**1. Introduction**

The gut has the most populous and diverse system of anaerobic and aerobic microorganisms in the human body [1–3]. It is composed mainly of bacteria. However, yeasts, archaea, or parasites living in the large area of the gastrointestinal tract often play a substantial role in this microenvironment [1,2,4,5]. The first years of life, including delivery, are crucial for the development of this complex system [6,7]. Especially at this time, selective pressure is induced by essential host and environmental factors such as breastfeeding or formula feeding, weaning age, diet, infections, and antibiotics [6,7].

The gut microbiota lives in homeostasis with its host. These interactions are regulated by an integral gut barrier and immune system [8,9]. The gastrointestinal tract communicates bidirectionally with the central nervous system via direct and indirect mechanisms [10]. This intricate interplay is called the gut–brain axis (GBA) [11] (Figure 1). Immunological, hormonal, and neural signals play vital roles in this interaction [10,12]. At the same time, the gastrointestinal response to central stimulation is influenced by microorganisms [11]. The microbiota participates in supplying the gut with necessary nutrients and maintaining its barrier integrity. Both terminals of the GBA use serotonin as a vital transmitter [13] Some behavioral changes regulated by serotoninergic transmission seem to depend on the microbiome [13]. Moreover, the GBA affects other systems [10,14–16].

**Citation:** Ustianowska, K.; Ustianowski, Ł.; Machaj, F.; Gor ˛acy, A.; Rosik, J.; Szostak, B.; Szostak, J.; Pawlik, A. The Role of the Human Microbiome in the Pathogenesis of Pain. *Int. J. Mol. Sci.* **2022**, *23*, 13267. https://doi.org/ 10.3390/ijms232113267

Academic Editor: Maria Teresa Mascellino

Received: 28 September 2022 Accepted: 25 October 2022 Published: 31 October 2022

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

**Figure 1.** Gut–brain axis with an interconnected net of dependencies. The cerebral function could be modified by gut microbiome and its influence on gut epithelium and immune response. This bidirectional axis uses cytokines and other soluble factors, but also neuronal communication. The short-chain fatty acids (SCFAs) produced by fiber-fermenting bacteria probably have immunomodulatory functions. By binding to G-protein coupled receptors (GPR41, GPR43, and GPR109A), SCFAs exert an anti-inflammatory response in the gut mucosa [17,18].

Disrupted homeostasis in the GBA was first associated with gastrointestinal symptoms and disorders such as inflammatory bowel disease (IBD) or irritable bowel syndrome (IBS) [19]. Moreover, alterations in the composition of the commensal bacterial species populating the gastrointestinal tract are risk factors for a variety of diseases, including cancer [10,14,20–23]. A plant diet has an opposite effect promoting colonization of the gut by protective bacteria and inducing the production of short-chain fatty acids (SCFAs) by species such as *Faecalibacterium prausnitzii* or *Roseburia intestinalis* [24,25].

Subsequently, studies connecting the microbiota with elements of pain pathogenesis were performed. SCFAs are microbial metabolites that affect T-regulatory cells controlling inflammation [26]. Microorganisms produce neurotransmitters that, together with ingested nutrients, stimulate enteroendocrine cells to produce multiple hormones [27,28]. There is growing evidence relating the microbiome to stress, anxiety, neurological diseases, and depression [29–31]. Brain functions affected by microorganisms might augment nociceptive transmission [32–35].

Initiation of pain transmission is induced by nociceptors, which convert noxious stimuli into nerve impulses [36,37]. Then, the signal is modulated by multiple neurons of different types and functions or non-neuronal cells such as glia [36–39]. Nevertheless, sustained pain depends on emotional or cognitive experience [36,40]. It is regulated peripherally and centrally by substances whose production is affected by the microbiome. Pain should serve as protection from tissue damage [37]. Nonetheless, chronic pain leads to a lower quality of life [32,41]. Thus, a better understanding of its mechanism is crucial

to improving the lives of millions of people worldwide. Moreover, targeting the gut microbiota seems to be a promising novel therapeutic approach for pain management.

As the aforementioned processes continue to receive increasing attention, we addressed the role of the gut microbiota in pain regulation and discussed the possibility of pain therapy by targeting the gut microbiota. In this narrative review, we collected results from in vitro and in vivo studies on the association between the GBA, pain, and its management.

