfaces with the best characterized and most well-studied microbiota is that of the intestine. Under healthy conditions, the composition of the intestinal microbiota is balanced and beneficial to the host, but in pathological situations, this balance can be disturbed, leading to dysbiosis, with potentially harmful consequences to the host[9]. Such a dysbiosis has been confirmed in patients suffering from inflammatory bowel disease (IBD)[9]. IBDs – Crohn’s Disease (CD) and Ulcerative Colitis (UC) – are characterized by chronic relapsing inflammation of the gastrointestinal tract, with various immunological, genetic and environmental factors involved in its pathogenesis[9]. One such environmental factor is the intestinal microbiota. Patients with IBDs have been shown to have a distinct, altered microbiota composition, lower in diversity compared to healthy individuals, and, in many cases, attempts to restore their microbiota composition to a healthy state have proved beneficial for the patient’s health. Such attempts have been through either fecal microbial transplantation, where fecal matter from healthy donors is transplanted into patients with IBD to restore their microbiota to a healthy state, or through probiotic supplementation[10].

One of the major beneficial effects of probiotics is their ability to modulate the immunological responses of the host. This can be accomplished by: (1) Displacement of potential pathogens via competitive exclusion; (2) offering protection to the host through the secretion of various defensive mediators; and (3) supplying the host with essential nutrients[7]. In this Editorial, we will analyze and discuss almost all the in vitro and in vivo studies published on the past 2 years which have investigated the possible favorable effects of certain genus of probiotics on gut immunological responses (Figure 1), in an effort to highlight which are the most beneficial in relation to intestinal inflammation.

Figure 1 The beneficial effects of probiotics on the immune system, intestinal microbiota and the human gut. Probiotics promote the upregulation of anti-inflammatory cytokines, induce the downregulation of pro-inflammatory ones, improve the colitis symptoms, enhance the immune system, reinforce the epithelial barrier and favor the enrichment of the gut microbiota.

THE GENUS LACTIPLANTIBACILLUS

Lactiplantibacillus (formerly known as Lactobacillus), is a genus of Gram-positive bacteria that has been associated with favorable outcomes for the host. Many of its members have been proved to be essential players in the food industry[11]. One of the most well-known is Lactiplantibacillus plantarum (L. plantarum), which has been found to thrive in a wide range of environments, including fermented foods, several types of meat and plants, as well the mammalian gastro-intestinal tract[12]. Regarding its effects on the gastro-intestinal tract, L. plantarum has been shown to boost wound healing and promote anti-inflammatory processes[13].

In a series of studies, any researchers have highlighted the immune-related properties of L. plantarum in the gastro-intestinal tract. Our research group has shown that L. plantarum may participate in the alertness of the intestinal immune system, as it seems to mildly upregulate specific chemokines in subepithelial myofibroblasts[14]. In a cyclophosphamide-induced immunosuppressive animal model, Zeng et al[15] observed that the administration of L. plantarum led to the enhancement of the immune system through the restoration of inflammatory cytokines and immune markers in the spleen[15]. In another animal model of antibiotic-induced diarrhea, Liang et al[16] showed that the administration of L. plantarum ELF051 significantly improved the animals’ health, by downregulating pro-inflammatory signaling pathways and cytokines, such as interleukin (IL)-1β, upregulating the anti-inflammatory ones, such as IL-10, and by enriching the diversity of the topical gut microbiota[16]. In the same way, L. plantarum YRL45, a bacteriocin-producing probiotic, has been reported to favorably regulate the immune system of mice by elevating the immunoglobulins sIgA, IgA and IgG levels and by upregulating the expression of epithelial markers mucin 2, zonula occludens-1 and junctional adhesion molecule 1[17].

Similar results come from a study investigating the immunoregulatory effects of L. plantarum CRL681 and CRL1506 in enterotoxigenic Escherichia coli (E. coli) infection in mice; both strains found to favorably modulate the intestinal innate immune response and increase resistance to E. coli, ultimately leading to reduced counts of E. coli in the gastrointestinal tract[18]. In another study, Li et al[19] showed that the administration of L. plantarum in E. coli-infected mice led to a significant improvement in disease status, since it significantly stopped weight loss and restored the flattened mucosa in the jejunum, findings probably related to the significant downregulation of the proinflammatory cytokines[19]. This improvement was further enhanced when L. plantarum was administered along with two other probiotics, Bifidobacterium longum and Pediococcus acidilactici[19], supporting the idea that a combined rather than a single regime of probiotics is more effective.

