1. Introduction
Microorganisms colonize the skin and mucous membranes from birth [
1]. Normally, commensal microorganisms protect against pathogens and promote the formation of an adequate immune response to pathogenic microorganisms and harmless antigens [
1]. Disruption of symbiotic and mutualistic relationships between the microflora and the host organism can lead to various pathological conditions, including allergic diseases [
2]. For example, it has been shown that allergic reactions are associated with the use of antibiotics in early childhood [
3]. It was found that the use of antibiotics during the first 2 years of life was a risk factor for the development of asthma, atopic dermatitis, and allergic rhinitis at the age of 5 [
4,
5,
6]. This is how the hygiene hypothesis arose, referring to the T-helper type 1/2 (Th1/Th2) model [
7,
8]. According to the hygiene hypothesis, allergic diseases are associated with insufficient activation of innate immune receptors in early childhood during the development of the immune system. This hypothesis was also confirmed by the discovery that children from urban areas are predominantly affected by allergic diseases compared to children from rural areas [
9]. It should be noted that along with the “hygiene hypothesis”, a “counter-regulatory” hypothesis emerged, according to which the epidemic of allergic diseases observed in recent years is a small price to pay for a significant reduction in child mortality, achieved through measures such as improved sanitation, access to drinking water and vaccination [
10]. In addition, society has come to understand the carcinogenic effects of sunlight and to avoid it, while ultraviolet radiation is a powerful inducer of IL-10 [
10]. Microbial colonization of mucosal tissues during infancy may have long-term consequences, such as promoting tolerance to environmental insults or contributing to the development of diseases in later life, including inflammatory bowel disease, allergy, and asthma [
11,
12]. With increasing understanding of the complexity of the regulation of allergic reactions, other theories have emerged, such as the “biodiversity” hypothesis, which shows a correlation between reduced biodiversity of the skin and mucosal microbiota and the incidence of allergic diseases [
13]. In addition, among siblings, younger siblings were found to be less likely to develop allergic diseases [
14,
15]. Moreover, this correlation was independent of time period and geographic region [
14]. With the discovery of bacteria in the meconium of newborns, it became obvious that acquaintance with microorganisms occurs in the womb and the problem of the impact of microflora should be analyzed from earlier periods than birth [
16,
17,
18,
19,
20].
Modern technologies have revealed associations of genetic polymorphisms of numerous genes involved in the implementation of innate immunity and the formation of tight junctions between epithelial cells with allergic and inflammatory diseases [
21,
22,
23]. However, the discovered polymorphisms could not fully explain the “non-heritability of asthma”, which necessitated determining the influence of environmental factors on epigenetic changes leading to allergic diseases [
24]. Among these factors are air pollutants associated with professional activities, allergens and infectious agents in the prenatal and postnatal periods. A modified strategy that includes gene polymorphisms in combination with environmental factors has shown a correlation with allergic diseases [
24]. The recently introduced “Epithelial Barrier Theory” includes even more parameters to explain the occurrence of allergic inflammation [
25,
26]. The Epithelial Barrier Theory combines all previous hypotheses and suggests that toxic substances in hygiene products, as well as microplastics and air pollution, damage the epithelium of our skin, lungs and gastrointestinal tract [
27,
28]. At the same time, epithelial barrier dysfunction can be mediated by genetic and environmental factors [
29,
30,
31]. In particular, serine proteases and metalloproteinases contained in pollen disrupt the integrity of the lung epithelial barrier by degrading the transmembrane adhesion proteins E-cadherin, claudin-1, and occludin, as well as the cytosolic complex zonula occludens-1 (ZO-1), which leads to an increase in transepithelial permeability [
32]. Disruption of tight junctions and an increase in transepithelial permeability facilitate the action of allergens on the epithelial sublayers, promoting sensitization to a wide range of allergens [
33]. Disruption of the epithelial barrier leads to a decrease in the biodiversity of the microbiome and colonization by opportunistic microorganisms, which interact with the immune system to initiate local and systemic inflammation [
25,
26].
The increase in allergic diseases since 1960 is also associated with the emergence of 350,000 new toxic compounds that, against the background of genetic susceptibility, contribute to disruption of the epithelial barrier and cause local and systemic inflammatory diseases [
31]. Evolutionarily formed relationships between commensal microorganisms and the host are under pressure from new, previously unknown chemicals to the immune system and can significantly modulate immune reactivity along with resident microorganisms. Thus, there is a need to summarize the possible ways in which microorganisms influence the immune system to determine their contribution to the prevention or aggravation of allergic inflammation. The aim of this review was to analyze the influence of bacteria inhabiting the gastrointestinal tract, skin, upper and lower respiratory tract, as well as bioregulators of bacterial origin on allergic diseases.
2. Bacteria Modes of Action on Host Cells
The human microflora includes bacteria, archaea, fungi, protozoa, and viruses that inhabit the skin and all mucous membranes [
34,
35]. Bacterial community is the most studied and most numerous in the gastrointestinal tract, where the number 10
13–10
14 [
36]. At the same time, it was found that resident microflora not only helps to ferment food [
37,
38], supply various nutrients and vitamins [
39,
40,
41,
42,
43], and prevent the colonization of pathogenic microflora [
44,
45], but also helps to strengthen the epithelial barrier [
46,
47]. Such a diverse effect of commensal bacteria on the human body is achieved due to the existence of many ways of activating host cells (
Figure 1). First of all, bacteria affect specific receptors of the innate immunity located on the surface and in the cytosol of host cells through pathogen-associated molecular patterns (PAMPs). These include peptidoglycans, lipopolysaccharides, flagellin, CpG nucleotides, muramyl peptides and lectins that activate TLR2, 4, 5, 9, NLR and CLR, respectively [
48,
49,
50,
51,
52,
53]. In this case, lipopolysaccharides and muramyl peptides, which are agonists of TLR and NLR respectively, may have structural differences characteristic of different strains of microorganisms [
54,
55,
56,
57,
58]. Activation of PAMP receptors of innate immunity TLR and NLR triggers a cascade of reactions with activation of the transcription factor NFkB, synthesis of proinflammatory cytokines and chemokines, expression of cellular receptors, markers of differentiation and maturation of immunocompetent cells, which normally leads to elimination of the pathogen [
59,
60,
61]. In this case, cross-interaction occurs between different classes of innate immunity receptors [
62,
63,
64]. In addition, innate immunity receptors cross-interact with other receptors on the cell surface. The microbial metabolite indole-3-propionic acid (IPA) has been shown to suppress enterocyte TNF-α via the pregnane X receptor (PXR). PXR-deficient mice (Nr1i2(-/-)) exhibit distinctly leaky gut physiology coupled with upregulation of the Toll-like receptor (TLR) signaling pathway. These epithelial barrier defects were corrected in Nr1i2(-/-)Tlr4(-/-) mice [
65]. Thus, the bacterial metabolite indole-3-propionic acid, via PXR, normally regulates TLR4 activity. Subsequently, PXR was shown to regulate the intestinal epithelial barrier during inflammation by reducing cytokine-induced myosin light chain kinase expression and c-Jun N-terminal kinase 1/2 activation [
66]. The resulting cross-talk and positive and negative feedback loops, depending on the intensity of the stimuli that cause them, ultimately determine the nature of the immune response [
53,
67].
In addition to specific activity, bacteria have a non-specific effect on host cells by producing amino acids, vitamins, hormones, short-chain fatty acids, bile acid intermediates, bacteriocins, and other chemical compounds [
39,
40,
41,
42,
43,
68,
69]. Gut bacterial metabolites exert their biological activity through specific recognition by G protein-coupled receptors (GPRs) [
70,
71]. GPRs are differentially expressed in different cell types, and the response to the same metabolite can vary depending on the specific roles of the cells [
72]. Therefore, the response to gut microbiota-derived metabolites exhibits a huge combinatorial diversity, which poses significant challenges in understanding their effects [
73]. Bacteria secrete not only small molecular compounds. Bacterial secretion systems, in particular type III and IV, can deliver proteins and nucleic acids directly into the host cell cytosol, directly influencing cellular metabolic processes [
74,
75,
76].
Another way bacteria influence host cells is through extracellular vesicles, when bacteria can introduce enzymes, RNA, toxins, components of the bacterial membrane, including LPS, into the host cell [
77,
78,
79,
80]. In this case, pathogenic bacteria can change intracellular pathways, which allow bacteria to evade the immune response and colonize eukaryotic cells [
81,
82,
83]. Extracellular vesicles of pathogenic bacteria have been studied in more detail than those of commensal bacteria; however, the formation of extracellular vesicles in commensals has been established, with the ability to modulate interaction with the host and immune training [
57,
84,
85,
86]. Moreover, it has been established that extracellular vesicles of Lactobacillus plantarum Q7 have an anti-inflammatory effect [
87], and bacterial tryptophan catabolites enclosed in extracellular vesicles can enhance the barrier functions of the intestinal epithelium and regulatory immune responses [
88,
89,
90].
Thus, bacteria inhabiting the skin and mucous membranes affect all systems and organs in numerous ways, normally contribute to the maintenance of immune homeostasis and can affect diseases [
91,
92,
93,
94,
95].
