Bioactive molecules of probiotic bacteria and their mechanism of action: a review

Abstract

The bacteria residing in the gut environment do play a pivotal role in metabolic activities of the host. The metabolites produced by these bacteria affect the physiology and health of the host. The gut bacteria are exposed to environmental conditions where multiple factors such as lifestyle, stress, antibiotics, host genetics and infections have an influence on them. In case of pathogenesis of a disease, the gut bacterial composition is altered which leads to a diseased state. This stage is due to colonization of bacterial pathogens in the gut environment. The pathological condition can be alleviated by administering probiotic strains into the gut environment. The probiotic strains produce therapeutic molecules such as amino acids, vitamins, bacteriocins, enzymes, immunomodulatory compounds and short-chain fatty acids. This review discusses recent evidences of the impact of bioactive molecules produced by probiotic bacteria and their mechanism of action in the gut environment to maintain homeostasis and health of the host without any effect on beneficial bacteria sharing the same niche. In addition, the manufacturing challenges of probiotic products for various applications are discussed here.

Introduction

Probiotics concept was introduced in early twentieth century by Russian scientist and Nobel laureate Metchnikoff. The scientist thought that these probiotic bacteria exert a positive influence on the host by improving the intestinal microbial balance and alleviating digestive disorders (Pandey et al. 2015). The current definition of probiotics is that these are live microorganisms which, when administered in appropriate doses, confer health benefit to the host (FAO/WHO 2002). They facilitate good environment for smooth functioning of various metabolic activities in the intestine through the production of proteins, carbohydrates, vitamins and enzymes. The intestinal pathogenic organisms are suppressed by the action of acids and proteolytic activity of lactic acid bacteria (Schepper et al. 2017). The colonized probiotic bacteria serve many beneficial effects to the host cell for several reasons. The probiotic bacteria eliminate potential pathogenic organisms and help the host by increasing resistance to new colonization. They have high metabolic activity, producing useful compounds such as vitamins which maintain the health, bacteriocins that fight against various pathogens and also production of immunomodulatory compounds that regulate host immune system (Conlon and Bird 2015; Jandhyala et al. 2015). Probiotics also promote healthy environment by producing saturated fatty acids and oligosaccharides (Pandey et al. 2015). The introduced beneficial microbes alter the microbial composition and it has been associated with a therapeutic strategy which combats the pathogenic bacteria in the intestinal niche (Langella and Martín 2019; Vitetta et al. 2015). These bacteria alleviate various disorders and maintain overall health of the host (Daliri and Lee 2015; Amara and Shibl 2015; Sanchez et al. 2017). Human body harbors different microbes inside the gut environment, particularly the bacteria, fungi and viruses (Patrice 2018). This gut microbiota is influenced by several factors such as diet, human health, life style, infections, aging and host immunity; as a result, the gut environment is altered (Conlon and Bird 2015). The human gut microbiome is a potential source for novel therapeutics in the form of probiotics that produce therapeutic molecules to address the issues such as obesity, metabolic disorders, microbial infections and host immunity (Patrice 2018). Probiotics play a pivotal role in the well-being of humans which includes: (1) metabolic activity of undigested carbohydrates (Rowland et al. 2018; Kerry et al. 2018), (2) prevent multiplication of pathogenic bacteria and viruses (Zhang et al. 2015), (3) influence on host immune system (Kober and Bowe 2015; Amara and Shibl 2015), (4) synthesis of various nutrients such as vitamins, amino acids and enzymes, (5) bioavailability of nutrients (Pandey et al. 2015) and (6) creating less desirable conditions for harmful microorganisms in the intestine by changing the pH and reducing the oxygen availability (Schepper et al. 2017). As a result, the probiotic microorganisms promote beneficial effects in a host which are due to the production of bioactive compounds represented in Table 1. The bioactive compounds produced by these bacteria (Saccharomyces boulardii, Lactobacillus acidophilus, Lactobacillus rhamnosus, Lactobacillus casei, Lactobacillus plantarum, Bifidobacterium longum, Bifidobacterium bifidium) include bacteriocins, enzymes, vitamins, amino acids, oligosaccharides, exopolysaccharides, short-chain fatty acids and immunomodulatory compounds (Vidya Prabhakar and Ramkrishna 2008). The number of viable probiotic cells present in live condition in a probiotic food must be 10⁸–10⁹ colony forming units (CFU) per day according to National Sanitary Surveillance Agency (ANVISA) (Costa and Miglioranza 2012). Conversely, the influences of probiotic microorganisms vary depending on species, the quantity of bacteria ingested and the physiologic conditions of the host (Campana et al. 2017; Sanders et al. 2018).

