Probiotic Yeast Saccharomyces: Back to Nature to Improve Human Health

Saccharomyces cerevisiae var. boulardii is best known for its treatment efficacy against different gastrointestinal diseases. This probiotic yeast can significantly protect the normal microbiota of the human gut and inhibit the pathogenicity of different diarrheal infections. Several clinical investigations have declared S. cerevisiae var. boulardii a biotherapeutic agent due to its antibacterial, antiviral, anti-carcinogenic, antioxidant, anti-inflammatory and immune-modulatory properties. Oral or intramuscular administration of S. cerevisiae var. boulardii can remarkably induce health-promoting effects in the host body. Different intrinsic and extrinsic factors are responsible for its efficacy against acute and chronic gut-associated diseases. This review will discuss the clinical and beneficial effects of S. cerevisiae var. boulardii in the treatment and prevention of different metabolic diseases and highlight some of its health-promising properties. This review article will provide fundamental insights for new avenues in the fields of biotherapeutics, antimicrobial resistance and one health.


Introduction
According to the latest definition of the World Health Organization, probiotics are active microbes that stimulate the growth of other probiotic bacteria in the gut and possess beneficial health effects to the host [1]. These microorganisms are able to produce anticarcinogenic, antioxidant and anti-mutagenic agents and induce protection against different bacterial diseases including diarrhea and respiratory tract infections. Saccharomyces cerevisiae var. boulardii is the most significant probiotic yeast species. S. cerevisiae var. boulardii is a eukaryotic organism that has been used in scientific investigations since the time of its discovery [2]. This model organism has unique importance because of its alterable and flexible genome. The genome of S. cerevisiae var. boulardii was completely sequenced in 1950 and a genome size of approximately 11.3 Mb was reported. It has approximately 6000 genes and 275 additional tRNA genes. Almost 23% of the S. cerevisiae var. boulardii's genome is homologous to the hominid genome. This specific yeast is best known for its role in treating gastrointestinal diseases [3,4].
S. cerevisiae var. boulardii has gained the importance of the scientific community due to the production of different bioactive compounds [5]. This specie is an excellent protein source with high amino acid content, which is essential for the production of various foods boulardii can produce beneficial health effects [22]. The dose of S. cerevisiae var. boulardii in case of chronic diseases should be increased to meet the treatment criteria. Recently, an upsurge in multidrug-resistant organisms is reported due to the excessive consumption of antimicrobials [23]. Global healthcare authorities are trying to create awareness all over the globe via antibiotic stewardship programs, but the severity of antimicrobial resistance is continuously increasing [24]. To cope with this alarming situation, probiotics, especially S. cerevisiae and S. cerevisiae var. boulardii yeast, can be considered as an alternative method for the treatment of bacterial and fungal infections. A number of research and review articles describing the probiotic potentials of yeast and bacteria have been published in the last decade. A comprehensive review is needed to highlight the probiotic potential of S. cerevisiae var. boulardii in various aspects. Therefore, the aim of this review is to explore the diverse probiotic potential of S. cerevisiae var. boulardii through the combination of different meta-analyses. Utilization of S. cerevisiae var. boulardii as an alternate to antibiotics for the treatment and eradication of different metabolic diseases is investigated. Moreover, details of commercially available probiotic strains of S. cerevisiae var. boulardii and their clinical and beneficial detailed effects are provided. Finally, the significance of S. cerevisiae var. boulardii against cancer signaling cascades and safety attributes regarding its consumption among humans and livestock animals are thoroughly discussed. and children (Figure 1) [20]. A study claimed that this yeast can also effectively wo against chronic permeability in patients with Crohn's disease when administered ora for 3 months [21]. In HIV-linked diarrhea, exposure to a 3 g per day dose of S. cerevis var. boulardii can produce beneficial health effects [22]. The dose of S. cerevisiae var. b lardii in case of chronic diseases should be increased to meet the treatment criteria. R cently, an upsurge in multidrug-resistant organisms is reported due to the excessive co sumption of antimicrobials [23]. Global healthcare authorities are trying to create awa ness all over the globe via antibiotic stewardship programs, but the severity of antimic bial resistance is continuously increasing [24]. To cope with this alarming situation, p biotics, especially S. cerevisiae and S. cerevisiae var. boulardii yeast, can be considered as alternative method for the treatment of bacterial and fungal infections. A number of search and review articles describing the probiotic potentials of yeast and bacteria ha been published in the last decade. A comprehensive review is needed to highlight probiotic potential of S. cerevisiae var. boulardii in various aspects. Therefore, the aim this review is to explore the diverse probiotic potential of S. cerevisiae var. boulardii throu the combination of different meta-analyses. Utilization of S. cerevisiae var. boulardii as alternate to antibiotics for the treatment and eradication of different metabolic disease investigated. Moreover, details of commercially available probiotic strains of S. cerevis var. boulardii and their clinical and beneficial detailed effects are provided. Finally, significance of S. cerevisiae var. boulardii against cancer signaling cascades and safety tributes regarding its consumption among humans and livestock animals are thoroug discussed.

Enzymatic Potential of S. cerevisiae var. boulardii
S. cerevisiae var. boulardii can produce different enzymes which play a significant r in various industrial processes ( Figure 2). Some active enzymes, i.e., maltase and inve ase, have the potential to enhance the flavor of fermented products specifically in the fo industry. Maltase is responsible for the conversion of malt sugar into normal sugar wh invertase converts granulated sugar into regular sugar. Another enzyme, zymase, tra forms normal sugar into CO2 and alcohol [25]. S. cerevisiae var. boulardii is able to produ intestinal enzymes including amylase, protease, cellulase and lipase and is unable to sy thesize galactosidase, DNAase and gelatinase ( Figure 2) [26]. S. cerevisiae var. boulardii h