3. Modulation of Allergic Reactions by Bacterial Regulators
Allergic reactions occur in tissues that are adjacent to the external environment and populated by various microorganisms, which, along with heredity and environmental factors, contribute to inflammatory reactions. There are many phenotypes and endotypes of allergic diseases, differing in the causes of occurrence, clinical signs and immune cells involved in pathological processes [
96]. Despite the differences in the manifestation of allergic reactions on the skin, mucous membranes of the gastrointestinal tract, upper and lower respiratory tract, genitals, etc., there are common signs of ensuring the homeostatic state of barrier tissues [
97,
98,
99]. Commensal bacteria play a significant role in maintaining the homeostatic state of barrier tissues, which normally keep innate immune cells active to quickly repel pathogen attacks and ensure tolerance to resident bacteria [
1,
100,
101]. The balance between excessive immune reactivity to harmless antigens and insufficient immune response to dangerous microorganisms can be disrupted by the negative impact of external factors. As a result, the integrity of epithelial barriers can be disrupted, the composition of microflora can be changed and, as a consequence, inflammatory, allergic and autoimmune reactions can occur [
102,
103,
104].
Depending on the localization, the epithelium of barrier tissues uses different methods of protection from external influences: antimicrobial peptides, mucins, IgA, various enzymes. However, when recognizing allergens and bacterial metabolites, common response mechanisms can be traced (
Figure 2). In the case of recognizing allergens by dendritic antigen-presenting cells (APCs), they are processed to form complexes of class II major histocompatibility complex (MHC II) molecules and the antigen, migrate to the lymph nodes and promote the differentiation of naive T cells into Th2 cells [
105,
106]. At the same time, bacterial bioregulators interacting with APCs through TLRs and NLRs trigger the activation of Th1 and Th17 types by producing cytokines TNF, IL-1β, IL-6, IL-12, IL-23, IFN γ [
107,
108,
109,
110]. When stimulated, naïve T cells secrete IL-2, interferon (IFN)-γ, lymphotoxin, and tumor necrosis factor (TNF)-α, as well as low levels of IL-4, IL-13, and IL-10 [
111]. The preferential activation of Th1, Th2, Th17, or Treg depends on the cytokine milieu of the microenvironment [
112]. The balance between these endogenously produced cytokines is determined by the phenotype of the lymphokine-producing primed cells. This balance depends on the genetic background, the nature and strength of the signal, and the activation state of DCs. Th2 migrate to the site of inflammation and produce proinflammatory cytokines (IL-4, IL-5, IL-13) to activate eosinophils and stimulate IgE synthesis by B cells and subsequent degranulation of mast cells [
113]. Allergen-stimulated epithelial cells secrete IL-25, IL-33, and thymic stromal lymphopoietin (TSLP) to activate innate lymphoid cells type 2 (ILC2), which secrete IL-5, act on eosinophils, and stimulate inflammatory responses [
114,
115,
116]. ILC2 are critical drivers of type 2 (T2) inflammatory responses associated with allergic inflammatory conditions and can secrete IL-4, IL-5, IL-9, and IL-13 [
117]. Interestingly, depending on the microenvironment, namely, under the influence of retinoic acid, IL-2, IL-4, IL-10, and IL-33, a subpopulation of KLRG1+ILC2 may appear which has the ability to produce the anti-inflammatory cytokine IL-10 and exert an anti-inflammatory effect [
117]. Cross-interaction of immunocompetent cells, their mutual influence and sensitivity to the microenvironment normally ensures homeostasis and maintains the integrity of the epithelium [
118].
In the case of a compromised epithelial barrier, allergens directly activate APCs and mast cells, enhancing inflammatory responses [
119]. In addition, damaged epithelium increases immune cell infiltration, further enhancing dysfunction [
119]. When microorganisms and/or their metabolic products affect epithelial cells, the latter produce cytokines (in particular, IL-1, Il-6, TNF), which activate APCs and T-regulatory cells [
120,
121,
122,
123]. Treg increase immune tolerance during inflammation, highlighting the role of the microbiota in maintaining homeostasis [
100]. In addition, Tregs can modulate the responses of other T-cell populations, changing the direction of the immune response [
124,
125,
126,
127]. Commensal bacteria are known to induce the production of type I IFN (IFNα and IFNβ) and IL-27 by dendritic cells to influence Tregs [
100]. Moreover, IL-27 plays a crucial role in the expression of IL-10 by T-regulatory cells [
128,
129,
130,
131]. The importance of IL10 is due to its ability to exert an anti-inflammatory effect, controlling the intensity of immune responses, and maintaining immune homeostasis [
132,
133,
134]. Interleukin 10 is produced by almost all populations of immunocompetent cells - T-helpers, cytotoxic and regulatory T-cells, B-lymphocytes, macrophages, NK-cells, monocytes, dendritic cells, neutrophils, eosinophils and mast cells [
135,
136,
137,
138]. At the same time, B-cells secreting IL-10 were isolated into a separate subpopulation, namely regulatory B-cells (Breg), which have anti-inflammatory functions in allergic diseases [
139]. The key role of IL-10 in ensuring tolerance is the effect on dendritic cells, which, after interaction with Tregs, promote allergen-specific and cross-reactive tolerance [
140,
141]. IL10 is also produced by keratinocytes, intestinal mucosal epithelial cells, and tumor cells [
142,
143]. The ability of IL10 to suppress the immune response is used by some pathogens to evade immune surveillance [
142,
143]. In particular, Helicobacter pylori stimulates IL10 to survive in the stomach, Mycobacterium tuberculosis induces B cells, and Streptococcus pneumoniae induces neutrophils to produce IL10, which allows the bacteria to colonize their niches [
142,
144,
145,
146]. Some viruses, such as HIV, hepatitis C, and hepatitis B virus, also stimulate IL10 production to suppress the immune response and promote viral persistence [
143,
147]. Importantly, bacteria and their cell wall fragments stimulate the production of both proinflammatory and anti-inflammatory cytokines [
11,
12,
148]. The ability of bacteria to activate anti-inflammatory responses, including IL10 production, limits tissue damage and is important for mediating an effective immune response necessary for host survival [
146,
149,
150].