Heat-killed fractions and proteins from L. plantarum 299v, now named postbiotics[20], have also proved to have anti-inflammatory properties, in the same way as live bacteria: In an lipopolysaccharides (LPS) in vitro model they were found to downregulate the pro-inflammatory cytokine IL-18[21], thus exerting immunomodulatory properties on the immune responses. The anti-inflammatory properties of L. plantarum are also highlighted by the proteomic study of Cufaro et al[22]: L. plantarum C904 was once again found able to downregulate in vitro, to a considerable degree pro-inflammatory cytokines, including IL-2, IL-5, IL-6, and interferon (IFN)-γ, in inflamed intestinal epithelial cells[22].

Another subspecies, the panda-derived L. plantarum BSG201683, when added to LPS-treated intestinal epithelial cell cultures, has been shown to strengthen their integrity, downregulate pro-inflammatory and upregulate anti-inflammatory cytokines, such as IL-10[23]. The undoubted anti-inflammatory effects of L. plantarum are further supported by the study of Ren et al[24] Mice with either acute or chronic dextran sulfate sodium (DSS)-induced colitis, when given L. plantarum, presented with overall improved health; the colitis symptoms improved, as did both the oxidative stress and inflammatory response[24]. The results from an LPS-induced colitis model in mice are similar; L. plantarum was able to ameliorate colitis, not only by counteracting its symptoms, but also by downregulating pro-inflammatory cytokines and by strengthening the epithelial barrier integrity[25].

THE GENUS LACTOBACILLUS

Lactobacillus acidophilus (L. acidophilus) is the most well-known probiotic species from the genus Lactobacillus, also having significant anti-inflammatory properties. We have previously shown that L. plantarum and L. acidophilus are involved in the immunological alertness of the intestinal tissue, as it slightly upregulated specific chemokines in subepithelial myofibroblasts[14]. One of the metabolites of L. acidophilus, the indole-3-lactic acid, has been shown to act beneficially during DSS-colitis in cesarean-born mice, as its administration led to decreased intestinal inflammation and increased type-3 innate lymphoid cells (ILC3) and IL-22 Levels[26]. When bone marrow dendritic cells were co-cultured with L. acidophilus, it was found that the probiotic promoted both the production of IL-17 by CD4⁺ T cells, and IL-22 by ILC3 cells[27].

In another study, L. acidophilus was found to improve DSS-induced colitis when in tandem with another probiotic strain, Veillonella ratti. By working together, these probiotics significantly restored lost body weight and colon length in mice, perhaps through short chain fatty acids (SCFA) production, while also improving overall disease activity, by downregulating pro-inflammatory cytokines and oxidative stress markers and upregulating anti-inflammatory factors[28]. The protective effects of L. acidophilus have also been emphasized by Aximujiang et al[29]: When L. acidophilus was given in combination with the Chinese medicine Huan Kui Le suspension, the protective effect against colitis was dramatically enhanced. The immune responses shifted towards immunoregulatory ones, and were supported by the upregulation of IL-13 and transforming growth factor-β and the downregulation of IFN-γ, the microbiota composition being once again enriched with beneficial bacteria[29].

THE GENUS LACTICASEIBACILLUS

Lacticaseibacillus rhamnosus, (formerly Lactobacillus rhamnosus, L. rhamnosus), has also been shown to exert anti-inflammatory and immunoregulating properties. Indeed, postbiotic (i.e. heat-killed fractions and proteins from the bacteria) of L. rhamnosus were also reported to downregulate the pro-inflammatory cytokine IL-18 in the LPS in vitro model of Magryś et al[21], previously described, but also to upregulate the anti-inflammatory cytokine IL-10, which L. plantarum failed to do[21].