Enzymatic Potential of S. cerevisiae var. boulardii
S. cerevisiae var. boulardii can produce different enzymes which play a significant role in various industrial processes ( Figure 2). Some active enzymes, i.e., maltase and invertase, have the potential to enhance the flavor of fermented products specifically in the food industry. Maltase is responsible for the conversion of malt sugar into normal sugar while invertase converts granulated sugar into regular sugar. Another enzyme, zymase, transforms normal sugar into CO 2 and alcohol [25]. S. cerevisiae var. boulardii is able to produce intestinal enzymes including amylase, protease, cellulase and lipase and is unable to synthesize galactosidase, DNAase and gelatinase ( Figure 2) [26]. S. cerevisiae var. boulardii has antibacterial properties due to the presence of extracellular protease enzymes and cell surface hydrophobicity [27]. This yeast enhances the concentration of the enzymes by the production of polyamines that trigger the cells of the intestine. Cell surface hydrophobicity is responsible for the adherence of S. cerevisiae var. boulardii yeast to the cell wall lining of the human intestine. S. cerevisiae var. boulardii is critically responsible for the production of ethanol in anaerobic conditions. This species can also show tolerance against a high level of ethanol and gastric discharge, including bile salts and intestinal acids, hence, eliminating toxic bacterial strains from the host body in the form of fecal matter [28]. Interestingly, this yeast can work against both Gram-positive and -negative bacteria and boost the host immunity. Despite its antibacterial properties, S. cerevisiae var. boulardii also showed resistance against all broad and narrow-spectrum antibiotic drugs and does not disturb the normal microbiota of the gastrointestinal tract of the host [28].
production of polyamines that trigger the cells of the intestine. Cell surface hydroph city is responsible for the adherence of S. cerevisiae var. boulardii yeast to the cell wall lin of the human intestine. S. cerevisiae var. boulardii is critically responsible for the produc of ethanol in anaerobic conditions. This species can also show tolerance against a h level of ethanol and gastric discharge, including bile salts and intestinal acids, hence, e inating toxic bacterial strains from the host body in the form of fecal matter [28]. Inte ingly, this yeast can work against both Gram-positive and -negative bacteria and b the host immunity. Despite its antibacterial properties, S. cerevisiae var. boulardii showed resistance against all broad and narrow-spectrum antibiotic drugs and does disturb the normal microbiota of the gastrointestinal tract of the host [28].

Factors Responsible for the Efficiency of S. cerevisiae var. boulardii as a Probioti
Probiotics are being used to enhance treatment efficacy and to produce signifi health benefits. S. cerevisiae var. boulardii is a unicellular, cost-effective active yeast spe that has probiotic potential and is often used as a nutritional additive [29]. Different mo of actions were observed in favor of the host and against the antigenic microorgani which include luminal action: (1) Antimicrobial activity: (a) Reduction in the intest bacterial growth [30], (b) lowering of gastrointestinal translocation of microbes [31] nullifying the effect of bacterial pathogenicity [32], (d) reducing the binding affinity o host cell with the bacterial population [33]. (2) Antitoxin effects: (a) obstructing the p ogenic receptor active sites [30], (b) enhancing the production of antibodies against C tridium difficile toxin A [34], (c) mediating the synthesis of the phosphatases enz against Escherichia coli (E. coli) [35], (d) cleavage of pathogenic enzymatic proteins [35] Trophic action associated with intestinal linings: (a) reducing the expression of tumor crosis factor-alpha (TNFα) gene and inhibiting programmed cell death [36], (b) enhan the synthesis of glycoprotein in the intestinal brush border [37], (c) inducing the prod tion of intestinal polyamines [37], (d) repairing fluid transport pathways [37], (e) sti lating the production of membrane enzymes (28). (4) Mediation of immune system stimulating the production of regulatory T cells [32], (b) enhancing the level of IgG a body against Clostridium difficile toxin A [34], (c) improving the adherence of WBCs (W Blood Cells) to the endothelial cells [38,39].
Probiotics have gained global beneficial additive status to use as a potential feed plement [40]. Human probiotic administration is based on the development and viab

Factors Responsible for the Efficiency of S. cerevisiae var. boulardii as a Probiotic
Probiotics are being used to enhance treatment efficacy and to produce significant health benefits. S. cerevisiae var. boulardii is a unicellular, cost-effective active yeast species that has probiotic potential and is often used as a nutritional additive [29]. Different modes of actions were observed in favor of the host and against the antigenic microorganisms which include luminal action: (1) Antimicrobial activity: (a) Reduction in the intestinal bacterial growth [30], (b) lowering of gastrointestinal translocation of microbes [31], (c) nullifying the effect of bacterial pathogenicity [32], (d) reducing the binding affinity of the host cell with the bacterial population [33]. (2) Antitoxin effects: (a) obstructing the pathogenic receptor active sites [30], (b) enhancing the production of antibodies against Clostridium difficile toxin A [34], (c) mediating the synthesis of the phosphatases enzyme against Escherichia coli (E. coli) [35], (d) cleavage of pathogenic enzymatic proteins [35].
(3) Trophic action associated with intestinal linings: (a) reducing the expression of tumor necrosis factor-alpha (TNFα) gene and inhibiting programmed cell death [36], (b) enhancing the synthesis of glycoprotein in the intestinal brush border [37], (c) inducing the production of intestinal polyamines [37], (d) repairing fluid transport pathways [37], (e) stimulating the production of membrane enzymes (28). (4) Mediation of immune system: (a) stimulating the production of regulatory T cells [32], (b) enhancing the level of IgG antibody against Clostridium difficile toxin A [34], (c) improving the adherence of WBCs (White Blood Cells) to the endothelial cells [38,39].
Probiotics have gained global beneficial additive status to use as a potential feed supplement [40]. Human probiotic administration is based on the development and viability of probiotics in the intestinal lumen of the host organisms. Probiotic yeast has more survival chances in the stomach due to the presence of digestive enzymes, bile and gastrointestinal juices in comparison to probiotic bacteria [41]. The Food and Drug Administration (FDA) has approved certain probiotic strains which are potentially used for the benefit of humans, but S. cerevisiae and S. cerevisiae var. boulardii are the only probiotic yeast species that are commercially used for human benefits (Table 1) [42]. S. cerevisiae var. boulardii has surpassed the affectivity of the commonest probiotic bacteria, i.e., lactobacillus due to its resistance against different antibiotics [43]. S. cerevisiae var. boulardii can be administered to patients as an alternative source of antibiotics due to its outrageous antibacterial properties. Probiotic consumption can also reduce the pathogenicity of harmful microbes present in the human gut [44]. The different strains of Saccharomyces sp., including S. boulardii, S. cerevisiae, and S. unisporus, also showed antibacterial and antiviral properties. These strains were used to enhance the probiotic potential of different human food supplements. These probiotic strains are also effective against acute and chronic diarrhea. The combination of S. cerevisiae var. boulardii with other probiotics of the same or different genus can also enhance the efficacy of human feed supplements [45].
Several intrinsic and extrinsic factors are directly implicated in the efficacy of S. cerevisiae var. boulardii as a probiotic ( Figure 3).
OR PEER REVIEW 6 of 20  (Table 2), while other S. cerevisiae var. boulardii strains are functional at 37 °C temperature and some can survive below 20 °C temperature. As a probiotic, S. cerevisiae var. boulardii is present in the form of capsules. The heat-dried S. cerevisiae var. boulardii capsules could not survive at 25 °C after opening due to their reduced potency. They retain their efficacy when stored at a 4 °C refrigerator. Lyophilized S. cerevisiae var. boulardii capsules can survive at room temperature and are viable for 1 year approximately. Studies suggested that S. cerevisiae var. boulardii can grow best at 37 °C. However, the death phase of this yeast usually appeared at 55-56 °C [46,47].