Table 1.

Bioactive compounds of probiotic bacteria and their health benefits

Bioactive compound	Health benefits inside the gut	Probiotic microorganism	References
Bacteriocins	Helps in survival of bacteria in GI tract. Kills various intestinal pathogens. Acts as signaling molecules	Lactococcus lactis	Sturme et al. (2002), Sankar et al. (2012)
Enterocins	Antimicrobial activity against Pseudomonas aeruginosa	Enterococcus casseliflavus MI001	Indira et al. (2018); Indira et al. (2019)
Exopolysaccharides	Antioxidant activity	B. coagulans RK-02	Vidya Prabhakar and Ramkrishna (2008), Kodali et al. (2013)
SCFAs
Lactic acid	Used as substrate for glucose, cholesterol, and lipids metabolism Lowers pH in vaginal environment	Lactobacillus sps.	Tachedjian et al. (2017); Leblanc et al. (2017)
Butyric acid	The butyric acid is the energy source for colonocytes and has anti-inflammatory and anti-tumour properties	F. prausnitzii, Roseburia spp. Butyricicoccus pullicaecorum	Geirnaert et al. (2017); Miremadi and Shah (2012)
Propionic acid	Propionate plays a role in gluconeogenesis	P. freudenreichii.	Vorobjeva et al. (2008)
Acetic acid	Acetate is metabolized in muscle and used to produce adenosine-5’-triphosphate (ATP). Defense functions in host epithelial cells	Bifidobacterium sps.	Fukuda et al. (2012)
Fructooligosaccharides
Inulins  Levans	Reduces fat absorption Reduces cholesterol absorption	L. gasseri strains DSM 20604 and 20077	Anwar et al. (2010)
Amino acids
Lysine	Essential amino acid for the host	Clostridium sps.	Metges et al. (2006); Dai et al. (2015)
Amino acid metabolites	Essential nutrients supports the growth	Enterobacteriaceae	Dai et al. (2015)
Arginine	Function on both female and male reproductive system	Fusobacterium varium	Dai et al. (2015)
Tryptophan and Tyrosine	Function on male reproductive system	Enterococcus sps, streptococcus sps, Bacillus sps	Dai et al. (2015)
Vitamins
Folate	Energy metabolism Biosynthesis of nucleic acids	B.adolescentis DSM 18350 B. pseudocatenulatum	Strozzi and Mogna (2008) Asrar, and O’Connor (2005) Rossi et al. (2011)
Vitamin B1 (Thiamin)	Role in synthesis of nucleic acids, steroids fatty acids and amino acid precursors. All these bioactive compounds are essential for functioning of brain	Lactobacillus sps.	Gu and Li (2016)
Vitamin B2 (Riboflavin)	Energy metabolism	L. fermentum CECT 5716 Lactococcus lactis	Cardenas et al. (2015)
Vitamin B6 (Pyridoxine)	Amino acid metabolism	Bifidobacterium sps.	Patel et al. (2013)
Vitamin B9	Energy metabolism	L. fermentum CECT 5716	Cardenas et al. (2015)
Vitamin B12	Helps in red blood cell formation and makes DNA	Lactobacillus reuteri JCM1112	Santos et al. (2008)
Enzymes
Amylase	Hydrolysis of starch	Lactobacillus sp G3_4_1TO2	Padmavathi et al. (2018)
β-Galactosidase	Hydrolysis of β-galactosides	P. freudenreichii	Vorobjeva et al. (2008)
Super oxide dismutase, Catalase	Antioxidant activity	Lactobacillus fermentum E-3 and Lactobacillus fermentum E-18, Lactobacillus casei BL23	Wang et al. (2017)