Chemotherapy to fight cancer is known to induce gastrointestinal tract inflammation[30,31]. When L. rhamnosus was given as pretreatment, it was shown to mitigate the inflammatory responses; Nenu et al[32] showed that the combinational therapy of regorafenib and L. rhamnosus for the treatment of hepatocellular carcinoma in mice resulted in a significant reduction of inflammation and gut permeability[32], while Alsholi et al[33] reported that the administration of L. rhamnosus alleviated cisplatin-induced mucositis in an animal model[33]. Lu et al[34] also reported that L. rhamnosus GG postbiotic and antiprogrammed cell death 1 (anti-PD1) immunotherapy had better results in colorectal cancer treatment in relation to anti-PD1 immunotherapy alone. The authors observed increased populations of MHC II⁺ DC cells, CD4⁺ and CD8⁺ T cells in the tumor sites[34], suggesting that the probiotic extracellular vesicles could boost the immune system in favor of the host, to fight the cancerous cells.

Apart from its effects on cancer-induced inflammation, L. rhamnosus has been also shown to have beneficial anti-inflammatory properties in DSS-colitis; Kim et al[35] found that the administration of L. rhamnosus KBL2290 in mice could ameliorate colitis by restoring body weight and colon length, reducing disease activity, and downregulating pro-inflammatory cytokines, while at the same time upregulating the anti-inflammatory IL-10[35]. The results from an LPS-induced inflammation in Caco-2 cell culture are similar; L. rhamnosus was found to counteract the detrimental effects of LPS on Caco-2 cells, thus enhancing their survival, reducing the inducible oxidative stress, inducing the expression of tight junction proteins and by downregulating pro-inflammatory cytokines[36]. Tomotsune et al[37] supported the option that the beneficial immunomodulatory effects of L. rhamnosus are not necessarily promoted through its adhesion to the epithelial barrier, but can also be exerted without binding to the mucus, possibly through its secreted products[37].

Finally, the beneficial effects of L. rhamnosus are not limited to the immune system. Chen et al[38] showed that this probiotic may favorably influence the epithelial barrier during sepsis, thus increasing survival time; which may occur through the increase of intestinal stem cells proliferation[38].

THE GENUS LIGILACTOBACILLUS

Ligilactobacillus [formerly known as the Lactobacillus salivarius (L. salivarius) group], are lactic acid producing, Gram-positive bacteria, commonly found in fermented foods[39]. L. salivarius has been characterized as a potential probiotic, due to its anti-inflammatory properties. Carbonne et al[40] showed that L. salivarius CNCM I-4866 was able to downregulate a number of pro-inflammatory markers both in vitro and in vivo, strengthen the epithelial barrier and inhibit the adherence of various intestinal pathogens to the host’s epithelial cells[40].

THE GENUS LIMOSILACTOBACILLUS

The genus Limosilactobacillus, (formerly known as Lactobacillus) comprises several species able to possibly exert favorable outcomes for the host[41]. The most well-studied Limosilactobacillus species is Lactobacillus fermentum (L. fermentum).

Two species of the Limosilactobacillus genus, L. fermentum MN410703 and MN410702, were used to investigate whether they effectively inhibit enteric pathogens to prevent environmental enteropathy. The authors concluded that both strains have strong anti-inflammatory properties, and thus prevent chronic gut inflammation through over-expression of IL-6 and IL-10 in vitro and downregulation of the pro-inflammatory cytokine IL-8. Additionally, both strains were found to exert strong antagonistic properties on pathogens, adhesion to HT-29 cells, and inhibition of pathogen adherence to HT-29 cells[42].

L. fermentum has also been shown to protect against chemotherapy-induced gut permeability by regulating the expression and localization of tight-junction proteins and LPS-induced inflammation and by downregulating various pro-inflammatory cytokines. A number of authors have suggested that L. fermentum treatment could alleviate the adverse effects of chemotherapy in patients with colon cancer[43]. In the case of DSS-colitis, L. fermentum has also shown promising results as it improved the overall health of mice by restoring their lost body weight and colon length, but also by strengthening the epithelial barrier integrity through the expression of the tight-junction proteins. Additionally, it shifted the immune responses by promoting the T regulatory and suppressing the T inflammatory cells[44]. In a similar animal model of colitis, L. fermentum was shown to counteract inflammation by targeting the nuclear factor kappa B (NF-κB) signaling pathway, and thus downregulating several pro-inflammatory cytokines, such as tumor necrosis factor alpha (TNF)-α and IL-1β[45].

In another study which compared the possible anti-inflammatory properties of Limosilactobacillus mucosae (L. mucosae) and Lactobacillus amylovorus with another lactic acid bacterium, the L. mucosae, it was found to predominate; although both probiotics downregulated several pro-inflammatory cytokines in mice with DSS-colitis, L. mucosae had better effects in alleviating the colitis symptoms[46].

THE GENUS LEVILACTOBACILLUS

Levilactobacillus (formerly also known as Lactobacillus) is a genus of Gram-positive bacteria found mainly in fermented foods and in the composition of the intestinal microbiota[47]. One of its species, Levilactobacillus brevis, has been found to have promising anti-inflammatory properties, as Kim et al[48] showed that, when given, in inflamed HT-29 intestinal epithelial cells it resulted in the reduction of IL-8 and NF-κB levels, possibly through targeting the extracellular signal-regulated kinase and Akt signaling pathways[48].

THE GENUS BIFIDOBACTERIUM

Bifidobacterium, a genus of Gram-positive bacteria, is found both in food and in the gastrointestinal tract. Several of its species are also considered as beneficial probiotic supplements[49]. In an experimental model of excisional cutaneous trauma in rats, we have previously shown that Bifidobacterium longum (B. longum) can promote wound healing and especially angiogenesis[13]. However, its gut mucosal effect on inflammation is still under discussion. Li et al[19] found that the administration of B. longum in E. coli-infected mice successfully restored the lost body weight and colon length; however, it only downregulated the pro-inflammatory cytokines IL-6 and TNF-α, but not IL-1β[19]. In a similar study, where the animals were infected with Plasmodium berghei, B. longum was able to counteract the infection by diminishing parasitemia, reducing the levels of the pro-inflammatory cytokines IFN-γ and TNF-α in the serum and by enhancing an anti-inflammatory profile in the animals[50].

On the other hand, B. longum BAA2573 was observed to ameliorate DSS-induced colitis in mice, restoring the body weight and colon length by means of decreasing a number of disease activity markers, such as neutrophil infiltration, while, at the same time, also improving the gut microbiota diversity[51]. This action pathway leads us to believe that B. longum may be beneficial only in certain types of inflammation, or that it may exert its anti-inflammatory properties only when combined with other probiotics. In the study of Yue et al[52], when B. longum was given with Bifidobacterium bifidum to mice with LPS-induced colitis, the beneficial result was even greater than when either one was given alone. Working together these two probiotics managed to boost the immune system by elevating the IgA levels in the serum, along with increasing populations of CD4⁺/CD8⁺ T and dendritic cells. They also succeeded in strengthening the epithelial barrier by means of promoting the expression of mucus and tight junction proteins and, additionally, downregulated various pro-inflammatory cytokines[52].

Finally, bioengineered probiotics have been reported to have even greater beneficial effects on the host than simple bacteria strains[53]. One such example is B. longum fortified with artificial enzymes, which can lead to much reduced intestinal inflammation and enriched microbiota diversity[54].

Bifidobacterium bifidum (B. bifidum), on its own, has also been shown to ameliorate DSS- and trinitrobenzene sulfonic acid-induced colitis in mice. In both studies, B. bifidum was found to improve the overall health of the mice by restoring their body weight and colon length, strengthening the epithelial barrier integrity and the immune profile, downregulating pro-inflammatory factors and upregulating the anti-inflammatory ones[55,56]. In the case of DSS-colitis, the favorable effects of B. bifidum were speculated to arise through the activation of the aryl hydrocarbon receptor in the intestine[56].

Bifidobacterium breve (B. breve) has also been reported to possess anti-inflammatory properties. In particular, Park et al[57] showed that the administration of B. breve in two different animal models of colitis (DSS and dinitrobenzene sulfonic acid) had favorable effects, leading to the amelioration of disease severity. It increased the number of goblet cells in the intestinal epithelium and also strengthened the epithelial barrier by upregulating the mRNA of tight-junction proteins[57]. In an animal model of ileitis, the administration of a four probiotic formula comprising Saccharomyces boulardii, L. rhamnosus, Lactobacillus acidophilus, B. breve plus amylase led to a significant improvement in overall health: Disease activity was reduced, gut microbiota was enriched, and the immune system significantly boosted[58]. Nonetheless, these beneficial effects were not observed when amylase was not given[58]. It is proposed that amylase might play a significant role in inhibiting the biofilm formation by the potentially harmful bacteria.

