The health of the brain is synchronized or regulated by the GI tract or it is directly proportional to the microbiome present in the human gut. Imbalance in the microbial community is associated with many diseases, but in the brain, it was responsible for neurodegenerative diseases such as AD, PD, and others disorders [23]. Brain and gut microbiota can interrelate with each other through several pathways such as neuroendocrine, neuroimmune, and autonomic nervous systems. Interactive partners that perform the mechanisms are the cell wall, neurotransmitters, vagus nerves, and metabolites [24]. The microbiome of the gut can synthesize the neurotransmitter that may help to maintain the homeostasis of the central nervous system, which can influence neurodegeneration, examples are tryptophan, brain-derived neurotrophic factor (BDNF), Gama-aminobutyric acid (GABA), and short-chain fatty acid (SCFA) [25]. Neurotrophins have a neuroprotective property that is important for the growth, development, and synaptic plasticity along with the differentiation and survival of the neuron. The decreased level of BDNF influences the neurodegenerative disease related to the cerebral cortex and directly relates to the gut-brain axis, which triggers other diseases too [26]. Gut microbes such as Faecalibacterium prausnitzil, Clostridium leptum and Eubacterium rectala, etc. produce short-chain fatty acids through a down regulation of pro-inflammatory cytokines that play a major role in neurodegeneration. Microbial-derived SCFAs are produced by bacterial fermentation and have neuro-active functions, they act as a modulator for serotonin (neurotransmitter) and some neuropeptides which help to facilitate the gut-brain axis at various stages. During the release mechanism which influences neuronal health and behavioral response [27]. Excessive release of SCFAs is responsible for neuronal health and behavioral responses. An essential amino acid called tryptophan plays an important role in the synthesis of serotonin and other neurotransmitters in the CNS. Imbalance in their levels leads to brain and gastrointestinal disturbances that may cause neurodegeneration, cognitive impairment, and mood disorders [28,29]. An important inducer that administered the excitation of the neurons, is a by-product of bacterial metabolism called GABA. Deregulation of GABA leads to various pathological imbalances that play a major role in neuro-cytotoxicity which accelerates several chronic neurological disorders. GABA is an example that proves how gut microbiota regulates brain chemistry [30].

People with gut disorders more prone to have AD in the future. Changes in complex ecosystems are co-related with many gastrointestinal disturbances that can implicate many inflammatory diseases including obesity, diabetes and inflammatory bowel disease. The dysbiosis of gut microbiota has an impact on the synthesis of proteins and metabolic processes which are related to the progression of the disease such as AD. Aging changes the gut microbial concentration which enhances pro-inflammatory bacterial growth more than anti-inflammatory bacteria that deteriorates the permeability of the blood-brain barrier and GIT (Gastro Intestinal Tract) functions [31]. Pro-inflammatory phylum such as Proteobacteria, Verrucomicrobia, genera such as Escherichia/Shigella, and species such as Pseudomonas aeruginosa, anti-inflammatory species are Clostridium spp., and Ruminococcaceae. Some study reveals that increased mRNA encoding initiates the release of pro-inflammatory cytokines such as, IL6, CXCL2, and NLRP3, Escherichia/Shigella are related to pro-inflammatory taxon [28]. The presence of Helicobacter pylori in the gut microbiota increases the release of inflammatory mediators which increases the amyloid β 40/42 ratio in the serum, other bacteria such as Borrelia burgdorferi and Chlamydia pneumoniae also participate in the hyper phosphorylation of tau which is an important hallmark of AD. IL-10 is an anti-inflammatory cytokine Eubacterium rectale is associated with an anti-inflammatory taxon [32].

PD is a multifocal neuronal disease that is distinguished by tremors, slow movement, akinesia, muscular rigidity, gait, and difficulty in walking. Instead of these symptoms, PD patients suffer from constipation which is one of the causes of increased intestinal permeability and inflammation that is directly related to the microbiota community of the small intestine [33]. Small intestinal bacterial outgrowth and helicobacter pylori infection has seen in diseased person that causes motor impairment and problem-related with stool [34]. In most cases, patients suffer from increased mucosal permeability and endo-toxic exposure caused by Coliform bacteria [35]. Comparably, some bacteria are reduced in feces such as Roseburia intestinalis, Roseburia hominis, Coprococcus eutactus, and Blautia faecis dysregulate the biosynthesis of lipopolysaccharides and are also responsible for GABA deregulation [36]. Escherichia coli is a Gram-negative bacteria that releases amyloidogenic protein which induces alpha-synuclein aggregation and which regulates disease in the gut and neurodegeneration in the brain [37].

HD is a genetic disease caused by overexpression of the huntingtin coding gene, new research suggests that an imbalance in gut microbiota dysregulates the cytokine levels and excessive production of hydrogen sulfide that negatively affects gut health [38,39]. Imbalance is seen in two microbial communities such as increased the majority of the bacterial phyla Bacteroidetes (4%) and Firmicutes (83%) which causes mortality and motor ability-related problems in HD conditions. Research suggests that irregular intestinal biomicrome decreases mucosal thickness and decreased neuropeptide formation with abnormal endocrine hormonal conditions [40,41,42]. ATP levels are associated with Prevotella scopos which has negative effects and also has a co-relation with decreased butyrate formation affected by Blautia producta [43].

In neurological disorders, the nerve function and structure become affected badly, causes of neuronal cell death, are Amyotrophic lateral sclerosis, Friedreich ataxia, Lewy body disease, spinal muscular atrophy, and Epilepsy, etc. Epilepsy, a neuro-psychiatric disorder is a result of environmental and genetic imbalance, several studies implicated dysbiosis of gut microbiota correlated with the disease, and imbalance of microbiome increases the pro, and anti-inflammatory effects, which leads to chronic inflammation and progression of the disease [44,45]. Bacteria out-growth downregulates lipid and glucose metabolism which disturbs the ATP binding cassette and transporter-associated pathways [46]. Amyotrophic lateral sclerosis is a degenerative condition that is caused by the mutation in dozens of genes which produces a misfolded protein that is found in motor neurons, responsible for voluntary muscular movement [47]. Bulbar function slowly deteriorated with the progression of the disease, and dysphagia (because of aspiration pneumonia and weight loss) have seen [48]. ALS is implicated by the deregulation of the resident and peripheral immune system. Gut microbiota connected with the intestinal immune system, because of invasion or dysbiotic leaky gut and disturbed molecular patterns, provoke cells to release pro-inflammatory cytokines that deregulate the Firmicutes/Bacteroidetes ratio [49]. Bacteroidetes are good gut bacteria, that are decreasing at the diseased condition that imbalances the cell homeostasis, is regarded as dysbiosis and several reviews suggest that the gut plays an impotent role in the progression of Lewy body disease, and intestinal pro-inflammation is the cause of the chronic phase of the disease. An imbalance in microbiota and translocation of the lumen through a leaky gut are key mechanisms of the disease [50].

2. Recent Evidence for Probiotics and Neurodegenerative Disorders

2.1. Invertebrate System Studies

2.2. