Rebuilding Microbiome for Mitigating Traumatic Brain Injury: Importance of Restructuring the Gut-Microbiome-Brain Axis

Abstract

Traumatic brain injury (TBI) is a damage to the brain from an external force that results in temporary or permanent impairment in brain functions. Unfortunately, not many treatment options are available to TBI patients. Therefore, knowledge of the complex interplay between gut microbiome (GM) and brain health may shed novel insights as it is a rapidly expanding field of research around the world. Recent studies show that GM plays important roles in shaping neurogenerative processes such as blood-brain-barrier (BBB), myelination, neurogenesis, and microglial maturation. In addition, GM is also known to modulate many aspects of neurological behavior and cognition; however, not much is known about the role of GM in brain injuries. Since GM has been shown to improve cellular and molecular functions via mitigating TBI-induced pathologies such as BBB permeability, neuroinflammation, astroglia activation, and mitochondrial dysfunction, herein we discuss how a dysbiotic gut environment, which in fact, contributes to central nervous system (CNS) disorders during brain injury and how to potentially ward off these harmful effects. We further opine that a better understanding of GM-brain (GMB) axis could help assist in designing better treatment and management strategies in future for the patients who are faced with limited options.

Introduction

Traumatic brain injury (TBI) occurs due to an external force causing skull damage which could invariably affect the brain [1]. The trauma leading to brain injury can be broadly categorized as an impact or a non-impact event depending upon whether the external object had a direct contact with the head (impact) or was it a non-impact force like the blast waves or a rapid acceleration, and deceleration (non-impact) with the head [2]. In the USA, frequency of TBI occurs every 15 s (roughly about 1.7 million new TBI cases/year) and costs more than US $77 billion/year [3]. In brief, TBI events are responsible for 50,000 deaths together with 80,000 individuals that are left with permanent disabilities each year [4–7]. It is believed that the frequency of brain injury is estimated to be higher than any other type of diseases such as Parkinson’s disease, multiple sclerosis, AIDS, and breast carcinoma [3]. For example, motor-vehicle or traffic-related accidents constitute 17% cases while walking-falls are responsible for 35% of cases in USA [4–7]. As per one estimate 130,000 children in the age between 5 and 18 years suffered from sport-related concussions [8]. Besides, blast injury was the most common cause of TBI-related event that was observed among the military personnel [9]. In recent years, several experimental animal models have been developed to replicate human TBI pathophysiological aspects employing the pre-clinical settings [10] including fluid percussion, weight-drop injury, and controlled cortical impact (CCI). These animal models are routinely used in simulating TBI-related events in small animals with characteristics of mild or severe TBI. In fact, these models remain the workhorses for studying characteristic features of the primary, as well as secondary brain injuries in humans [11].

An acquired insult during TBI could potentially change various structural components of the brain resulting in temporary or even permanent brain impairment [12, 13]. Interestingly, GM and its role(s) in various system disorders has recently been the major focus area of research worldwide. For example, previous work reveals that GM plays important roles in neurogenerative processes such as formation of BBB, myelination, neurogenesis, and microglial maturation [14]. It has been shown that microbiome also modulates many aspects of our behavior since GM is involved in the modulation of cellular and molecular processes by balancing microbial eubiosis and dysbiosis condition and also involved in the progression of TBI-induced pathologies including BBB permeability, immune response to neuroinflammation, astroglial activation, and mitochondrial dysfunction (Fig. 1) [14]. Currently, efficacious treatments for TBI patients are acutely lacking [15–17]. Additionally, gut dysbiosis is known to exacerbate behavioral impairment as shown in studies that employed animal models of TBI and the spinal cord injury [18–21]. Furthermore, the dysbiotic milieu negatively affects the post stroke recovery [22–25]. Treatments for TBI and related disorders are severely limited, but recent research shows that microbiome transplants could mitigate CNS damage and functional impairments in spinal cord injury and stroke in animals [18]. In addition, probiotics were shown to reduce the rate of infections and time spent in intensive care units of hospitalized patients suffering from the brain trauma [18, 26, 27]. Thus, establishment of a protective, that is, eubiotic GM, is a promising therapeutic avenue since the brain injuries induce dysbiosis (Fig. 2). Reiner and colleagues 2014 reported that Novel CB2 Inverse Agonist SMM-189 reduce motor, visual, and emotional deficits after closed-head mild traumatic brain injury mouse model via mitigation of microglial inflammatory action [28]. ER stress was found to be increased early in juvenile rats exposed to TBI and that these rats developed tau oligomers over the course of 30 days and had significant short-term and spatial memory deficits following injury [29]. Treangen and colleagues suggested that acute bacterial dysbiosis within the gut microbiome was observed after TBI post-injury in mice [30]. The overall researched layout is represented in Fig. 2. However, post-TBI associated ocular and brain dysfunction via direct regulation of altered gut microbiome homeostasis is still needed to be demonstrated. This review discusses how GM alterations during post TBI contribute to CNS dysfunction and how to potentially target GM for therapeutic benefits in patients.