Bifidobacterium lactis (B. lactis) is known to exert promising probiotic properties. Our research group has highlighted that this probiotic may contribute to the immunological alertness of the intestinal tissue through the upregulation of specific chemokines in subepithelial myofibroblasts[14]. In the study by Lan et al[59], the administration of B. lactis in mice with DSS-induced colitis led to the amelioration of the disease (weight loss and disease activity scores were reduced), but more significantly, several pro-inflammatory cytokines were downregulated[59], strongly highlighting the anti-inflammatory properties of B. lactis.

Bifidobacterium pseudocatenulatum (B. pseudocatenulatum) is a less studies species. Wang et al[60] reported that the administration of B. pseudocatenulatum in mice with DSS-induced colitis led to an overall amelioration of the disease, the integrity of the epithelial barrier was found to be strengthened through the upregulation of tight-junction proteins and mucus production, oxidative stress was decreased by the promotion of the expression of several antioxidant enzymes and inflammation was decreased through the downregulation of pro-inflammatory cytokines and the upregulation of the anti-inflammatory IL-10[60].

Finally, Bifidobacterium animalis (B. animalis) subsp. lactis BLa80 has been investigated for its possible therapeutic properties in IBD. B. animalis subsp. lactis BLa80 was shown to have a favorable effect in DSS-induced colitis in mice, as it not only reduced the histological disease scores and restored the colon length, but it also downregulated pro-inflammatory cytokines and enriched the microbiota composition[61].

THE GENUS PEDIOCOCCUS

Pedioccocus is a Gram-positive, lactic acid bacterium which plays a significant role in the food fermentation process[62]. Li et al[19] found that the administration of Pediococcus acidilactici in E. coli-infected mice successfully restored body weight and colon length, as well as downregulating the pro-inflammatory cytokines IL-1β, IL-6 and TNF-α[19].

Another species, Pediococcus pentosaceus (P. pentosaceus) CECT8330, was investigated for its possible anti-inflammatory properties in a mouse model of DSS colitis[63,64]. The administration of P. pentosaceus resulted in the restoration of lost body weight and colon length, and in the reduction of disease activity and inflammation[63]. However, it is of exceptional interest to mention that P. pentosaceus is involved in an early switch in macrophage phenotype from the pro-inflammatory M1 to M2, in parallel with the downregulation of IL-1β levels. This finding is evidence of the acceleration of the inflammatory phase during the healing process[64].

THE GENUS AKKERMANSIA

Akkermansia is an aerotolerant anaerobe, Gram-negative bacteria, capable of growing on a viscus substrate such as mucin[65]. Although it was first isolated in 2004, it is known that it has the ability to degrade intestinal mucin glycoproteins, a process leading to the production of SCFA. Additionally, it promotes mucin turnover, thus strengthening the mucosal barrier and reducing gut permeability[66]. Liu et al[67] highlighted the beneficial properties of Akkermansia muciniphila (A. muciniphila) in a mouse model of antibiotic-induced diarrhea. A. muciniphila was found to reduce diarrhea incidents, to enhance the epithelial barrier integrity and to enrich the microbiome and metabolome profiles. Regarding the inflammation status, it resulted in the upregulation of anti-inflammatory markers, GPR109A and SLC5A8, and the downregulation of pro-inflammatory TNFα, IFN-γ, IL1β, and IL6[67]. The favorable anti-inflammatory effects of A. muciniphila have also been underlined by Daniel et al[68], being found to ameliorate the emulsifier-induced inflammation in mice through the reduction of inflammatory cell infiltration and histology scores[68].

Finally, another study underlines the favorable immunomodulatory effects of A. muciniphila on the gut-brain axis. A culture medium of Caco-2 intestinal epithelial cells, when pre-treated with A. muciniphila, exhibited an inhibitory effect on pro-inflammatory cytokine production in human microglial cells[69].

