2. Mechanisms Driving Postbiotic Efficacy
2.1. Protective Modulation against Pathogens
Although postbiotics influence microbiota temporarily, for the most part, they could indeed play a significant mechanistic role. According to in vivo research, molecules contained in postbiotics, namely lactic acid and bacteriocins, may have direct antimicrobial properties. For instance, organic acids belonging to lactic acid bacteria, bifidobacterial and other postbiotic strains primarily exert antimicrobial efficiency against Gram-negative pathogens, which has a dose-dependent effect
[16]. The antibacterial action of the cell-free supernatants is thought to be mostly due to bacteriocins
[17], for example, supernatants derived from the genera
Lactobacillus and
Bifidobacterium were also verified to have antibacterial properties against the invasion of enteroinvasive
E. coli [18]. Different
Bifidobacterium strains have produced bifidocins, which have a wide spectrum of bactericidal action against both Gram-positive and Gram-negative bacteria as well as certain yeasts. Additionally, when exposed to exopolysaccharides (EPS) isolated from
Bifidobacterium bifidum, lactobacilli and other anaerobic bacteria grew more readily while enterobacteria, enterococci, or
Bacteroides fragilis are inhabited
[19]. The well-known antibacterial metabolite reuterin, which is generated by
Lactobacillus reuteri, is assumed to function by oxidizing thiol groups in pathogenic gut bacteria
[16][20]. Co-aggregation with
Helicobacter pylori has reportedly been suggested as another potential underlying mechanism for such an action of lactobacilli-contained postbiotics products
[21].
The biofilms of pathogenic bacteria are one of the serious hazards to the medical fraternity. Biofilm appears to be the primary cause of pathogenesis and treatment failure because of the antimicrobial resistance enclosed in the biofilm matrix
[22]. Through inhibiting the production of biofilms and deconstructing already-formed biofilms, the pure teichoic acids isolated from
Lactobacillus strains have exhibited inhibitory effects on biofilm formation of oral or enteric pathogens including
Streptococcus mutans,
Staphylococcus aureus, and
Enterococcus faecalis [23][24][25]. Biosurfactants that are produced extracellularly or attached to cell walls have the amphiphilic feature, which helps deconstruct existing biofilms or prevents biofilm formation. Additionally, the features of wetting, foaming, and emulsification prevent bacteria from adhering to, establishing themselves in, and subsequently communicating in the biofilms
[26].
2.2. Fortify the Epithelial Barrier
Certain postbiotics enhance mucosal barrier function through the alteration of secreted proteins. When administrating the active and heat-killed
L. rhamnosus to mice with colitis, protection against the rise in mucosal permeability and restoration of barrier function can be observed, which may be attributed to the upregulation of myosin light-chain kinase and zonula occludens-1 in intestinal epithelial cells
[27]. Synergism of mucosal protectors and postbiotics has been verified in intestinal cell models. The same combination, which resulted in an increase in transepithelial electrical resistance (TEER) and a decrease in paracellular flux, was also evaluated in CacoGoblet
® cells that had been exposed to
E. coli [28]. Moreover, short-chain fatty acids (SCFAs) modulate the trans-permeability in Caco-2 cells through similar mechanisms, enhancing TEER values and the expression of tight junction protein genes
[29][30].
Purified EPS from lactic acid bacteria and bifidobacteria demonstrated the defense against infections in several previous research papers
[17][31]. According to some publications, the antibacterial properties of EPS-containing postbiotics may be connected to the formation of a protective biofilm that protects the host epithelium from pathogens or their toxins
[31].
2.3. Modulation of Immune Responses
According to research by Tejada-Simon and Pestka, probiotic bacteria’s whole inactivated cells, cell components, as well as cytoplasmic fractions activate macrophages to produce cytokines and nitric oxide, thus indicating that bioactive substances may be present throughout the probiotic cells
[32]. These contents discuss the mechanism of the postbiotics regarding the modulation of immune responses in three dimensions: whole inactivated cells, bacterial components and metabolites.
Certain whole-cell postbiotic products of
Lactobacillus have demonstrated the anti-inflammatory (downregulation of IL-6, TNF- α and upregulation of IL-10) and anti-oxidative (removal of free radicals) properties in vitro and in vivo experimental animal models
[33]. ILs are immune-glycoproteins and are involved in inflammatory responses by modulating multiple growths and the activation progress of immune cells
[34]. For example, heat-treated
Bifidobacterium longum as a whole-cell postbiotic has demonstrated various barrier protection properties, such as antioxidation, anti-inflammation, and the inhibition of bacterial colonization
[35].