Fig. 1.

TBI induced dysbiosis via the gut microbiome brain (GMB) axis. The GMB-axis could potentially contribute, and further worsen the injury profile by promoting dysbiosis over eubiosis wherein harmful microbes in the gut can lead to an increase in neuroinflammation, mitochondrial dysfunction, oxidative stress, microglial activation, behavioral, and cognitive impairment, and intestinal wall permeability.

Fig. 2.

Resolution of the gut dysbiotic environment. Treatment with probiotics may help break the vicious dysbiotic cycle thus reducing the impact of brain injury, and hence improve substantially the TBI-related biochemical, pathological, and behavioral markers.

An Overview of Traumatic Brain Injury (TBI)

TBI is defined as an alteration in brain functions that is provoked often by an external force. Unfortunately, it remains the main cause of injury-related death, disability, and mental disorders, thus representing a major public health issue globally [31–34]. In the year 2013 alone, a total of approximately 2.8 million TBI-related emergency department visits, hospitalizations, and deaths were registered in the USA [35]. Unfortunately, it is also one of the most prevalent injury types in many industrialized countries [36]. Although there has been an overall decline in TBI mortality because of an improved treatment modality over the years; however, there is substantial increase in the number of individuals living with disabilities as a direct result of TBI [37–41]. At cellular levels, TBI pathophysiology is characterized by acute necrotic or delayed apoptotic neuronal death, cytokine and chemokine production, infiltration of peripheral immune cells, and activation of astrocytes and microglia that lead to a variety of brain, and visual problems [42–45]. The survivors often suffer from debilitating secondary injury conditions including cognitive deficits pertaining to their memory, attention, executive function, speed of information processing, and personality changes that are best characterized as dysexecutive syndromes involving social comportment, cognition, and motivated behavior and increased relative rates of psychiatric disorders, particularly depression and anxiety along with ocular dysfunction [15–17, 46, 47]. Despite past research accomplishments in TBI pathophysiology, much remains to be discovered regarding mechanistic understanding into the heterogeneous nature of brain injury-related neuropathologies, behavioral, and cognitive impairments. Again, as discussed earlier efficacious therapeutics for TBI-induced maladies are lacking [48–50]. Therefore, it is vital to consider novel treatment strategies to combat TBI-related disabilities, and in this context manipulation of GM by gut eubiotic therapeutic modalities could serve as means of captivating this expanding avenue, against the TBI-induced medical condition [51] (Figures 1 and 2).