THE GENUS ROUXIELLA

The genus Rouxiella was first described back in 2015 by Le Flèche-Matéos et al[70]. One of its members, the R. badensis subsp. acadiensis (Canan SV-53), is thought to exert immunomodulatory effects, as Shahbazi et al[71] showed that the administration of R. badensis subsp. acadiensis in healthy mice resulted in an increase in serum IgA, a decrease in several pro-inflammatory cytokines and an increase in the anti-inflammatory subset population of cells in the small intestine[71], all probably contributing to immune system boosting. In the next experiment by the same research group on LPS-induced colitis in mice, the administration of R. badensis subsp. acadiensis led to the amelioration of disease severity through anti-inflammatory effects that mainly targeted the epigenetic mechanisms of several genes involved in the differentiation of Th17 cells[72].

THE GENUS ROSEBURIA

Roseburia are Gram-positive anaerobic bacteria. R. intestinalis exerts immunomodulatory effects and is a major butyrate producer in the gut[73]. Kang et al[74] showed that R. intestinalis may favor cytotoxic CD8⁺ T cell populations during colorectal cancer, and thus boost the immune system towards fighting tumorous cells[74].

THE GENUS TETRAGENOCOCCUS

The genus Tetragenococcus comprises Gram-positive facultative anaerobic lactic acid bacteria[75], and among its members, Tetragenococcus halophilus (T. halophilus) has been shown to potentially protect against intestinal inflammation. In a DSS-induced colitis animal model, the administration of T. halophilus led to an overall amelioration of disease activity, but more significantly, it also reduced the activation of several immune-related cell populations, leading to decreased production of the pro-inflammatory cytokine IL-1β[76].

THE GENUS FAECALIBACTERIUM

The genus Faecalibacterium is an extremely oxygen-sensitive commensal butyrate-producer, populating the most anaerobic parts of the GI tract of mammals and is generally recognized as a biomarker of intestinal health[77]. Faecalibacterium prausnitzii (F. prausnitzii) has been found to be decreased in patients with IBD[78], and thus its (further) supplementation may lead to favorable outcomes. Indeed, in an animal model of DSS-induced colitis, the administration of Treg cells exposed to F. prausnitzii resulted in the amelioration of colitis, with decreased disease scores and significantly reduced inflammation[79].

THE GENUS ENTEROCOCCUS

Enterococcus is a genus of lactic acid bacteria, the species of which exert both harmful and beneficial effects[80], thus some species may be considered as possible probiotic supplements. Zheng et al[81] showed that Enterococcus faecium (E. faecium) could have a protective effect against the enterotoxigenic Escherichia coli infection, since it enhances the expression of tight-junction proteins and downregulates pro-inflammatory cytokines[81]. Benvenuti et al[82] suggested that the administration of E. faecium in obese mice resulted in reduced inflammation and improvements in the integrity of the epithelial barrier[82]. Although these results seem promising this genus has not yet received the status of “Generally Recognized As Safe” (GRAS) due to a number of potential health risks[80].

THE GENUS CLOSTRIDIUM

Clostridium is a genus of Gram-positive anaerobic bacteria and, similar to the Enterococcus genus, it includes both harmful and beneficial species[83]. Clostridium butyricum (C. butyricum), one of its species, exerts possible anti-inflammatory effects in experimental colitis. Indeed, extracellular vesicles (postbiotic) from this species have been shown to have anti-inflammatory properties both in vitro and in vivo, by suppressing the pro-inflammatory signaling pathways of mitogen-activated protein kinase and NF-κB through the restoration of the expression of miR-199a-3p[84]. The reduction in inflammation by means of suppression of the myeloid differentiation primary response 88 and NF-κB signaling pathways upon administration of C. butyricum, was also observed in an animal model of colorectal cancer[85], suggesting that C. butyricum could indeed be considered as a possible anti-inflammatory probiotic supplement.