## **2. Neuropathic Pain and Central Mechanisms of Pain Regulation**

Neuropathic pain occurs as a result of nerve-damaging trauma or somatosensory nervous system disease, including its central and peripheral components [42]. Various conditions, such as diabetes, alcoholism, hypothyroidism, or spinal stenosis, contribute to the development of neuropathic symptoms [42]. This type of pain manifests as abnormal sensations usually felt by patients for the first time. They perceive areas of skin with a sensory deficit, paraesthesia, either spontaneous or evoked pain and thermal or mechanical hypersensitivity [42]. Some drugs used in chemotherapy treatment, such as platinum, vincristine, or toxoids, may cause chemotherapy-induced peripheral neuropathy (CIPN) [43]. Over 30% of patients fighting cancer suffer from such severe CIPN-related pain that they are not receiving sufficient treatment dosages [44].

The gastrointestinal tract consists of various microorganisms, which are reported to play a significant role in neuroinflammatory responses. Neuroimmune activation is considered one of the primary mechanisms determining the central sensitization of chronic pain. It was shown in recent studies that the periphery, including gastrointestinal cells, might arouse brain cells [45]. The gut microbiota particularly regulates microglial function [46]. By affecting the activity of different cells, such as astrocytes, endothelial cells, microglia, monocytes, macrophages, pericytes and T-cells, the gut microbiota may regulate neuroinflammation (Figure 2). When those cells are activated, they start to produce multiple pro-inflammatory mediators such as C–C motif chemokine ligand 2 (CCL2 or MCP-1), CXCL-1, interleukin-1β (IL-1β), interferon-γ (IFN-γ), MMP-2/9, and tumor necrosis factorα (TNF-α) [12]. Cytokines and chemokines secreted by microglia or astrocytes influence synaptic neurotransmission by increasing glutamate and decreasing gamma-amino-butyric acid (GABA) levels, resulting in pain hypersensitivity [47,48]. Taking all the data under consideration, the gut microbiota can play a major role in central sensitization underlying chronic pain associated with neuroinflammation; hence, it may contribute to the development of diverse neurological diseases [49]. Ding et al., in their article, examined the influence of the gut microbiota on neuropathic pain in chronic-constriction injury of the sciatic nerve (CCI) and whether it is associated with T-cell immune responses. CCI is an animal model widely used to represent neuropathic pain. The study showed that the gut microbiota, via modulation of both pro- and anti-inflammatory T-cell responses, induces the development of neuropathic pain. Moreover, the gut microbiota also has an impact on nociceptive behavior in sciatic nerve CCI. The study found that changes in the gut microbiota caused by the administration of oral antibiotics reduced CCI neuropathic pain. It manifested as weakened mechanical allodynia and thermal hyperalgesia [50]. Another study reported that the gut microbiota might lead to peripheral nerve trauma-induced neuropathic pain. Yang et al. showed that rats with spared nerve injury (SNI) and gut microbial dysbiosis might be prone to neuropathic pain and depression-like phenotypes, including anhedonia [46]. By contrast, in the study by Huang et al. in rat models, no significant association between oral probiotics such as *L. reuteri* LR06 or *Bifidobacterium* BL5b and anti-nociceptive effects on CCI-induced neuropathic pain was demonstrated [51]. Recent studies showed that the gut microbiota is involved in the pathogenesis of CIPN pain and modifies the effects of chemotherapeutics on tumor growth [52,53]. Shen et al. found that the gut microbiota takes part in the evolution of mechanical hyperalgesia induced by chemotherapy. In their study, mice after antibiotic treatment and germ-free mice both experienced reduced mechanical hyperalgesia after oxaliplatin administration. Moreover, restoration of the germ-free mouse microbiota revoked the protective effect [54]. Another study reported that neuropathic pain induced by paclitaxel therapy might be relieved with a DSF probiotic (high concentration of *L. plantarum*, *S. thermophilus*, *B. breve*, *L. paracasei*, *L. delbrueckii*, *L. acidophilus*, *B. longum*, *B. infantis*). Castelli et al. implied that the use of a probiotic as an adjuvant during chemotherapy might be beneficial in counteracting pain associated with CINP [55].

**Figure 2.** The role of gut microbiota in neuroinflammation which contributes to central sensitization underlying chronic pain; IL-1β—interleukin-1β, IFN-γ—interferon-γ, TNF-α—tumor necrosis factorα, CCL2—C–C motif chemokine ligand 2, CXCL1—C-X-C motif chemokine 1 [12].

#### **3. Inflammation and Inflammatory Pain**