Bioactive compounds produced by Probiotic bacteria

Bacteriocins

Bacteriocins are ribosomally synthesized antimicrobial peptides produced by one organism and kill other organisms (Zacharof and Lovitt 2012). These bacteriocins are produced by two domains such as bacteria and some members of archea. In bacterial group, the bacteriocins are produced by both gram-positive and gram-negative bacteria (Indira et al. 2015, 2018, 2019; Gillor et al. 2008). The beneficial bacteria present in the gut environment produce bacteriocins and prevent pathogenic microbial colonization. This defensive role helps probiotic bacteria to occupy a specific niche and also limits the advancement of pathogens to neighboring cells. Recent reports have been proposed for probiotic bacteria that they are mediating quorum sensing signals in bacterial biofilms (Dobson et al. 2012; Mukherjee and Ramesh 2015). Bacteriocins have a narrow spectrum of activity and they are bactericidal in nature. The bactericidal mechanism of bacteriocin action is located in the cytoplasmic membrane region of receptor binding on bacterial surfaces. Moreover, these bacteriocins are non-toxic peptides, sensitive to proteases compared to antibiotics (Yang et al. 2014; Mokoena 2017).

SCFAs (short-chain fatty acids)

The human body harbors 100 trillions of microorganisms in both small and large intestine. The host cells lack a few enzymes that play a role in digestion of carbohydrates. However, the probiotic bacteria ferment these undigested carbohydrates and produce energy which is utilized by host to carry out various functions. The probiotic bacteria co-habit with the colonocytes and maintain a symbiotic relationship between gut flora and humans (Chatterjee et al. 2017). The probiotic bacteria converts these undigested sugars into SCFAs such as butyrate, acetate, propionate and other byproducts namely heat and gases (CH₄, CO₂ and H₂) (Leblanc et al. 2017; Thursby and Juge 2017; Yoo and Kim 2016). The short-chain fatty acids produced as end products of saccharolytic fermentation have antimicrobial activity. The organic acids are six carbon compounds that help in protection of host from microbial infections (Ciarlo et al. 2016).

The typical reaction of short-chain fatty acids production and over all stoichiometry has been summarized for a hexose sugar and is shown as follows:

59 C₆H₁₂O₆ + 38 H₂O → 60 CH₃COOH + 18 CH₃CH₂CH₂COOH +22 CH₃CH₂COOH + 96 CO₂ + 268 H⁺ + Heat (Topping and Clifton 2001).

In humans, 10% of the daily caloric requirement is from short-chain fatty acids produced in large intestine. Among all short-chain fatty acids, 60–70% of the energy is from butyrate produced in colonocytes (den Besten et al. 2013). The conversion of butyrate to acetyl co-A, followed by ketone bodies and CO₂ production was found in butyrate oxidation. Thus, butyrate acts as a fuel source for colon, muscle and brain cells (den Besten et al. 2013). Apart from production of short-chain fatty acids, the probiotic bacteria can accomplish various functions in the intestinal environment. The growth of pathogenic bacteria is prevented, regulation of immune system and physiological conditions of the gut, stimulation of epithelial cell growth by producing vitamins and hormones (Mach and Fuster-Botella 2017). SCFAs, particularly butyrate, have a therapeutic effect in various diseases such as inflammatory bowel disease, antibiotic-associated diarrhea, colon cancer and heart diseases (Sharma and Shukla 2016; Tominaga et al. 2018; Gill et al. 2018; Moss et al. 2018). In a previous study by Kimura et al. (2013), it was found that the receptors (GPR43) of short-chain fatty acids are linked with the metabolic activity of the gut microorganisms. These receptors maintained homeostasis in the host cells by controlling the energy utilization of the host. Further, they found that GPR43-deficient mice are obese on normal diet compared with over expression of GPR 43 mice.

Exopolysaccharides

Various genera of probiotic bacteria have the ability to produce exopolysaccharides (EPSs) in large quantities (Kodali et al. 2009; Indira et al. 2016). Among all, the genus lactic acid bacteria are under GRAS status (generally recognized as safe). The enzymes such as glycosyltransferases and glycantransferases convert the sugar nucleotide precursors into exopolysaccharides. Recently, the microbial exopolysaccharides (EPSs) have gained a lot of attention due to their health benefits (Ates 2015). The exopolysaccharides from lactic acid bacteria have immunostimulatory activity, antitumor effect, antioxidant activity and blood cholesterol lowering ability (Ghan et al. 2014; Tsai et al. 2014). In another study, the exopolysaccharide showed a good emulsifying activity which is an important feature to be used in food formulations (Peele et al. 2016). In addition to these, the exopolysaccharide produced from Lactic acid bacteria showed immune modulation, antiulcer and cholesterol lowering activities (Julendra et al. 2017). The antitumor activity of cell bound exopolysaccharide produced by Lactobacillus helveticus MB2-1 was tested against liver (HepG-2), gastric (BGC-823), and especially colon (HT-29) cancer cells (Li et al. 2015).