Water Activity aw and Relative Humidity
Water activity aw and relative humidity can produce synergistic effects. Survival of S. cerevisiae var. boulardii can be influenced by water activity. A study reported that the cells of S. cerevisiae var. boulardii showed a 0.98% value of water activity when refrigerated at −20 °C, which ultimately increased the survival rate of the yeast S. cerevisiae var. boulardii (Table 2). However, reduced water activity conditions can deteriorate the viability of S.  (Table 2), while other S. cerevisiae var. boulardii strains are functional at 37 • C temperature and some can survive below 20 • C temperature. As a probiotic, S. cerevisiae var. boulardii is present in the form of capsules. The heat-dried S. cerevisiae var. boulardii capsules could not survive at 25 • C after opening due to their reduced potency. They retain their efficacy when stored at a 4 • C refrigerator. Lyophilized S. cerevisiae var. boulardii capsules can survive at room temperature and are viable for 1 year approximately. Studies suggested that S. cerevisiae var. boulardii can grow best at 37 • C. However, the death phase of this yeast usually appeared at 55-56 • C [46,47].

Water Activity a w and Relative Humidity
Water activity a w and relative humidity can produce synergistic effects. Survival of S. cerevisiae var. boulardii can be influenced by water activity. A study reported that the cells of S. cerevisiae var. boulardii showed a 0.98% value of water activity when refrigerated at −20 • C, which ultimately increased the survival rate of the yeast S. cerevisiae var. boulardii (Table 2). However, reduced water activity conditions can deteriorate the viability of S. cerevisiae var. boulardii's cells. Water activity can also be influenced via relative humidity, specifically in the case of opened or uncovered foods. The viability of S. cerevisiae var. boulardii is reduced when the rate of water activity and environmental humidity decreases [48].

pH and Acidity
S. cerevisiae var. boulardii showed resistance against less pH and more acidic conditions. However, some yeast species are too fragile to bear such a hard environment. The ideal pH range for S. cerevisiae var. boulardii development and maturation is 2-8 ( Table 2). Yeast species belonging to a genus other than Saccharomyces showed tolerance against extreme acidic and alkaline environments. Overall functionality of the yeast is better in the lyophilized form [29,47,51].

Antimicrobial Agents
S. cerevisiae var. boulardii showed antimicrobial properties due to these subsequent reasons: (i) synthesis of the extracellular enzyme, i.e., protease, it aids in the formation of colonic mucosa, (ii) excretion of toxins and SO 2 gas, it can halt the efficacy of toxins released by Clostridium difficile, (iii) secretion of enzyme-based proteins, (iv) cell surface hydrophobicity and autoagglutination, it is responsible for the attachment of S. cerevisiae var. boulardii to the patient's intestinal lining. The viability of different Gram-positive and -negative bacteria can easily be reduced by the negative influence of S. cerevisiae var. boulardii on the host organism [26].

Nutrient Media for the Growth of S. cerevisiae var. boulardii
Yeast can grow on different nutrient media including Yeast extract peptone dextrose media, Sabouraud dextrose agar, but Oxytetracyclin yeast agar media (OGA) is considered as the best medium for its growth ( Table 2). Standard OGA media can be prepared by adding 8 g of yeast extract, 9 g of glucose, 12 g of nutrient agar and 0.1 uL of oxytetracycline in 500 mL of distilled H 2 O. Carbon (glucose, maltose, sucrose and fructose), nitrogen (Urea, peptone and powdered yeast extract) and a trace amount of minerals (zinc, copper, magnesium, sulfur) are required to enhance the growth of S. cerevisiae var. boulardii [49,50]. All these factors play a significant role in maintaining the viability of S. cerevisiae var. boulardii. The potential of this beneficial yeast may be disturbed when the optimum conditions of both intrinsic and extrinsic factors changes. Antibiotic-associated diarrhea (AAD) occurs due to the continuous consumption of antibiotics for a longer period. The use of probiotics, mainly S. cerevisiae var. boulardii, is the commonest method for the treatment against AAD. A total of 8 (80%) out of 10 (100%) controlled experiments confirmed the efficacy of S. cerevisiae var. boulardii for the prohibition of AAD specifically in adult patients ( Table 3). The beneficial impact of S. cerevisiae var. boulardii and the relative decline in AAD comparable to the control are categorized in the range of 7.4% and 25%, respectively [29]. The affectivity of S. cerevisiae var. boulardii against AAD in the pediatric population has also shown positive outcomes. Results of two meta-analyses confirmed the potential of S. cerevisiae var. boulardii against AAD with a pooled risk ratio of 0.47 and 0.43 and a 95% confidence interval [52].