With the exception of external bacterial products, the structural elements, especially the cell envelope, which is the outermost structure that immune system cells initially interact with, should play a significant role in mediating immunomodulatory activity. In immunomodulation, toll-like receptors (TLRs) are appropriate and the most common targets for ligand-drug discovery strategies, which make postbiotic products possible in inflammatory diseases and autoimmune disorders
[36]. Peptidoglycan (PGN) and lipopolysaccharide (LPS) extracted from bacteria have been the subject of several investigations, which have shown that both molecules stimulate the immune system in a receptor-dependent manner. The primary sensors of the innate immune system are specialized conserved pattern recognition receptors (PRRs) on host cell membranes, involving TLRs and the nucleotide-binding domain (NOD) proteins (or NOD-like receptors, NLRs), which recognize PGN and LPS as ligands associated with pathogens
[37]. Teichoic acids (TAs) can be covalently bonded to the cytoplasmic membrane or peptidoglycan (wall teichoic acids, WTAs) (lipoteichoic acids, LTAs). It has been suggested that TLR2 is the mechanism by which TAs from
lactobacilli cause proinflammatory reactions. Additionally, it has been indicated that symbiotic intestinal bacteria and Gram-positive probiotics regulate the immune response to pathogens via their TAs, limiting an excessive inflammatory response
[38]. The surface layer (S-layer), consisting of the self-assembly of protein or glycoprotein subunits on the outer surface, allows
lactobacilli to stimulate the host immune system. In a study conducted by Konstantinov,
L. acidophilus SlpA was recognized and bound to a C-type lectin receptor existing on both macrophages and dendritic cells
[39]. Indeed,
lactobacilli absent of S-layer proteins showed relatively inadequate adhesion ability to the enterocyte.
Finally, genomic DNA also enables postbiotics to interact with the host immune system. Unmethylated CpG sequences contained in prokaryotic DNA have immunogenicity properties in vitro and in vivo, according to convincing published literature
[40]. Researchers from the same period observed that bacterial genomic DNA extracted from pure bifidobacterial cultures of VSL#3 (a probiotic commercial product) affected cytokine production in peripheral blood mononuclear cells (PBMCs), with the tendency towards a low level of IL-1b and a high level of IL-10
[41]. An in vivo mouse investigation also validated the anti-inflammatory properties of genomic DNA from VSL#3, revealing that TLR9 signaling was crucial in mediating such anti-inflammatory response
[42].
3. Applications of Postbiotics in Different Fields
3.1. Applications in the Food Industry
Fermentation is the most prevalent process with applications of postbiotics, and strains of
Lactobacillus and
Bifidobacterium are commonly used as producer strains
[43]. The dairy industry benefits greatly from the EPS of specific strains of dairy starter cultures because EPS has significant control over the rheological characteristics of fermented dairy products and lowers their moisture content
[44]. Moreover, postbiotics from
Lactobacillus plantarum can exert efficacy as a bio-preservative to extend the shelf life of soybeans
[45]. Combining the above two kinds of application, MicroGARD is a commercial preparation made by Danisco that has received FDA approval and is utilized as a premier biopreservative in extensive dairy and food matrices. It is a fermented version of
Propionibacterium freudenreichii subsp.
Shermanii found in skim milk
[46]. Other novel approach involves increasing vitamin B and decreasing toxic components during probiotic-induced fermentation
[43].
3.2. Pharmaceutical Applications
Many postbiotics products in the experimental research stage and not being applied to the clinic yet, also show great application potential. Giordani B et al. in 2019 conducted research and discovered that the biosurfactants of
L. gasseri had antibiofilm capacity against methicil-lin-resistant
S. aureus (MRSA). Another example is a heat-stabilized
acidophilus containing medication called Lacteól Fort (Laboratoire du Lacteól du docteur Boucard, France), which has been proven effective in the treatment of acute diarrhea and irritable bowel syndrome (IBS)
[47] by randomized controlled trials. At present, industries are intending to place postbiotics into a regular pharmaceutical product matrix because of their stable pharmacodynamic features and beneficial effects in clinical application
[48].
3.3. Biomedical Applications
Postbiotics enhancing the effects of vaccination in the elderly has been proven by increasingly more evidence, the underlying mechanism of which includes sustainable antibody production and NK-cell activities. Research in 2016 demonstrated that the concentration of antibody to type A/H1N1 and B antigens were improved in an elderly subgroup with heat-killed
Lactobacillus paracasei jelly
[49]. Moreover, similar to parent probiotics, the use of postbiotics is a promising strategy to treat pediatric infectious diseases in under-five-year-olds because it exerts immunomodulatory as well as antimicrobial effects
[50]. Postbiotics are also able to serve as a novel strategy for food allergy in pediatrics because of their unique characteristics against parent live cells
[51].
Research demonstrates that the microbiome-metabolome axis in the gastrointestinal tract is affected and therefore graft versus host disease colitis can be alleviated through probiotics or postbiotics application
[52]. Similarly, the combination of postbiotic butyrate and active vitamin D could be a possible treatment for infectious and autoimmune colitis
[53]. Apart from gastrointestinal diseases, extensive research has proven that specific postbiotic metabolites affect the differentiation and function of CD4+ T cells, with results indicating that postbiotics could be a promising perspective to treat allergic rhinitis
[54]. Moreover, urinary tract infections (UTIs) should be further explored based on the results suggesting that mucosal protectors might lessen the intestinal reservoirs of uropathogenic
E. coli strains
[55]. Accordingly, some research indicates that metabolites generated by
lactobacilli (hydrogen peroxide and lactic acid) act in concert to eradicate uropathogenic organisms in vitro
[56][57] and could serve as the foundation for the creation of UTI management products with postbiotics.
Furthermore, various postbiotic molecules have attracted interest because of their wide modulation effects in obesity, coronary artery diseases, and oxidative stress through the capacity to trigger the alleviation of inflammation reactions and pathogen adherence to gastrointestinal tract, etc. Presently, postbiotic preparations have also been granted patents as bio-therapeutics for a specific health benefit of “immune-modulation”
[58].
4. Advantages of Postbiotics Compared with Probiotics
The use of non-viable postbiotics as a safer option has gained popularity as safety concerns over the use of live strains have surfaced in certain patient populations, including immunodeficient subjects, infants and vulnerable patients
[3][6][59]. They could significantly reduce consumer risk of microbial translocation and infection
[60].
Calculating the percentage of dead cells in a probiotic culture that is still viable will be difficult. Therefore, changing percentages of dead cells may be the origin of the variation in responses usually found with living probiotic products. However, it is simple to demonstrate that postbiotics are devoid of any living organisms. Postbiotics-based products would be long-lasting and extremely simple to standardize, making them easier to store, have a longer shelf life and facilitate logistics under extreme environmental conditions.
[35][59].
By lowering the likelihood of the transmission of antibiotic-resistant genes, using inactivated bacteria can have significant advantages. Probiotic use is now discussed in terms of antimicrobial resistance prevention techniques
[61][62] and the need to stay away from long-term pharmaceutical treatments and their negative effects
[61]. The use of non-viable probiotics as an alternative therapy is increasingly accepted due to the high incidence of antibiotic resistance in live probiotic applications (
Figure 1).
Figure 1. Comparison between probiotics and postbiotics in application.
5. Drawbacks of Postbiotics
Postbiotic products have been proven to be a relatively weaker influence on the modulation of intestinal metabolism or gene expression affecting nutrition metabolism when compared with corresponding probiotics. For example, live cells of
Bifidobacterium breve M-16V displayed enhanced immunomodulation effects in contrast to postbiotics, which is mainly reflected in the inhibition of pro-inflammatory cytokines in spleen cells and more significant alteration of intestinal metabolism
[59].
The type of technology in the inactivation process might relate to products with variable functionality in comparison with the progenitor microbial product according to the microbial inactivation degree achieved. For example, it has been demonstrated that different heat treatments ranging from air drying, freeze drying to spray drying can significantly impact the viability and immunomodulatory properties when dehydrating probiotics
[63] On the other hand, volatility in the nutritional value, sensory characteristics and flavors caused by traditional thermal processing including pasteurization, tyndallization and autoclaving frequently occurs, thus establishing a reliable controllable range, and the acceptance of the original product audience requires more investigation
[1]. Thermal processing, therefore, may not be the best option, especially when a postbiotic product is used as a food supplement. Emerging technologies such as electric field, ultrasonication, high pressure, ionizing radiation, pulsed light, magnetic field heating and plasma technology
[64] could possibly be applied to inactivate microorganisms and generate postbiotics to obtain safe and stable foods with retained overall quality and value.
The composition and quantity of a postbiotic product must be described and measured using appropriate methods. These techniques should be accessible for both quality control at the production site, and for a precise product description that enables duplicate research. An emerging technique, flow cytometry, is now gradually substituting traditional technology such as plate counting for microbial counting and enumeration
[65].
Inconsistent, vague, and frequently reliant on patient requests, postbiotic product recommendations also suffer from similar issues, according to a recent study on healthcare providers’ probiotic prescription practices. This means that the patient or the pharmacist made the postbiotics choice based merely on their own experience for a significant portion of the time
[14]. It is vital to address the insufficient, clear and specific clinical recommendations and the absence of supporting data from clinical research.
In the group receiving the inactivated
L. acidophilus with micronutrients, side effects appear involving severe to moderate dehydration, abdominal distension, and vomiting ranging from mild to severe
[66]. Postbiotic therapies’ safety and potential risks have not been thoroughly researched or understood. To ascertain the effects and safety of various postbiotics, additional multicenter studies are required.