Oligosaccharides

Oligosaccharides are the non-digestible cross-linked polymers composed of monosaccharide units. The nature of the oligosaccharide is characterized by the number of units and type of its glycosyl moieties (Meyer et al. 2015). These are one of the major food sources for probiotic bacteria and gut bacteria. The non-digestible oligosaccharides enrich nutrients to the gut flora and behave as dietary fibers and prebiotics (Patel and Goyal 2012). The positive effect on the beneficial bacteria, thus, induces the host health (de Moura et al. 2015). Oligosaccharides have enormous potential for stimulating the growth of bacteria and production of bioactive compounds such as antibodies, short-chain fatty acids and organic acids. Therefore, modulation of the intestinal flora achieves homeostasis in the gut environment (Pan et al. 2009). The examples of oligosaccharides are inulin, fructooligosaccharides (FOS), glucooligosaccharides (GOS) and xylooligosaccharides (XOS) (Yoo and Kim 2016). Many clinical studies reported that the combination of probiotic bacteria and prebiotics has therapeutic effect in obesity (Cerdo et al. 2019), colon cancer (Zackular et al. 2013), irritable bowel syndrome (Dai et al. 2013), bacterial infections (Sarowska et al. 2013) and tumors (Markowiak and Slizewska 2017). The study of previous researchers reported that the glucooligosaccharide (GOS) has a protective role in development of tumors and their multiplicity (Pericleous et al. 2013; Hou et al. 2013). A different approach for nutritional therapy in case of constipation is the use of prebiotics such as fructooligosaccharides and galactooligosaccharides (Patel and Goyal 2012). The laxative effect of galactooligosaccharides reduces the symptoms of chronic constipation and irritable bowel syndrome in the elderly people (Rao et al. 2015). Oligosaccharides support the synthesis of immunoglobulins which play a role in natural defense mechanism in the host (Newburg 1996). Majority of the findings reported that the increased levels of IgA synthesis are induced by the use of fructooligosaccharides in animal models (Nakamura et al. 2004). Addition of probiotic foods to beneficial bacteria has the potential to increase the levels of secretory IgA in the body (Hardy et al. 2013). Oligosaccharides promote the absorption of minerals through the consumption of prebiotics. In a recent study by Baye et al. (2017), it was found that the prebiotics increase the absorption of iron and calcium from the colon.

Enzymes

Probiotic bacteria have the ability to carry out various metabolic activities due to the production of enzymes such as lipases, esterases and amylases (Markowiak and Slizewska 2017). Human beings lacking the enzyme lactase develop intolerance to lactose levels. The probiotic bacteria produce enzyme lactase that digests lactose present in the milk and converts it into glucose and galactose. Further, these sugars get converted into short-chain fatty acids (den Besten et al. 2013). The amount of lactose in yogurt product is reduced by hydrolytic capacity of probiotic strains (Vonk et al. 2012). Some of the species of lactic acid bacteria lessen the lactose intolerance through their enzyme β-galactosidase (Montalto et al. 2006). Amylases and peptidases produced by probiotic organisms play a role in biochemical reactions of the host metabolism. The genera producing these enzymes are lactic acid bacteria and bifidobacteria (Savaiano 2014).

Amino acids

The gut bacteria produce several amino acids by de novo process and they act as precursors for the synthesis of short-chain fatty acids (Feng et al. 2018). The produced short-chain fatty acids help in the fermentation of undigested carbohydrates. Amino acids and short-chain fatty acids alter the physiology of the host (den Besten et al. 2013). The amino acids and amino acid-derived molecules regulate the metabolism of carbohydrates and lipids and further produce metabolites and in turn regulate health of a host (den Besten et al. 2013). Mainly, the aromatic amino acids act as substrates for the production of various metabolites (Dai et al. 2015; Dodd et al. 2017). Fermentation of these amino acids by probiotic bacteria in the gut produces phenols and indole. They maintain energy balance and resistance to pathogen infections due to immune signals. Many lactic acid bacteria (LAB) produce small peptides and amino acids by proteolysis of casein molecules (Rowland et al. 2018). The biochemical conversion of amino acids results in the production of alcohols, esters, aldehydes and organic acids that gives aroma and flavor. The most potent flavor compounds are aldehydes, alcohols, esters and acids. The amino acids are methionine, threonine, phenylalanine and branched-chain amino acids. They play an important role in flavor formation for various food and dairy products (Smit et al. 2005). The amino acids produced by probiotic microorganisms play an important role in alleviating various disorders. d-tryptophan acts as an immunomodulatory substance which reduces hyperactivity in allergic reactions (Kepert et al. 2017). Some small molecules produced by gut bacteria play a role in biofilm formation and quorum sensing. These molecules are acylhomoserine lactones and autoinducer peptides derived from S-adenosyl methionine, an important intermediate from amino acid methionine (Dai et al. 2011).