Clostridium Difficile Infection (CDI)
Clostridium difficile (C. difficile) is a Gram-positive anaerobic rod-shaped bacteria that may cause antibiotic-associated Clostridium difficile diarrhea. It is responsible for the colon infection, it shows diarrhea (mild) to colon damage (severe) symptoms. Meta-analysis of six randomized control trials of different Saccharomyces strains including S. cerevisiae var. boulardii showed efficacy against CDI with a total risk ratio of 0.59 [53]. The beneficial impact of S. cerevisiae var. boulardii and the relative decline in CDI comparable to the control were categorized within the range of 19% and 33.3%, respectively (Table 3) Clostridium difficile (C. difficile) is a Gram-positive anaerobic rod-shaped bacteria that may cause antibiotic-associated Clostridium difficile diarrhea. It is responsible for the colon infection, it shows diarrhea (mild) to colon damage (severe) symptoms. Meta-analysis of six randomized control trials of different Saccharomyces strains including S. cerevisiae var. boulardii showed efficacy against CDI with a total risk ratio of 0.59 [53].
The beneficial impact of S. cerevisiae var. boulardii and the relative decline in CDI comparable to the control were categorized within the range of 19% and 33.3%, respectively (Table 3)

Acute Diarrhea
S. cerevisiae var. boulardii administered to patients involved in two randomized control group trials showed clinical efficacy against acute diarrhea as compared to the control (Table 3). S. cerevisiae var. boulardii consumption among 100 patients of an age less than 15 for 7 days resulted in reduced stool frequency and stabilized the normal stool condition [54]. A meta-analysis conducted among more than 600 patients that administered the S. cerevisiae var. boulardii probiotic strains for 60 days significantly reduce the rapid stool frequency [55]. Another meta-analysis of seven randomized controlled trials claimed to stabilize the childhood diarrhea consistency within 24 h as compared to the placebo treatment [56].

Persistent Diarrhea
Two randomized controlled trials suggested that S. cerevisiae var. boulardii significantly enhances treatment efficacy specifically in children with persistent diarrhea ( Table  3). The beneficial impact of S. cerevisiae var. boulardii and the relative decline in persistent

Acute Diarrhea
S. cerevisiae var. boulardii administered to patients involved in two randomized control group trials showed clinical efficacy against acute diarrhea as compared to the control (Table 3). S. cerevisiae var. boulardii consumption among 100 patients of an age less than 15 for 7 days resulted in reduced stool frequency and stabilized the normal stool condition [54]. A meta-analysis conducted among more than 600 patients that administered the S. cerevisiae var. boulardii probiotic strains for 60 days significantly reduce the rapid stool frequency [55]. Another meta-analysis of seven randomized controlled trials claimed to stabilize the childhood diarrhea consistency within 24 h as compared to the placebo treatment [56].

Persistent Diarrhea
Two randomized controlled trials suggested that S. cerevisiae var. boulardii significantly enhances treatment efficacy specifically in children with persistent diarrhea ( Table 3). The beneficial impact of S. cerevisiae var. boulardii and the relative decline in persistent diarrhea comparable to the control was 50%, respectively. However, a meta-analysis of S. cerevisiae var. boulardii against persistent diarrhea among pediatric and young populations has not been performed up till now [57].

Enteral Nutrition-Related Diarrhea
Diarrhea is the major complication associated with total enteral nutrition (TEN) and can also cause fluctuations in short-chain fatty acids (SCFA). Diabetes, gastrointestinal infection and malabsorption-related disorders are responsible for diarrhea-associated TEN ( Figure 5). Schneider et al. reported that patients who received S. cerevisiae var. boulardii can significantly enhance the levels of short-chain fatty acids in 10 TEN patients as compared to the normal controls. This treatment could increase the SCFA level in high stool frequency. The beneficial impact of S. cerevisiae var. boulardii and the relative decline in TEN-associated diarrhea comparable to control are categorized within the range of 5% and 8.2% in three randomized control trials, respectively [37].

Enteral Nutrition-Related Diarrhea
Diarrhea is the major complication associated with total enteral nutrition (TEN) and can also cause fluctuations in short-chain fatty acids (SCFA). Diabetes, gastrointestina infection and malabsorption-related disorders are responsible for diarrhea-associated TEN ( Figure 5). Schneider et al. reported that patients who received S. cerevisiae var. bou lardii can significantly enhance the levels of short-chain fatty acids in 10 TEN patients as compared to the normal controls. This treatment could increase the SCFA level in high stool frequency. The beneficial impact of S. cerevisiae var. boulardii and the relative decline in TEN-associated diarrhea comparable to control are categorized within the range of 5% and 8.2% in three randomized control trials, respectively [37].

Traveler's Diarrhea
Traveler's diarrhea is a common digestive illness that is responsible for frequent stoo discharge. It occurs due to the intake of contaminated food or water ( Figure 5). Twelve randomized control trials of S. cerevisiae var. boulardii and other probiotic strains signifi cantly reduced the severity of the infection caused by Traveler's diarrhea in children (Ta ble 3). The beneficial impact of S. cerevisiae var. boulardii and the relative decline in Trav eler's diarrhea comparable to the control are categorized within the range of 5% and 11% in two randomized control trials, respectively [58].

Cancer
S. cerevisiae var. boulardii is being potentially used to inhibit cancer cell developmen and progression (Figure 4). It was observed that this probiotic yeast can reduce the tumor igenic effects of colorectal cells in humans. In vivo, high-throughput metagenomic analy sis of 281 stool samples confirmed that S. cerevisiae var. boulardii has significantly inhibited colorectal cancer metastasis by stimulating cancer cell apoptosis and promoting gastroin testinal health via immune modulation. S. cerevisiae var. boulardii significantly downregu lates the expression of various tumor-causing genes including TNFα, Interleukin-1β and Interleukin-17, the expression of NF-kB and mTOR signaling cascades was also inhibited ( Figure 4). However, the activity of different cytokines was not affected by S. cerevisiae var boulardii treatment. HCT116 and DLD1 colorectal cell lines were used to analyze the apop totic behavior of colorectal cells after the administration of S. cerevisiae var. boulardii. Re sults confirmed the presence of an enhanced percentage of apoptosis in probiotic yeast treated cells [59].