It should be noted that activation of innate immune receptors in experimental models protects animals from lethal infection with a wide range of pathogens and has therapeutic activity in allergic and oncological diseases [
151,
152,
153]. At the same time, stimulation with commensal bacteria through the induction of type 1 interferons triggers not only antibacterial and antiviral activity, but also the activation of dendritic cells and Treg cells, providing tolerance to commensals [
100].
Various combinations of PAMPs can enhance or inhibit the synthesis of cytokines induced by the activation of innate immune receptors, as well as cause activation or suppression of various populations of immunocompetent cells [
64]. In particular, PBMCs, monocytes, and dendritic cells secreted high levels of IL-10 when TLR5 was stimulated with flagellin, whereas TLR9 stimulation with CpG oligodeoxynucleotides did not induce IL-10 secretion in any of the three cell types but synergized with flagellin in this induction [
64]. Moreover, TLR5 stimulation completely abolished the NK cell cytotoxicity induced by TLR9 stimulation [
64].
Hundreds of genes are involved in the regulation of inflammatory processes, including ubiquitinases and deubiquitinases, the activity of which can be modulated by bacterial effectors [
77,
154]. Bacterial ligases, such as the E3 SopA, NleL, and IpaH families, can act on the principle of molecular mimicry and modulate ubiquitination processes in eukaryotic cells in the direction necessary for bacteria to manipulate the host signaling to facilitate infection [
155,
156,
157,
158,
159].
5. Concluding Remarks
New approaches to the diagnosis and therapy of asthma and associated diseases are being developed, including metagenomic studies, transcriptomics, systems biology and cell therapy [
154,
240,
241,
242,
243,
244]. At the same time, it is necessary to take into account a large number of factors influencing the occurrence of allergic diseases, in particular, the influence of the mother’s microbiome, her diet and the use of antibiotics during pregnancy on the formation of the allergic phenotype of her future child [
166,
245]. In addition, it is necessary to take into account the influence of viruses, fungi and archaea that are part of the human microbiome and enter into complex mutualistic, symbiotic and antagonistic interactions. It is important to determine not only the phyla and genera of bacteria associated with diseases, but also to study the species that can compete with each other for the habitat, as is shown by the example of
S. epidermidis and
S. aureus (phyla
Firmicutes, genera
Staphylococci).
The accumulated knowledge about the features of the impact of various microorganisms on humans makes it possible to prevent allergic diseases using both whole bacteria and their fragments and metabolites [
166,
246,
247,
248]. Currently, active research is underway to study the effect of microorganisms or their fragments on the host, while determining the effect of microorganism metabolites on humans requires more complex technologies. The developed modern methods for registering low-molecular compounds, as well as modern technologies, including machine learning, inspire hope for significant achievements in the near future. The ability of microbiota and bacterial metabolites to strengthen the integrity of the intestinal barrier, provide protection against inflammation, improve nutrient uptake pathways, and improve protection against age-related diseases has been associated with increased lifespan and health [
249,
250].
Understanding the mechanisms by which the immune response to various microorganisms and the substances they produce can help to discover how our immune system can detect commensal and pathogenic bacteria and develop new strategies to combat pathogens and allergic diseases. The numerous ways in which microorganisms affect the host require the consideration of many thousands of signaling pathways to identify associations with diseases. It is clear that maintaining health requires the participation of a large number of factors, provided by biodiversity. A thorough analysis of the influence of all possible bioregulators of bacterial origin, and their cross-interaction, taking into account specific genetic characteristics and environmental factors, will allow us to accurately assess the events taking place and develop personalized therapies and strategies for the prevention of allergic diseases. Our knowledge of the influence of microflora on the functioning of all systems and organs changes our food preferences and lifestyle.