Vitamins

The essential nutrients for the growth and development of a multi-cellular organism are vitamins. Humans have lost the ability to synthesize vitamins by de novo process, which leads to deficiencies, malnutrition and stunned growth starting from infant to elderly stage. These vitamins are supplied in the form of diet. All the vitamins are grouped under water-soluble and fat-soluble categories (Fitzpatrick et al. 2012; Ravisankar et al. 2015). Many bacteria produce B-group vitamins which are soluble in water and absorbed into the intestine, whereas fat soluble vitamins are absorbed with the help of lipids as micelles in the intestinal tract. The major sources of vitamins are dairy products which contain B-complex group of vitamins (Said 2011). The natural sources of vitamins are plants and animals but some vitamins are synthesized chemically (Fitzpatrick et al. 2012). The vitamins are produced by microbial fermentation processes. Vitamin is the key factor which plays an important role in growth, development, reproduction, red blood cell formation, antibody production, and lactation of the human beings. It is required by the body to carry out various metabolic activities of amino acids, fatty acids, carbohydrates and synthesis of nucleic acids. In the digestive tract, pyridoxine and iron are absorbed due to riboflavin which enhances the absorption rate and helps in the maintenance of red blood cells (LeBlanc et al. 2017). The biosynthesis of riboflavin has been studied in plants, bacteria and filamentous fungi. Extensive studies have been carried out on riboflavin production in B. subtilis and Escherichia coli, respectively (Lin et al. 2014). Another group of health promoting bacteria under the genus is Bifidobacteria which produces B-complex vitamins maintaining the overall health in the gut of a host (Markowiak and Slizewska 2017). In an experiment, rats with tamoxifen-mediated endometrial carcinoma were treated with B-complex vitamins such as Riboflavin and niacin combination with ascorbic acid (Sundravel et al. 2006). The most important vitamin under the B-complex group is the folic acid which is naturally produced by the gut microbiota in large quantities. It is one of the most important vitamins for synthesis of nucleic acids, conversion of amino acids and having antioxidative nature for removal of free radicals from the body. Various strains have been screened for folate production from the genus lactic acid bacteria; except L. plantarum, all other strains were incapable of folate production. These probiotic cultures cannot synthesize folate due to the lack of genes encoding for the folate production (Patel et al. 2013). The vitamin which plays a role in early development of the nervous system and growth of the embryo is the pyridoxine (vitamin B6). This vitamin has a functional role in the link between one carbon metabolism and antioxidative activity (Dalto and Matte 2017). The main producers for this vitamin are Bifidobacterium sps (Patel et al. 2013). The vitamin which is not synthesized by plants is Vitamin B₁₂ and it plays an important role in the functioning of nervous system and blood formation. The vitamin production is only through bacterial fermentation. A probiotic strain of L. reuteri belonging to LAB is able to produce the cobalamin (Mohammed et al. 2014). Salmonella typhinurium consists of multiple genes for the synthesis of vitamin B₁₂ by de novo process. Transfer of these genes to E.coli allows the production of vitamin B₁₂ in industries (Fang et al. 2017). Another organism, Propionibacterium shermani is also capable of producing vitamin B₁₂, propionic acid and other metabolites which are of industrial importance (Piwowarek et al. 2018). Another vitamin under B-complex group is thiamine (vitamin B₁) essential for the synthesis of nucleic acids, fatty acids, aromatic amino acids and other bioactive compounds essential for brain function. Masuda et al. (2012) investigated the production levels of folate, vitamin B₁₂ and thiamine from lactic acid bacteria and found that the thiamine level is very low compared with other two vitamins. Furthermore, the high level of thiamine production was reported in Bifidobacterium species. Vitamin K facilitates the coagulation process of blood and it is responsible for clot formation. In green plants, this vitamin exists as phylloquinone (Vitamin K₁) and in intestinal bacteria it is produced as menaquinone (K₂) form. For human consumption, the phylloquinone form is obtained from green plants and the menaquinone form produced by bacteria, which are used to supplement vitamin K to alleviate vitamin K deficiency (Patel et al. 2013). Through genetic engineering, a probiotic E.coli strain Nissle 1917 (EcN-BETA) was developed for the production of β-carotene. In malnourished children, the supply of this probiotic bacterium increases the vitamin A in the intestine. Vitamins production by probiotic bacteria in gut environment is a natural way of enrichment of vitamins in human beings (Miller et al. 2013).