Traveler's Diarrhea
Traveler's diarrhea is a common digestive illness that is responsible for frequent stool discharge. It occurs due to the intake of contaminated food or water ( Figure 5). Twelve randomized control trials of S. cerevisiae var. boulardii and other probiotic strains significantly reduced the severity of the infection caused by Traveler's diarrhea in children ( Table 3). The beneficial impact of S. cerevisiae var. boulardii and the relative decline in Traveler's diarrhea comparable to the control are categorized within the range of 5% and 11% in two randomized control trials, respectively [58].

Cancer
S. cerevisiae var. boulardii is being potentially used to inhibit cancer cell development and progression (Figure 4). It was observed that this probiotic yeast can reduce the tumorigenic effects of colorectal cells in humans. In vivo, high-throughput metagenomic analysis of 281 stool samples confirmed that S. cerevisiae var. boulardii has significantly inhibited colorectal cancer metastasis by stimulating cancer cell apoptosis and promoting gastrointestinal health via immune modulation. S. cerevisiae var. boulardii significantly downregulates the expression of various tumor-causing genes including TNFα, Interleukin-1β and Interleukin-17, the expression of NFk B and mTOR signaling cascades was also inhibited ( Figure 4). However, the activity of different cytokines was not affected by S. cerevisiae var. boulardii treatment. HCT116 and DLD1 colorectal cell lines were used to analyze the apoptotic behavior of colorectal cells after the administration of S. cerevisiae var. boulardii. Results confirmed the presence of an enhanced percentage of apoptosis in probiotic yeast-treated cells [59].

Ulcerative Colitis
Broad-spectrum antibiotics are conventionally used to treat ulcerative colitis but due to antibiotic resistance, their efficacy has been reduced by a substantial level. Probiotics, especially S. cerevisiae var. boulardii and its derivatives, act as an alternative method for the maintenance of normal gut microbiota and help to treat chronic colitis diseased patients [60]. Studies suggested that the pathogenic strain of E. coli, known as adherentinvasive E. coli (AIEC), showed a strong binding affinity with the small intestinal lining of Crohn's disease patients. This Gram-negative bacteria can easily invade the intestine of patients. Patients with Crohn's disease showed strong adherence between AIEC bacteria due to its FimH adhesion potential and overexpressed mannose residues, which are present on the surface of intestinal glycoprotein CEACAM6 (carcinoembryonic antigen-related cell adhesion molecule). In vivo results reported that S. cerevisiae var. boulardii significantly blocked the adherence potential of LF82 to the intestinal brush border. Probiotic yeast also lowered the pro-inflammatory cytokine level and was confirmed to treat the pathogenesis of ulcerative colitis [61].

Crohn's Disease (CD)
Typically, Crohn's disease is a part of chronic inflammatory bowel disease, which can cause digestive inflammation, abdominal pain, weight loss, watery stool and malnutrition. Individuals with chronic Crohn's disease may also come across inflammation of skin, liver, joints, anemia, kidney stones and maldevelopment. Bacteria associated with Crohn's disease can damage the gastrointestinal tract (GI), especially the small intestine, colon and can cause erratic and multiwall GI inflammation. Dalmasso and colleagues reported that consumption of S. cerevisiae var. boulardii can significantly reduce the level of CD, can control chronic inflammation and reinforce epithelial reformation [62]. In a pilot study, 31 patients with Crohn's disease were randomly treated with S. cerevisiae var. boulardii or an antimicrobial drug for 12 weeks. Patients treated with probiotics considerably reduced colonic permeability as compared to the antimicrobial drug-treated patients. Another pilot study of 20 patients with Crohn's disease, who were administered S. cerevisiae var. boulardii for 49 days, showed remarkable improvement in the patient's health (Table 3). S. cerevisiae var. boulardii consumption after steroidal therapy does not produce health-promoting effects on Crohn's disease patients [22].

Vaginal Candidiasis
Vaginal Candidiasis can be considered the most common fungus-associated vaginal infection globally. Candida albicans is the major causative agent of this disease. Vaginal Candidiasis is typically caused by large antibiotic consumption, which produces fluctuations in the normal composition of the vaginal microbiota. Studies have reported the efficacy of oral and intramuscular administration of S. cerevisiae var. boulardii-based probiotics [63] ( Table 3). Vaginal inoculation of S. cerevisiae var. boulardii live yeast or inactivated whole yeast can significantly lower the growth of Candida albicans in mice vaginas. Both these yeast types cause S. cerevisiae var. boulardii and fungus interaction which results in prohibiting the cohesion of Candida albicans to the vaginal epithelial cells. Probiotic administration can significantly reduce the pathogenicity of Candida albicans by lowering its ability to transform itself from yeast to mycelium and the capability of exhibiting aspartyl proteases. However, the efficacy of live yeast is greater as compared to the inactivated whole yeasts [63].