THE GENUS SACCHAROMYCES

Saccharomyces is a yeast and Saccharomyces cerevisiae (S. cerevisiae) is one of the most well-known species, widely used in the food industry – brewer’s yeast - but additionally serves as a very potent probiotic[86]. Regarding its role in regulating inflammation, Kil et al[87] showed that the administration of S. cerevisiae to mice with DSS-induced colitis resulted in the downregulation of neutrophil infiltration and the pro-inflammatory cytokine, TNF-α, and in upregulation of the expression of tight-junction proteins and the anti-inflammatory cytokine IL-10[87]. In transgenic rats subjected to ileocecal resection for Crohn's disease, when postoperative recurrence occurred, the administration of S. cerevisiae resulted in a reduction in the macroscopic and histological lesions in the anastomosis area, a decrease in E. coli LF82 adherence, as well as in downregulation of the pro-inflammatory IL-23 and IL-17 cytokines and upregulation of the anti-inflammatory IL-10[88].

When four different yeasts were tested both in vitro and in vivo, S. cerevisiae predominate as shown to have the strongest anti-inflammatory properties. S. cerevisiae has also been shown to protect against experimental colitis, as the overall health of animals improved and several inflammation markers were downregulated[89].

Another yeast of the same genus is the Saccharomyces boulardii. We have also previously documented in relation to subepithelial myofibroblasts that it is implicated in the immunological alertness of the intestinal tissue, through a mild upregulation of specific chemokines[14].

CONCLUSION

Our assessment of the most studied probiotics led us to conclude that all genera and their species, in different ways, downregulate intestinal inflammation and enhance immune response, as follows (Table 1): Almost all genera downregulate pro-inflammatory cytokines production; less genera, mainly of Lactobacillaceae family upregulate the anti-inflammatory cytokines. L. plantarum, L. acidophilus and L. rhamnosus, B. breve and B. lactis, R. badensis ssp.acadiensis and R.intestinalis as well as S. boulardii enhance immune function through different pathways. Almost all genera strengthen the epithelial barrier integrity by restoring the expression of tight junction proteins and/or by promoting of mucus expression. L. plantarum and L. salivarius, B. longum, B. breve, and B. animalis ssp. lactis as well as A. muciniphila enhance the disturbed - due to disease - intestinal microbial diversity. L. rhamnosus and B. pseudocatenulatum promote the expression of antioxidant enzymes. L. rhamnosus promotes the proliferation and differentiation of intestinal stem cells; B. breve relieves intestinal inflammation through augmenting goblet cell regeneration; and P. pentosaceus shifts macrophage polarization toward the anti-inflammatory M2 phenotype. Muciniphila seems to exert neuroprotective effects through the 'gut-brain' axis. Almost all genera - through different mechanisms and pathways - alleviate experimental colitis symptoms.

Table 1 Probiotics on intestinal inflammation and immune responses.

Probiotics	Immune system↑	Inflammatory cytokines		Epithelial barrier↑	Colitis symptoms↓	Micro-biota diversity↑	Oxidative stress↓	Stem cells↑	Goblet cells↑	M2 macrophages↑	Gut-brain axis↑	Ref.
		↓Pro-	↑Anti-
L. plantarum	+	+	+	+	+	+						[14-19,21-25]
L. acidophilus	+	+	+		+							[14,26-29]
L. rhamnosus	+	+	+	+	+		+	+				[21,32-36,38]
L. salivarius		+		+		+						[46]
L. fermentum		+	+	+	+							[42-45]
L. brevis		+										[48]
B. longum		+	+	+	+	+						[19,50-52,54]
B. bifidum		+	+	+	+							[55,56]
B. breve	+			+	+	+			+			[57,58]
B. lactis	+	+			+							[14,59]
B. pseudo-catenulatum		+	+	+	+		+					[60]
B. animalis ssp. lactis		+			+	+						[61]
P. acidilactici		+			+							[19]
P. pentosaceus		+			+					+		[63,64]
A. muciniphila		+	+	+		+					+	[67-69]
R. badensis ssp.acadiensis	+	+	+		+							[71,72]
R. intestinalis	+											[74]
T. halophilus		+			+							[76]
F. prausnitzii		+			+							[79]
E. faecium		+	+	+								[81,82]
C. butyricum		+										[84,85]
S. cerevisiae		+	+	+	+							[87-89]
S. boulardii	+											[14]

The up↑ and down↓ arrows mean ‘increase’ and ‘decrease’, respectively.