Immunomodulatory compounds

The immune system of the host is modulated by probiotic bacteria in the gut environment. The probiotic bacteria modulate the immune system and regulate the production of antibodies, interleukins, cytokines and lymphocytes (Nagpal et al. 2012). The probiotic bacteria react with intestinal epithelial cells and initiate a host immune response by producing immunomodulatory molecules. Due to this interaction, a controlled production of cytokines and chemokines takes place in the gut environment (Hardy et al. 2013). The probiotic bacteria modulate immunity and inflammatory gene expression results in the production of interleukins such as IL-1β and IL-8 (Plaza-Diaz et al. 2014). In a recent study reported by Kawashima et al. (2018), it is shown that the production of IgA antibody induced via IL-10 in mucosal sites of host is a host defence mechanism against the pathogens. The genetically engineered Lactococcus lactis strain-produced anti-inflammatory mediator such as IL-10 and IL-12 showed reduction of dextran sodium sulfate-induced colitis in mice models (Gupta et al. 2014). In another study reported by Ozdemir (2010), it was found that there is a reduction of IgE levels in allergic diseases. Hence, immunomodulation through probiotics is an alternative treatment option for various diseases.

Mechanism of action of bioactive molecules in the gut environment

Modulation of the micro-biome in the gut atmosphere has become a promising approach or an integral part of precision medicine approach to improve host health. This strategy protects the host from a variety of infections and diseases by producing bioactive compounds and regulates the host metabolism and immune system. These bioactive molecules regulate the host health by activating various mechanisms represented in Fig. 1. To begin with, probiotic organisms after oral administration reach the gut environment where they activate the production of quorum sensing molecules. These molecules form the biofilm, a protective layer made up of lipopolysaccharide which helps as a growth substrate for colonization of the useful bacteria and prevents the pathogenic bacteria colonization (Dobson et al. 2012; Mukherjee and Ramesh 2015). Secondly, they activate dendritic cells, which in turn activate naive T cell. The produced T cells further activate the B cells resulting in production of secretory IgA. The secretory IgA helps in elimination of pathogenic bacteria resulting in humoral response. In another way, the naïve T cell differentiates into mature T cell, which produces cytokines and results in immune modulation (Derrien and van Hylckama Vlieg 2015). The mechanism by which the probiotic bacteria benefit the host by producing bacteriocins, antimicrobial peptides kills the pathogenic bacteria results in the elimination of harmful organisms, in the gut environment. The secretory IgA, bacteriocins and mucus act as primary defense mechanism in the intestinal epithelium and protect the host from invading pathogens (Corthesy et al. 2007; Derrien and van Hylckama Vlieg 2015). In fourth mechanism, they produce short-chain fatty acids (butyrate, acetate and propionate) and organic acids (lactic acid and acetic acid) help in lowering of pH in the gut environment; this condition is unfavorable for growth of the pathogenic bacteria (Schepper et al. 2017). In fifth mechanism, they produce nutrients and growth factors such as vitamins, precursors to the enzymes helping in regulation of metabolism through biochemical reactions. The vitamins produced by these bacteria are B-complex vitamins (especially vitamin B₁₂, which is not synthesized by humans) and vitamin K which help in the growth and establishment of useful bacteria in the gut environment. Finally, the SCFAs regulate G-protein coupled receptors. In this line, the propionate binds with GPR43 receptor expressed on lymphocytes and triggers the production of IL-10, helping in resolving inflammatory responses. Butyrate also triggers the differentiation of T cell leading to the production of Interleukin such as IL-10 (Kota et al. 2018; Vitetta et al. 2015). Butyrate binds to the PPAR-γ (peroxisome proliferated activated receptor- γ) leading to β-oxidation and oxygen consumption. Hence, the reduced levels of oxygen in the gut lumen result in the anaerobic environment, which is undesirable for pathogenic bacteria (Patrice 2018). Thus, overall, the Probiotic bacteria completely influence metabolism, immune system, composition and functioning of the gut microbes.