Homocysteine, its Metabolism, and TBI Complications

Homocysteine (Hcy) is a sulfur containing non-proteinogenic amino acid derived from the essential amino acid methionine (Met) (Fig. 3). The catabolism of Met can be disrupted by factors like lifestyle, stress, aging, or genetic abnormalities leading to hyperhomocysteinemia (HHcy). Dietary Met is first converted to S-adenosyl methionine (SAM), which is changed to S-adenosyl Hcy (SAH). Then SAH leads to the production of Hcy, which is further re-methylated back to Met wherein vitamin B12 serves as a co-factor, and the cycle continues [52]. Under the condition of either low cysteine or saturation of Hcy back to re-methylation, then Hcy can be further catabolized via the trans-sulfuration pathway into cysteine. In this rate-limiting step, Hcy is first converted to cystathionine with the help of cystathionine β-synthase (CBS) where vitamin B6 is an essential co-factor. Subsequently, cystathionine is converted to cysteine by cystathionine γ-lyase (CSE). Cysteine is then broken down to taurine, glutathione, and hydrogen sulfide (H₂S) (Ref-9 (Fig. 3). In its excess amount when it starts accumulating, Hcy is highly neurotoxic to astrocytes and neurons [53, 54]. Normally, it is continuously eliminated either via its re-methylation back to Met or via trans-sulfuration to cystathionine production. However, Hcy can accumulate under certain circumstances such as during folate deficiency, aging, occurrence of a mutation in methylenetetrahydrofolate reductase (MTHFR) gene, or under physical or emotional stress conditions [55–60]. When Hcy builds up (also known as HHcy), it invariably leads to BBB dysfunction and microvascular disorders [61]. It also increases NAD(P)H oxidase activity which in turn triggers microglia activation thus stimulating the secretion of pro-inflammatory molecules [61–64].

Fig. 3.

Hcy metabolic pathways. Hcy resides at the intersections of re-methylation and trans-sulfuration pathways. In the re-methylation pathway, tetrahydrofolate (THF) is converted to methyl (CH₃) THF by methylenetetrahydrofolate reductase (MTHFR). The methyl group is donated to Hcy, and in the presence of methionine synthase (MS), and B12, it is converted to Met. Met is used further in many methyl transfer reactions. When the diet is replete with Met, Hcy is converted via trans-sulfuration pathway to cystathionine by cystathionine β synthase (CBS), and then again to cysteine via the action of cystathionase (CSE) in the presence of B6. Finally, cysteine is converted to several beneficial downstream products such as glutathione (GSH), and taurine which are essential for brain, and retinal functions. Abbreviations: SAHH; S-adenosyl homocysteine hydrolase, SAH; S-adenosyl homocysteine, R; represents a carbon-based group, DNA; deoxyribonucleic acid, RNA; deoxyribonucleic acid.

A recent study demonstrated that Met-treated HHcy in a TBI mouse model exhibited increased oxidative stress and BBB dysfunction. The phenotype promoted infiltration of inflammatory cells into the cortex as also emphasized here in Figs. 1 and 2 [63–65]. The study also suggested that HHcy was implicated in visual dysfunction. The previous study has clearly indicated that HHcy is a potent risk factor for retinal arteriosclerosis [51, 66], exudative age-related macular degeneration (AMD) [67], and macular and optic atrophy due to retinal vascular occlusion or non-arteritic ischemic optic neuropathy [68] and glaucoma [69]. In fact, cross-sectional studies have demonstrated that there is a strong association between HHcy and exudative neovascular AMD [70]. Thus, HHcy appears to exacerbate TBI outcome suggesting that Hcy dysregulation may be a significant biological variable that could contribute to TBI pathophysiology and ocular disease conditions (Fig. 3). In addition, Zinno and colleagues suggested that dietary supplementation of dairy matrices containing natural folic acid could mitigate the plasma homocysteine (Hcy) and SAM levels and found to restore the fecal microbiota composition in the hyperhomocysteinemic (HHcy) mouse model (Fig. 3) [71]. However, the direct connection between HHcy, associated post-TBI outcomes such as neuroinflammation, BBB dysfunction, and cognitive deficits via modulation of gut microbiota homeostasis needs to be explored. Therefore, further research is warranted to understand the mechanism(s) of HHcy-associated multifactorial post-TBI linked outcomes.

Gut Microbiome Brain (GMB) Axis

The human intestine consists of more than 1000 types of microbes from at least 4000 different species [72–75]. Hence, alterations in their relative composition have been known to play definitive roles in the pathogenesis of many system disorders like diabetes, obesity, inflammatory bowel disease, Crohn’s disease, Alzheimer’s disease, anxiety, and depression [76–83]. In recent years, there has been an increasing interest in studying interactions between the brain, gastrointestinal (GI) tract, and its microbiome and the bidirectional relationship between these systems [84] (Fig. 1). A deeper insight into the brain-gut crosstalk revealed the existence of a complex communication channel that not only ensures proper wellbeing of the gastrointestinal homeostasis but also likely to have multiple effects on the overall brain functioning such as higher cognitive function and motivation. The complexity of these interactions between the brain and gut is presumably through “gut-brain axis” ([85]. By realizing the importance of microbiome in modulating health, the gut-brain axis has been renamed as the GMB axis, which represents a complex network of communication between the gut, intestinal microbiome, and brain that seem to modulate immune system, gastro-intestinal tract, behavior, stress response, and CNS functions [15–17, 85–94] (Fig. 2). This bidirectional communication includes the enteric nervous system, autonomic nervous system, central nervous system, and the hypothalamic-pituitary-adrenal axis; however, the hypothalamic pituitary adrenal axis is considered as the core stress efferent axis [85, 95–97]. It was previously shown that GM regulates intestinal function and health but later accumulating evidence indicated that it could also influence the immune and nervous systems and vice versa [98].

Elderly individuals with lower diversity of microorganisms have strong connection with various system disorders [99]. Although quite a literature has already been published, it is still an evolving area of great significance where important links between diet, microbes, and cognition should be emphasized in detail as we make further progress. The influence of the microbiome on obesity and metabolic syndrome is also being increasingly recognized. It has been proven in the meantime that gut and brain do communicate with each other via several routes including the vagus nerve, immune system, enteric nervous system, and or by way of various microbial metabolic processes [100–102]. These microbiome-dependent processes have also been shown to include myelination, adult hippocampal neurogenesis, and microglia activation [103–105]. Reports show that animals featuring normal intestinal microbiome that are subject to no external influences develop inflammation in their brain [101, 102]. There are, in fact, already reports of the link between an intestinal dysbiosis and the non-infectious uveitis. Furthermore, the microbiome alteration with either germ-free rearing or treatment with oral metronidazole and ciprofloxacin resulted in reduced uveitis [106–110]. Another study showed that fecal microbial transplantation from Behcet’s disease patients into an autoimmune uveitis-prone mouse model worsened intraocular inflammation [111, 112].

Interestingly, obesity has been called a psychiatric disease and is highly linked with depression and other neuropsychiatric disorders [113–115]. Similarly, schizophrenia has been linked to intestinal inflammation and gastro-jejunal ulcers [115, 116]. Furthermore, deregulation in the GM that is associated with age-related decline in sensory, motor, and higher cognitive functions leads to age-related neurodegenerative disorders [117–121]. In recent years, researchers have proposed a potential role for pathogenic microbes, including those derived from gut in the development or exacerbation of Alzheimer’s disease [122–125]. There are many studies depicting alterations in the GM that are associated with neurological disorders, multiple sclerosis, and the Parkinson’s disease [126–133]. Similarly, unhealthy microbiome has been associated with the disruption of the ocular tissues, exacerbation of diabetic retinopathy, age-related macular degeneration, choroidal neovascularization, uveitis, glaucoma, and Sjogren’s syndrome [134, 135]. On the other hand, Treangen and colleagues in 2018 suggested the overall impact of TBI on bacterial dysbiosis, and they went on to show that microbial changes occur 24 h after TBI in mice, indicating that CCI causes a rapid shift in relative abundance of many species including Lactobacillus gasseri, Ruminococcus flavefaciens, and Eubacterium ventriosum that are commonly seen in the human GM [30]. These results suggested that probiotics administration could be a therapeutic strategy for individuals with post-traumatic stress disorders such as TBI.

GMB-Axis in CNS Injuries

TBI-related pathophysiological effects have been increasingly studied that are directly associated with intestinal dysfunction and these effects represent an important consequence to the host because the GMB-axis supposedly ensures major bidirectional communication pathway between the brain and gastrointestinal tract incorporating both afferent and efferent signals which involve neuronal, hormonal, and immunologic cross-talk. Such bidirectional interactions can result in sequelae such as chronic fatigue of the gastrointestinal system including its disability to function properly [136–139] [140, 141] (Fig. 1). In the mouse model of brain, and spinal cord injury, CNS injury does upset the intestinal wall motility, and its permeability along with changes in the GM composition which ultimately led to gut dysbiosis [136, 137, 142–144].

Conversely, it is also reported that gut dysbiosis influences the traumatic CNS injury and its pathophysiology [145, 146]. In a traumatic spinal injury model, a study showed that gut microbiota composition was altered wherein there was a decrease in the population of Bacteroidetes but an increase in Firmicutes amount, and it was associated with an impairment in locomotor function [147]. Further, in a CCI rodent model of moderate TBI, gut microbiota composition (decrease in Firmicutes and increase in Bacteroidetes and Proteobacteria) was altered following 2 h of injury along with increased post-injury lesion volume [148]. On the other hand, the study explains that gut eubiosis controlled reversible DNA methylation through activating DNA methyltransferase (DNMT) and phosphatidylethanolamine methyltransferase (PEMT) and that allows normal gene regulation. But the irreversible DNA methylation causes HHcy during dysbiosis condition and disrupts the normal gene regulation which could be responsible for cardiovascular metabolic syndrome (141) (Fig. 4). However, understanding the relationship between TBI-induced gut dysbiosis and altered gene regulation needs to be studied. Howard and colleagues suggested that severely injured patients with polytrauma established the relation between gut dysbiosis and post-injury in human patients. They have shown that 72 h of post-injury was associated with a decrease in Bacteroidales, Fusobacteriales, and Verrucomicrobiales but an increase in Clostridiales and Enterococcus populations [149]. These findings provide the complex relation between and multiple disease outcomes, revealing the notion that bacterial populations indeed influence the post-injury mediated neuro-pathophysiology and functional impairment via dysbiosis-dependent mechanisms [150–156].

Fig. 4.

Eubiosis versus dysbiosis: Epigenetic mechanisms favor reversible and irreversible DNA methylation pattern in a given cell. During eubiosis condition, DNMT- and –mediated reversible DNA methylation allows normal gene regulation. However, irreversible DNA methylation causes HHcy due to dysbiotic condition and disrupts normal gene expression. Abbreviations: PE phosphatidylethanolamine, PC phosphatidylcholine, PEMT phosphatidylethanolamine methyltransferase, SAM S-adenosyl methionine, SAH S-adenosyl homocysteine, SAHH S-adenosyl homocysteine hydrolase, mDNA methylated DNA, and DNMT DNA methyltransferase

Interplay of Transforming Growth Factor/Bone Morphogenetic Protein Signaling During TBI and Its Possible Link with the GM

Transforming growth factor (TGF) superfamily members are altered following TBI in the rodent species [157–163]. It has been reported that an increased TGF-β1 and TGF-β2 protein levels were described in human spinal cord injuries and BMP7 mRNA and protein levels in rat spinal cord injury [164–166]. Despite accumulated evidence, the precise role of this signaling system, and the interplay between the TGFβ, and BMP branches in the context of TBI pathophysiology are still not fully understood.

An early expression of BMP-6 in neurons of the hippocampus and cortex in normal adult rat brains that were subjected to TBI was demonstrated which was followed by pronounced expression in astroglia located to the lesion became obvious 48 h post-injury. The glia cells were found to be distributed around lesion, thus demarcating the injured tissue from the normal brain area. Further, double labeling by immunohistochemistry revealed that the major glial sources for BMP-6 were reactive astrocytes together with a few ED1(+) or W3/13(+) cells that also co-expressed BMP-6 protein. Subsequently, it was noticed that BMP-6 expression in neurons located to hippocampus and cortex of the lesioned hemisphere was upregulated 3 days post-injury suggesting that BMP-6 might be involved in astrogliosis following TBI [167]. Similarly, it was shown by others the distribution of TGF-β1 and BMP-6 in the brain of rats subjected to a mild and reversible ischemic damage produced by a 20-min occlusion of both carotid arteries without occlusion of the vertebral arteries. The researchers selected this model to study how the expression of trophic factor of the TGF-β superfamily changes in neurons that recover from a transient insult. Immunocytochemical analysis showed a loss of TGF-β1 in neurons of all hippocampal subfields immediately after the ischemic period, followed by a recovery of immunoreactivity in CA1 and CA3 neurons after reperfusion. BMP-6 immunoreactivity was also lost in most of the hippocampal neurons, but immunostaining became particularly intense in the interstitial space after both ischemia and reperfusion. An interstitial localization of BMP-6 was also observed in the cerebral cortex, particularly after reperfusion. Interestingly, mild ischemia also induced substantial changes in the expression of TGF-β1 and BMP-6 within the cerebellar cortex. In control animals, these factors appeared to be localized in granule cells and Purkinje cells, whereas the molecular layer was not immune-positive. Both TGF-β1 and BMP-6 were highly expressed in the interstitial spaces of the cerebellar cortex either 20 min after ischemia or 20 min after reperfusion. Taken together, these results suggest that a mild and reversible ischemia stimulates the release of BMP-6 from neurons into the interstitial space [168].

Disruption of TGF-β/BMP signaling cascade alters GM size, and its composition via ligand binding, and activation of heterodimer receptors, downstream Sma and Mothers against decapentaplegic (SMAD) homologs transcriptional regulators and co-activators [169]. The above signaling regulates intestinal immunity and also manages control of the intestinal bacterial proliferation that is important for the regulation of lifespan in Caenorhabditis elegans since it is involved in the shaping of the GM [169–182]. In corroboration with previous findings, in the present review, we thus hypothesize that targeting TGF-β/BMP signaling cascade could prove as a milestone for effective future treatment strategy in the mouse model of TBI. In short, we believe that through modulation of GM via probiotics administration in the injured mouse model could counteract the downstream cellular and molecular events (neuronal death, inflammation, and BBB damage) of TGF-β/BMP signaling, and thereby it might be able to restore the brain and ocular damaging events (Fig. 5).

Fig. 5.

Diagram illustrating TGF-β/BMP-6-mediated effects during TBI. Activation of the NF-kB results in neuronal cell death, inflammation, BBB break-down followed by vascular dysfunction. Many of these deleterious effects could be mitigated by timely probiotics treatment. Abbreviations: TBI

Gut Microbiome (GM) as Potential Diagnostic and Therapeutic Target for TBI

Understanding the gut microbiota composition and its alteration during dysbiosis may help in designing therapeutics having more efficacy and the relevant diagnostic tools for treating post-TBI complications and outcomes in affected TBI patients. Houlden and colleagues in 2016 found a positive correlation between the degree of gut dysbiosis and post-TBI injury in the “closed-head-impact” pre-clinical mouse model. In this regard, manipulation of the GM via microbiota transplants and pre- or probiotics consumption could, in fact, provide an exciting treatment against brain injuries (Fig. 2). As previous studies suggested that GM was substantially changed during the 24–72 h window period post-TBI injury; therefore, eubiotic therapeutic strategies could fundamentally shift the altered GM to a beneficial one and thus mitigate post TBI-associated secondary injury and the exaggerated damage response. [147–149].