Antibacterial and Antiviral Properties
The efficacy of S. cerevisiae var. boulardii on gastrointestinal microbiota has been critically investigated. S. cerevisiae var. boulardii can opt for different modes of action for antibacterial and antiviral activities in the human gut, which includes: (i) direct inhibition of pathogenic intestinal microbes and normalizing the pH of the gastrointestinal tract by reducing the pathogenicity of toxic microorganisms, (ii) producing an indirect impact on the gut microenvironment, (iii) producing an immunomodulatory effect on the host body [64]. The antibacterial effects of S. cerevisiae var. boulardii against different Grampositive and -negative bacterial and viral pathogens including Bacillus anthracis, Shigella, E. coli, Vibrio cholera, Helicobacter pylori, C. difficile, Salmonella and Rotavirus have been previously reported [65]. S. cerevisiae var. boulardii can adhere to the toxin released by Vibrio cholera and inhibit its activity. The enhanced fluidity of sodium and chloride produced by Vibrio cholera can significantly be reduced by S. cerevisiae var. boulardii via inhibition of cyclic adenosine monophosphate-induced chloride secretion. Therefore, probiotic yeast can directly treat C. difficile disease by targeting its toxins and receptors. This infection can also be prevented by the action of the protease enzyme of S. cerevisiae var. boulardii against receptors and bacterial toxins. Moreover, this yeast can also block the pro-inflammatory pathways which are triggered by the toxins of C. difficile. It can also inhibit the expression of IL-8 and Erk1/2 genes and the activity of the NF-K B pathway (Figure 4) [66]. Anthrax is a bacterial infection that is produced by virulence factors with a protective antigen, lethal factors and edematogenic factors, these peptides are responsible for causing morphological changes in the epithelial cells of the host [67]. The probiotic potential of S. cerevisiae var. boulardii against Salmonella enterica Typhimurium has been analyzed previously. Studies suggested that probiotic yeast can reduce the morbidity and mortality rate of the disease caused by pathogenic S. Typhimurium bacteria. It can limit the entry of bacteria into the host intestinal epithelial cells by inactivating the Rac pathway ( Figure 4). S. cerevisiae var. boulardii sticks to the surface of pathogenic bacteria and reduces its multiplication and growth by accelerating bacterial excretion via the stools [68]. S. cerevisiae var. boulardii can also produce antibacterial properties against peptic ulcer disease caused by Gram-negative Helicobacter pylori bacteria, which cause gastrointestinal tract infection and chronic gastric inflammation in the infected stomach. S. cerevisiae var. boulardii decreases the cytokine and chemokine levels into the stomach and significantly produces IgA antibodies against the toxic Helicobacter pylori [66]. This probiotic yeast is also effective against infection caused by viruses including rotavirus. It can suppress the level of oxidative stress in the host cells that are infected with rotavirus and also reduce the Clexcretion caused by rotavirus [69].

Immune System Modulation
The probiotic effect of S. cerevisiae var. boulardii on the human immune system has been thoroughly investigated. The mechanisms that are mediated by the action of S. cerevisiae var. boulardii yeast are: (i) stimulation in the host immune activity, (ii) production of immunoglobulins, (iii) synthesis of cytokines and chemokines, (iv) assistance in the development of immune cells and (v) stimulate immune priming [70]. In a clinical study, S. cerevisiae var. boulardii administered to a child suffering from gastroenteritis showed a considerable rise in the IgA levels and a reduction in CRP (C-reactive protein) levels. After a 7-day treatment, a significant increase in the rate of CD8 lymphocytes in the S. cerevisiae var. boulardii-treated group as compared to the control group (Figure 4) [71]. A combination of yeast and bacterial probiotic is capable of treating child-associated diarrhea. Results showed a significant increase in immune system modulation and CD3+, CD4+ and Th1/Th2 levels. Moreover, the clinical outcomes of the diarrheal disease were remarkably increased in the probiotic-treated group [72]. When the pathogenic bacteria enter into the gastrointestinal tract of the host, S. cerevisiae var. boulardii releases IgA antibodies which bind to the bacteria and excrete it from the host's body via feces. S. cerevisiae var. boulardii consumption frequently increased the release of IgA antibodies when the host is exposed specifically to C. difficile toxin A [73]. A study on germ-free mice reported that yeast raised the level of IgM, cytokines and the total number of liver macrophages and cleared the infection caused by pathogenic bacteria from the host intestine in the treated group [74]. Briefly, S. cerevisiae var. boulardii can mediate different hormonal and molecular responses which are responsible to inhibit the activity of intestinal pathogens. Generally, the defensive mechanism of probiotic yeasts against several toxins is executed by stimulating the production of cytokines and interleukin (IL)-1β, IL-12, IL-6, TNFα, and IL-10 [75]. The in vitro and in vivo studies of S. cerevisiae var. boulardii showed significant modulation in the host early immune response, through this, the host body can show resistance against most microbial communities. It can also keep the equilibrium between pro and anti-inflammatory immune responses by the upregulation of several cytokines and inhibit the immune cell proliferation and maturation [8].

Antioxidant Properties
S. cerevisiae var. boulardii showed comprehensive antioxidant properties in the previous studies. S. cerevisiae var. boulardii extracted from the fermentation of guajillo pepper showed 66.1% alleviation in cholesterol level when placed in the incubator for 2 days. In a study, DPPH (1,1-diphenyl-2-picryl-hydrazyl free radical) assay calculated 63% of the total antioxidant potential of S. cerevisiae var. boulardii [34]. A DPPH scavenging assay of S. cerevisiae var. boulardii yeast also showed 2.3 mgTE/L antioxidant activity, which is beneficial for the manufacturing of beer with enhanced probiotic potential. Moreover, this assay also exhibited a 40% antioxidant level of S. cerevisiae var. boulardii yeast extracted from different Brazilian local fermented foods [76,77]. Studies suggested that S. cerevisiae var. boulardii whole cells possess superior antioxidant properties as compared to its extracts, it can be due to the presence of a high level of 1/3-b-D-glucan in the S. cerevisiae var. boulardii cell wall structure. Insoluble glucan and metabolites including phenyl ethyl alcohol, vitamin B6, cinnamic acid, vanillic acid and erythromycin are responsible for high antioxidant properties [78]. In a comparative study, raw and miscellaneous S. cerevisiae var. boulardii extracts were investigated for antioxidant level by DPPH test, superoxide radical scavenging assay by the total number of bioactive compounds. Results of this study confirmed the maximum antioxidant potential of S. cerevisiae var. boulardii raw extracts as compared to other extracts. S. cerevisiae var. boulardii antioxidant properties also showed beneficial effects on clinical therapeutics [79]. Another study demonstrated that S. cerevisiae var. boulardii can induce antioxidant activities of gastrointestinal-induced oxidative stress. In a human organ culture study, S. cerevisiae var. boulardii showed a reduction in the level of oxidative stress specifically in the rotavirus infected cells via human gastrointestinal examination [80].

