Encyclopedia
Please note this is a comparison between Version 1 by Emidio Scarpellini and Version 2 by Vicky Zhou.
Gut microbiota (GM) is a complex ecosystem containing bacteria, viruses, fungi, and yeasts. It has several functions in the human body ranging from immunomodulation to metabolic. GM derangement is called dysbiosis and is involved in several host diseases. Pre-, probiotics, and symbiotics (PRE-PRO-SYMB) have been extensively developed and studied for GM re-modulation.
- gut microbiota
- prebiotics
- probiotics
- symbiotic
- postbiotics
1. Introduction
The human microbiota, now considered as a “functional organism“, consists of a complex community of microorganisms (bacteria, yeasts, fungi, archea, protozoa, and viruses), living on our skin and mucosal tissues, hence forming an efficient ecosystem together with the body [1][2][1,2].
In humans, gut microbiota is crucial in metabolism, immune tolerance, and nutrients absorption, within and outside the gut [3]. Thus, an imbalance of gut microbiota, potentially generated by diet, use of antibiotics, or infections, can be detrimental for host equilibrium [4].
The latter, namely “dysbiosis”, can express as intestinal and extra-intestinal diseases [5]. The first group includes liver steatosis (NAFLD) until the liver cirrhosis stage [5], inflammatory bowel diseases (IBD), irritable bowel syndrome (IBS), celiac disease, and gastro-intestinal cancers [4]. The second group includes obesity, atopic and autoimmune diseases, autism, and systemic sclerosis [4].
The main measures to re-establish unbalanced gut microbiota are the use of diet, antibiotics, pre-, and probiotics [6]. However, emerging evidence shows how the use of living organisms, namely probiotics, is not devoid of virulence emergence and antibiotic-resistance development over time [6]. Thus, there is a strong need for use of safer and equally effective gut microbiota modulatory agents. One very promising chance is represented by bacterial products and the components of probiotics, namely “postbiotics” [7].
2. From Pre- and Probiotics to Post-Biotics
2.1. Gut Microbiota Composition and Main Functions
The human GI tract hosts over 100 trillion microbes, the vast majority being bacteria [3].
The latter are divided in phyla, classes, orders, families, genera, and species [8][9]. Main gut microbial phyla are: Firmicutes, Bacteroidetes, Actinobacteria, Proteobacteria, Fusobacteria, and Verrucomicrobia. However, the two phyla Firmicutes and Bacteroidetes account for almost 90% of the entire gut microbiota. Firmicutes includes more than 200 different genera (e.g., Lactobacillus, Bacillus, Clostridium, Enterococcus, Ruminicoccus, and Clostridium) and Bacteroidetes has two of the most abundant genera (Bacteroides and Prevotella) [1][8][1,9].
The entire genome of the gut microbiota is called “microbiome” and is about 150-fold bigger than that of a human cell [9][10].
One-tenth of the total colonizing bacterial species per individual constitutes a dynamic “microbial fingerprint”: ageing, dietary changes, and use of antibiotics, prebiotics, and probiotics can make it fluctuate [10][11]. Indeed, there is an intestinal microbial ‘core’ that includes 66 species conserved in over 50% of the general population [11][12].
The use of culture-based methods has limited the study of gut microbiome composition. However, the advent of new metagenomic technologies has paved the way to the definition of inter/intra-individual variability of the gut microbiome [10][11].
Upon delivery, microbial life starts with a limited and unstable repertoire of microorganisms that evolves and adapts to environmental changes during our lifespan [12][13][14][13,14,15].
Gut microbiota is crucial for nutrient absorption and fermentation, regulation of intestinal permeability (IP), host metabolism (e.g., carbohydrates absorption and processing, proteins putrefaction, bile acids formation, insulin sensitivity), and modulation of intestinal and systemic immunity. This is crucial in childhood and adulthood, for the maintenance of antigen tolerance and the containment of pathogens expansion [15][16].
Thus, there is ever-growing interest in the remedies and approach to re-establish gut microbiota composition through diet and antibiotics as well as with pre- and probiotics [3][16][3,17].
2.2. Pre-, Probiotics and Symbiotics
Prebiotics are food components beneficially affecting gut microbiota. The main examples of their group are human milk oligosaccharides, lactulose, fructo-oligosaccharides, and inulin [17][18]. Their impact on gut microbiota is relevant. They have been extensively used in functional food manufacturing and as an add-on treatment for dysbiosis [17][18]. However, adjusted dose-finding and treatment-duration studies are needed to evaluate and assess the optimal scheme of administration [17][18].
Prebiotics are very often administered together with probiotics, namely living microorganisms (e.g., bacteria, yeasts), beneficially affecting host health [16][17]. Several studies on probiotics have been analyzing their real impact on human health and diseases [4]. More robust data vs. prebiotics are available on their efficacy and dose- and strain-effect relationship [18][19]. Pre- and probiotics, namely symbiotics, can be used together with an empowering mutual efficacy and multi-effect profile [18][19].
The spectrum of human diseases where pre- and probiotics are usable alone or in combination encompasses GI (e.g., NAFLD, cancer, celiac disease, IBD, IBS, diarrhea, functional dyspepsia, constipation) and extra-intestinal diseases (obesity, diabetes, autoimmune disease, atopic conditions, neurological disorders, psychiatric disorders) [18][19].
However, use of probiotics is not devoid of potential harmful issues, such as antibiotic resistance development as well as the growth and selection of virulent strains for humans [18][19].
2.3. A New Concept among the “Bugs”: Postbiotics
Postbiotics are defined as beneficial substances, resulting from microbiota metabolism and having a beneficial effect on the microbiota itself as well as the host [7][19][7,20]. Therefore, after looking extensively into the real origin of “the egg or the chicken”, and landing on the probiotics concept as the long-lasting “chicken”, we arrived at the discovery and usage of “the egg”.
There is still debate about whether this exclusive definition is accurate enough. For example, Tsilingiri et al. consider postbiotics “any substance released by or produced through the metabolic activity of the microorganism, which exerts a beneficial (direct or indirect) effect on the host” [20][21]. Salminen et al. claimed the concept of postbiotics to be inclusive of inactivated microorganisms beneficially affecting host health [19][20]. However, this definition has not been accepted by the rest of the scientific community [21][22]. Thus, we assume that postbiotics include all substances of bacterial/fungal origin with a beneficial effect on the host. In addition, they fail to meet the criteria for probiotics and prebiotics [21][22].
Main representatives of postbiotics are: cell-free supernatants, exopolysaccharides, enzymes, cell wall fragments, short chain fatty acids (SCFA), bacterial lysates, and gut microbiota metabolites.
2.4. Actual and Future Applications of Postbiotics
Although postbiotics can be considered pleiotropic agents on humans’ health, there are different effects that deserve a deeper description.
2.4.1. Immunomodulation and Anti-Cancer Effects
Immunomodulatory effects of postbiotics have been reported in the last 30 years. In detail, SCFAs, and, more in detail, propionate, are able to up-regulate Tregs [22][85]. Supernatant, cell wall fragments from Bacillus coagulans culture also promote T helper (Th)2-dependent immune response [23][86]. In addition, supernatant from Bifidobacterium breve culture is able to limit the Th1-mediated and enhance Th2-mediated responses [24][25][87,88]. These effects are often observed in mice models of atopic diseases [25][88].
The SCFA propionate is able to selectively induce apoptosis in gastric cancer cells [26][89]. Interestingly, SCFAs can also modulate onco- and suppressor genes expression through epigenetic modifications: L. rhamnosus GG supernatant increases ZO-1 expression (responsible for intercellular permeability) and decreases MMP-9 expression that facilitates cancer cell penetration [7] (Table 1).
2.4.2. Anti-Infectious Effects
Some postbiotics can competitively bind to receptors for pathogenic bacteria, are able to change the expression of host genes, and modulate host environmental components [27][90]. Starting from this paradigm, it is interesting to note that the newer combination of postbiotics and probiotics can efficaciously prevent rotavirus-associated diarrhea [28][91]. However, this finding about a new kind of “symbiotic” is only preclinical. In a group of children aged 12–48 months, the daily intake of L. paracasei postbiotic leads to reduction in the incidence of diarrhea [29][92], acute gastroenteritis, pharyngitis, laryngitis, and tracheitis [30][93].
2.4.3. Metabolism Modulation and Anti-Atherosclerotic Effects
Postbiotics can regulate and reshuffle lipid metabolism. This can result in a significant reduction of cardiovascular risk and related accidents. Propionate is well-known, having a “statin-like” effect (similarly to nutraceuticals, e.g., curcumin, K-monacolin) [31][94]. It is interesting to note that the case of kefiran conjugates antiatherogenic (e.g., prevention of cholesterol accumulation in macrophages and reduction of lipid concentration) and anti-inflammatory actions [32][95]. Lactobacilli BLs reduce the levels of triglycerides and LDL cholesterol, while increasing the level of beneficial HDL cholesterol in an obese mouse model [33][96]. These effects are explained by activation of the peroxisome proliferator-activated receptor (PPAR), the same therapeutic target of fibrates [33][96].
2.4.4. Detoxification and Wound Healing Effects
Autophagy is a self-degradative process, cleaning out cells and their components from tissues. It is an efficacious response to various stress stimuli such as those from diet. Bacterial peptidoglycan promotes autophagy through the NOD1 receptor [34][97]. More in detail, L. fermentum postbiotics trigger autophagy in hepatic cells HepG2, resulting in a protective effect towards induced liver toxicity [35][98]. Furthermore, urolithin A inhibits mitophagy, namely autophagy of mitochondria, and can potentially prevent or delay muscle ageing [36][99].
Non-experts often ignore that oxytocin, an important gynaecologic neuropeptide, can also stimulate and accelerate wound healing. BLs obtained by sonication of L. reuteri significantly increase the number of oxytocin-producing cells in the hypothalamic periventricular nuclei, raising hormone blood concentration in animal models [37][100]. The subsequent probiotic-generating postbiotic administration to both animal and human models confirmed these results, with a good safety profile [37][100].
2.4.5. Functional Foods Preparation
Functional foods (FF) resemble dietary components with a clear nutritional value and other beneficial health effects. Postbiotics and probiotics are already part of the preparation of functional foods.
One of the main advantages of FF enrichment by postbiotics is the host’s immune-stimulation. For example, the cell-free fraction of fermented milk is able to prevent Salmonella infection in mice [38][101]. Furthermore, postbiotics from B. breve and Streptococcus thermophilus are currently used in the production of modified milk, in order to obtain a long-lasting reduction in the incidence of food intolerance and/or respiratory allergy in the first months of the life of children [39][102]. Importantly, only mild diarrhea was recorded in these trials [40][103].
2.4.6. Future Perspectives and the COVID-19 Issue
As postbiotics are able to beneficially affect the maturation of the immune system, improve the regulation of intestinal permeability, and, last but not least, modulate gut microbiota composition, their future uses are expanding. They could be used to prevent and, further, treat several diseases devoid of an efficacious cure, such as Alzheimer’s disease and multiple sclerosis. Preliminary reports on the first clinical trials on the use of postbiotics for the abovementioned diseases seem promising [7].
Gut microbiota modulation with probiotics seem to affect the inflammatory storm of COVID-19 and to directly counteract replication of SARS-CoV-2 [41][104]. Thus, GM re-modulation via postbiotics can be crucial in preventing SARS-CoV-2 infection in predisposed humans [42][105]. In addition, GM remodulation can change the natural story of COVID-19 towards a milder form [42][105]. Similarly to probiotics, postbiotics could directly inhibit SASR-COV-2 replication.
In an in vitro and in silico study by Rather et al., an extract from L. plantarum Probio-88 (P88-CFS) was able to significantly inhibit the replication of SARS-CoV-2. In addition, P88-CFS-treated cells showed a significant reduction of inflammatory cytokines. Based on the in silico molecular docking data, it was unraveled that the antiviral activity of this strain derives from plantaricin E (PlnE) and F (PlnF), which is able to bind on the SARS-CoV-2 helicase. Thus, these postbiotics can act as a “blocker” of viral ss-RNA during its replication. This strain and/or its postbiotic substances can be used as an integrative approach, along with a vaccine, to contain the spread of SARS-CoV-2 and, most importantly, its variants [43][106].
A special mention is deserved for the emergence of “biological doping”. GM modulation can change humans’ metabolism and the availability of energy sources for a better sport performance [7]. For example, an increased abundance of Veillonella in our gut, able to metabolize lactic acid to propionate, significantly increases animals’ physical performance [43][106]. Similarly, enteral administration of propionic acid leads to similar results [43][106]. Another issue is represented by the recognition of this form of doping, which is emerging as an efficient road to skipping health controls for athletes.