As mentioned above, pre-clinical studies support the concept that microbiota transplants could be a valuable therapeutic tool to reduce brain lesion size and can help improve health outcomes in animal models such as shown in a mouse model of ischemic stroke to restore the microglial function [183, 184]. Probiotic and the microbial-derived metabolites and short-chain fatty acid (SCFA) products like butyrate, propionate, and acetate may also help modulate mitochondrial homeostasis, and energy production [185]. Interestingly, along with dietary ketones, these metabolites work as the alternative energy sources for the injured brain towards improving mitochondrial bioenergetics in post-TBI and SCI injury models [186]. The gut microbiota-derived SCFA, especially the butyrate, acts as a potential histone deacetylation (HDAC) inhibitor, thereby offering a robust neuroprotection following post-TBI insults [187]. For example, probiotic Clostridium butyricum-derived butyric acid in fact did improve neurological deficits and attenuated neurodegeneration, BBB impairment, and reduced brain edema through the GMB axis in mouse model of weight-drop impact head injury and cerebral ischemia [188]. Also, VSL#3, is the mixture of eight friendly bacterial strains (4 strains of Lactobacillus viz. Lactobacillus acidophilus, Lactobacillus plantarum, Lactobacillus casei, and Lactobacillus delbrueckii subspecies bulgaricus, 3 strains of Bifidobacterium viz. Bifidobacterium breve, Bifidobacterium longum, and Bifidobacterium infantis, and 1 strain of Streptococcus viz. Streptococcus salivarius subspecies thermophilus. VSL#3 is considered as an advanced probiotic medical food [189]. It mainly improves gut microbial eubiosis and reduces intestinal barrier function, through improving tight junction protein (TJP) expression, and modulation of anti-inflammatory cytokine expression that is required for the physiological function of the host [190]. When VSL#3 was provided to the spinal cord injury, mouse model on the same day of injury was shown to improve the post-injury neuropathology. Furthermore, this probiotic improved the locomotor recovery and triggered an increase in the number of Treg cells, thus offering a protective immune environment [19].

In human preclinical trials, post-TBI injury patients when supplemented with Lactobacilli-rich probiotics within the first 48 h of hospital admission, and it was demonstrated that patients had a substantial decrease in their gastrointestinal dysfunction along with less incidence of ventilator-associated pneumonia, thus reducing the nosocomial infection rate [27, 191]. Till date we have not come across any study that has been shown to provide positive effects in cognition and behavioral outcomes following post-TBI injury events. Therefore, future study is warranted to understand the beneficial effects of probiotic supplementation for patients with TBI injury that could help reduce the mortality rate also.

Future Perspectives and Conclusion

The eubiotic gut is closely linked with good human health, and an imbalance of the GM either because of HHcy-led dysfunction of the 1-carbon metabolism or any other reason is associated with various system disorders (Figs. 1 and 2) [150, 153–156, 192–194]. Because of the importance of bi-directional relationship between the GM and TBI-associated pathophysiology, management of gut dysbiosis could serve as paradigm for a likely therapeutic target (Figs. 1 and 2). Probiotics consisting of butyrate-producing gut bacteria appear to be the most beneficial as a mode of eubiotic therapy that may enhance benefits of GMB axis and its health-promoting functions through anti-inflammatory and the positive mitochondrial energetic properties (Figs. 3 and 4). Advances in the next-generation sequencing, and bioinformatics tools have revealed an expansive and diverse microbial community that certainly offers a promising avenue for developing new class of therapeutics for a host of medical conditions, including TBI.

It is becoming clear that perturbations in the GM can contribute to neuro-physiological disorders. Thus, a further understanding of the role of GMB axis and its influence on brain and ocular function and its links with neurologic and neurodegenerative disorders will provide not only better treatment options but also superior managemental strategies in the coming future (Figure 5). Coordinated research efforts to understand the mechanism(s) involved in the dysbiosis could help us on how the connectivity between CNS injury, and microbiota regulates the diseases and their outcomes. Insights into these mechanisms could provide further options for early intervention for TBI in patients of all ages. Identification of therapeutic eubiotic microbes and their potential metabolites would offer promise for devising the effective treatment modalities for the patients. Furthermore, pathogenic mutations that cause genetically governed HHcy-related 1 carbon metabolic disorders and the attendant TBI severity and their relationships with intestinal dysbiotic flora remain to be investigated and thus warrant continued investigation.

Authors’ contributions

MS conceived the idea of collecting, assembling, and reviewing the literature for this manuscript before finalizing its submission. AKG helped putting the draft of the manuscript along with figure generation. JB, NT, and SCT assisted in developing the framework for the work, while RPH was responsible for language interpretation and sequence arrangement along with reference management for the manuscript. All authors read and approved the final version of this manuscript.

Funding

A part of this work was supported by NIH grants AR-71789, HL139047, and DK116591.

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The authors declare no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Further, the authors received no financial support for the research, authorship, and/or publication of this article.

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