Control of Antibiotic Resistance
S. cerevisiae var. boulardii showed resistance against both broad and narrow-spectrum antibiotic drugs. However, it cannot resist antifungal drugs and therapies. It is the most suitable and active therapeutic agent for the prevention and medication of all diarrhealassociated diseases which are specifically caused by the fluctuation in the normal gastrointestinal microbiota in patients with continuous antibiotics consumption for a longer run. Furthermore, bacterial probiotics are unable to exhibit such properties [34]. In order to increase the quality of functional food, different probiotic strains have been added to human foods and dietary supplements. The safety of human administered probiotics is significant due to their resistance against various antimicrobials [43]. Antibiotics are considered the fundamental tool to fight against pathogenic bacteria. These pathogenic microorganisms can acquire antibiotic resistance genes against advanced antibiotic drugs [81]. This resistance mechanism can negatively affect the treatment strategy against common bacterial infections [82]. Studies suggested that probiotic bacteria and yeast can act as the reservoir of antibiotic resistance genes. Antibiotic resistance in probiotic strains due to intrinsic or extrinsic mutations does not harm the host gastrointestinal tract. Moreover, they are useful to regain the lost gut microbiota of the host after continuous antibiotic intake. However, these probiotics can horizontally transpose resistance genes in harmful microbes. Tetracyclin and vancomycin resistance genes have been observed in various food and gut microbes [83].

Mechanisms of Action of S. cerevisiae var. boulardii Yeast
The responsibility of the host gut microbiota is not limited to just providing protection against pathogenic microbes [84]. It can also contribute to various mechanisms including cellular adhesion, reestablishment of lost gut microbiota, mediation of cancer signaling cascades, competition with pathogenic microbes, mucin production and regulation of nutritional and trophic effects. Adequate administration of S. cerevisiae var. boulardii can target and eliminate disease-causing microbes from the gastrointestinal tract of the host [85].

General Mode of Actions
Host gut dysbiosis due to the pathogenic microbial attack may reduce the overall probiotic bacterial load in the host gastrointestinal tract which may cause inflammation and secondary infections [86]. The adhesion potential of S. cerevisiae var. boulardii against pathogenic microbes may actively contribute to neutralizing the mechanism of antigen translocation from the gastrointestinal tract to other parts of the host body [87]. Continuous administration of S. cerevisiae var. boulardii for several weeks can stabilize the host gut microenvironment by reducing the severity of the disease and eventually eradicating the disease from the host body. Some probiotics are frequently eliminated from the host body, but before the elimination, they would have significantly modulated the host immune system. While other probiotics may recognize and bind to the active sites of the host intestinal mucosal layers [88]. Mucin production by intestinal epithelial cells of the host may also be influenced by the presence of probiotics in the host gut. Both pathogenic and beneficial microbes compete for binding to the gastrointestinal tract of the host. Cell wall proteins and mannose residues of S. cerevisiae var. boulardii are responsible for the direct binding of probiotic yeast to the intestinal receptors and reducing the probability of pathogenic microbes binding to the active sites [89]. However, if pathogens already adhere to the active sites, then probiotic administration may significantly induce the expression of exogenous sugars which can obstruct the binding of pathogenic microbes to the intestinal mucosal layers.

Mechanisms of Cancer Signaling Cascades
Cancer is considered as a major public health concern globally [90]. It is the leading cause of death not only in developing countries but also in developed countries. This deadly disease is a combination of more than 100 different diseases [91]. There are different types of cancer; all types have the same origin which is the abnormal growth of the cells. The growth rate of healthy and cancerous cells are different, healthy cells grow and proliferate in a controlled manner resulting to keep the body alive, while tumor cells grow in an abnormal fashion leading to cause anti-apoptotic effects [92]. To reduce the expression of oncogenes and protooncogenes, several clinical therapeutics including anticancer drugs, chemotherapy, radiotherapy and other strategies are conventionally used, however, the use of probiotics acts as an alternative treatment method for cancer prevention.
S. cerevisiae var. boulardii can significantly induce cancer signaling cascades by upregulating the expression of apoptotic proteins and downregulating the expression of protooncogenes and oncogenes. A recent study investigated the anti-tumorigenic activity of S. cerevisiae var. boulardii against gastric cancer cell lines and analyzed total cellular viability, apoptotic effects and activity of survivin gene after 3 days. Results of this study reported that targeted probiotic yeast significantly reduced the level of cellular viability, which stimu-late apoptosis and lowered the activity of the survivin gene in gastric cancer cells (Figure 4). This study strongly recommends the use of S. cerevisiae var. boulardii as a potential antigastric-cancer treatment therapy [93]. The probiotic potential of this yeast was also reported against human colorectal cancer cell lines (HT-29) and animal models. To evaluate the efficacy of S. cerevisiae var. boulardii on cell growth, development and apoptosis, this yeast was thoroughly spread over the HT-29 cells by using 4 ,6-diamidino-2-phenylindole (DAPI) dye and 3-(4,5-dimethylthiazoyl-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The expression profiles of PTEN/caspase-3, Bclxl and RelA genes were evaluated by real-time PCR [94]. After 24 h, the activity of PTEN and caspase-3 gene was increased. However, the expression of Bclxl and RelA genes was significantly reduced (Figure 4). After 2 days, the MTT assay showed inhibition in the growth of probiotic treated HT-29 cells. In another study, 1,3-beta-glucan part of the S. cerevisiae var. boulardii yeast showed anti-neoplastic effects on rat colon cancer cells when treated with dimethylhydrazine and S. cerevisiae var. boulardii orally [94]. Chen et al. reported that S. cerevisiae var. boulardii consumption can significantly block the activity of epidermal growth factor receptors when exposed to the targeted yeast and inhibit the Erk and Akt pathway ( Figure 4). According to the results, S. cerevisiae var. boulardii reduced the growth and proliferation of cancer cells and induces cancer cell apoptosis [19].