Fig. 1.

Schematic presentation of the overall mechanisms on protection of gut environment by bioactive molecules produced by Probiotic bacteria (Kim et al. 2013; Derrien and van Hylckama Vlieg 2015). First, the ingested bacteria reach the gut environment where they then produce lipopolysaccharides for colonization. Secondly, they activate the dendritic cells which in turn activate the T cell triggering the activation and differentiation of B cells. The produced B cells secrete IgA, and helps in the elimination of pathogens. Thirdly, they produce bacteriocins which are antimicrobial peptides and kill the pathogenic bacteria. Fourthly, they prevent pathogenic bacteria by decreasing the luminal pH results from the production of organic acids. This creates undesirable conditions for growth of pathogens. Fifthly, they produce growth factors, including vitamins and exopolysaccharide (EPS) which helps for the growth and metabolic reactions in the gut environment. Finally, the metabolites acetate, butyrate and propionate trigger immune defense mechanisms

Industrial applications of probiotics and manufacturing challenges for probiotic products

In recent years, the use of probiotics has become a fascinating area due to applications in various fields such as diary, food and beverages, agriculture, health care and aqua culture (Zielinska and Kolozyn-Krajewska 2018). Once desired strain is identified for developing a probiotic product, first we have to see if the strain can be cultured at large scale, process design and control, product recovery, formulation technology, packaging of a product and finally successful incorporation into the consumer products (Fenster et al. 2019). The important criteria required for selection of an organism considered as probiotic food application include: The strain has to survive in process conditions, formulation and packaging of a product, shelf life of the product, stability of the organism, quality and safety of the organism, adherence and colonization of the intestinal epithelium, stressful conditions in the gastrointestinal tract, anti microbial activity against pathogenic strains, immune system activation, biological function in the GI tract and finally health benefits to the host (Misra et al. 2019; Fenster et al. 2019; Sanders et al. 2019). For industrial producers, the important parameters which affect the manufacturing and marketing are stability and viability of microorganisms (Sutton 2008). In case of manufacturing probiotic supplements, the viable count of the cells and shelf life of a product are important. The survival ability of the organism during process depends on the quality of raw materials, culture medium selection, cell protectants, environmental parameters and aseptic maintenance of the process (Vinderola et al. 2011; Fenster et al. 2019). The functional aspects of probiotic organism include viability in sufficient dosage levels and the production will be strain dependent (Fenster et al. 2019). In case of dietary supplements, the probiotic products are manufactured as freeze-dried powders and formulated as capsules, powders and tablets (Govender et al. 2014; Fenster et al. 2019). The parameters which influence manufacture of these dietary supplements include viable count of the cells and water activity (Vinderola et al. 2011; Grzeskowiak et al. 2011). For application of probiotic as food and beverages, the products are prepared in vegetative cell form and added to the food products (Misra et al. 2019). Hence refrigerated conditions are required for maintaining the shelf life of a product (Ranadheera et al. 2017). The fermented probiotic products maintain viable cell count, but on the other side, care should be taken to maintain the sensory profile of a product, mitigation of health risk due to pathogens and maintenance of aseptic conditions. The use of probiotics as functional food may enhance the energy source and greatly influences the performance of a product (Khaneghah and Fakhri 2019; Ranadheera et al. 2017).

Conclusions

The gut microbiota can be modulated to recover host health through some interventions, i.e., administration of probiotic organisms. This review highlighted the data related to the relationship between bioactive molecules produced by probiotic bacteria, their mechanism of action in in vivo and its impact on the health of host. This is a new and attractive phase of research for formulating probiotic bacterial composition to modulate the host immune system for protection and treatment of various ailments. In future, identification of new variants and consumption of newly formulated probiotics may be a good strategy to promote good health in future. Therefore, probiotics can be recommended as alternative bio-therapeutic agents for treatment of various infections.

Abbreviations

CFU

Colony forming units

ANVISA

National Sanitary Surveillance Agency

FOS

Fructooligosaccharides

GOS

Glucooligosaccharides

XOS

Xylooligosaccharides

LAB

Lactic acid bacteria

PPAR-γ

Peroxisome proliferated activated receptor- γ

Compliance with ethical standards

Conflict of the interest

The authors declare no conflicts of interest.