Discussion
Probiotics are non-digestible constituents of food, and when added in food or diet, confer useful and healthy effects to the host and stimulate the growth of a confined quantity of colon bacteria [95]. Natural strains of S. cerevisiae var. boulardii observed harsh environmental conditions as compared to the strains artificially cultured in the lab. This probiotic yeast has advanced conventional survival strategies which ensure its viability for the long run [96]. Mostly, the natural strains of this beneficial yeast are present in the nutrient-enriched soil environment. Some other environmental habitats of S. cerevisiae var. boulardii are the leaves and trunk surfaces of different medicinal and non-medicinal plants. It is also naturally present in intact grapes and other citrus fruits [27]. The natural transmission of this yeast to the human body is possible by the consumption of grapes, grape wine and different fruits. Studies suggested that this yeast is also insect-borne and is observed in wasps, Drosophila and other insects. These insects absorbed S. cerevisiae var. boulardii by feeding on the grapes and other fruits [97]. S. cerevisiae var. boulardii has shown direct and indirect effects on functional (fermented) food stuff. Direct effect indicates host-organism relationship, while indirect effects demonstrate the biogenic upshot (due to taking of microbial metabolites as a result of fermentation). This advances towards the efficient consequences of probiotics that seem to be applied in non-dairy food items as products related to chocolate, chewing gum, biscuit, honey, cereals, cakes, dressing, sweetness and tea [98]. In general, S. cerevisiae var. boulardii in the food industry somehow has difficulty in its multiplication and survival rate because of the distress conditions of the gastrointestinal tract [99].
To ensure the shelf-life of probiotics, novel probiotics are being designed through microencapsulation technology that opposes environmental conditions. Various factors can contribute to the beneficial aspects of probiotics but its proper mechanism of action is still vague. Studies suggested that lactation performance of the dairy animals was improved by S. cerevisiae var. boulardii yeast supplementation. It is found that the increased milk yield might be due to the stimulatory effect of probiotic yeast on the animal microbiota, which in turn increases cellulose digestion [100].
The presence of functional food in animal diets is responsible for increasing the productivity of livestock. Livestock can significantly improve human nutrition by providing essential nutrients in the form of milk, meat, and eggs [101]. An inadequate diet can drastically damage the health of livestock and reduced the overall yield. Due to poor feeding, animals are generally suffering from digestive and respiratory diseases leading to insufficient digestion and consequently retarded growth and productive performance.
Dietary supplementation of S. cerevisiae var. boulardii is a viable and safe option for farmers to enhance the production of lactating dairy cattle and heifers [102]. This yeast has gained the Generally Recognized As Safe (GRAS) status from the Food and FDA, thus, can significantly be used to improve the animal feed supplements [103]. Moreover, the probiotic dose administered to animals is dependent on the (i) composition of feed, (ii) age of the animal, (iii) physical health of the animal and (iv) nature of the digestive system of the animal [29].
S. cerevisiae var. boulardii showed its applications in the wine industry for the benefit of humans. The natural grape was considered as a potential habitat of S. cerevisiae var. boulardii due to its high sugar content and acidic pH [104]. This yeast can significantly cope with all the fermentation stresses of the environment and has gained "the wine yeast" status and it is considered an important component of the wine industry globally. Moreover, the biosynthesis of primary and secondary alcohols is responsible due to their fermenting ability [105]. Theobroma cacao grains are significantly used for the manufacturing of chocolate. Fermentation of cacao grains can reduce the bitter and acrid effects of these grains. Direct exposure of cacao grains to probiotic yeast can induce fermentation reaction resulting in the production of ethanol and useful secondary metabolites [106]. The pectinolytic enzymes of S. cerevisiae var. boulardii can potentially metabolize citric acid produced by cacao grains. The increased growth of S. cerevisiae var. boulardii under high pH and stress conditions contributed to ethanol production [104]. S. cerevisiae and S. cerevisiae var. boulardii have remarkable applications in the bread and bakery industry. Sourdough, water and flour mixture are required for the manufacturing of bread. Various types of flours are commercially available which include spelt, barley, maize, einkorn, rye, khorasan, sorghum and many others [107]. Probiotic yeast and lactic acid bacteria are the main components of sourdough. A total of 2% of the fermenting yeast is added for the biosynthesis of bread. Atmospheric oxygen enters into the dough during dough mixing, which is adequately consumed by the yeast cells. Moreover, in an oxygen-limited environment, the rate of yeast cell reproduction was hindered and dough started to rise due to the fermentation process [108].

Conclusions
Continuous upsurge in multidrug-resistant organisms is responsible for causing millions of deaths annually. To control the spread of antimicrobial resistance, probiotic yeast S. cerevisiae var. boulardii can be considered as an alternative method for the treatment of bacterial and fungal infections. Several clinical and therapeutic studies confirmed the efficacy of S. cerevisiae var. boulardii against different pathogenic gastrointestinal diseases. The probiotic nature of this yeast has surpassed the effectiveness of different probiotic bacteria due to its gut microbiota protection potential. This probiotic yeast can actively participate in the manufacturing of bread, bakery products, wine, chocolate and large-scale bioethanol production. The consumption of S. cerevisiae var. boulardii in adequate amounts can also enhance the overall yield of milk and meat in poultry and livestock. The prescribed S. cerevisiae var. boulardii dose can also reduce the probability of co-morbidities that are caused by the continuous consumption of antibiotics for a long period. The combination of S. cerevisiae var. boulardii with other probiotics can enhance the treatment efficacy and reduce the pathogenicity of the disease. Despite its beneficial aspects, the use of this probiotic yeast should be according to the prescription of a physician. Moreover, this study will open up new insights for the development of novel probiotic strains, which will reduce the transmission of antimicrobial resistance genes among humans and farm animals.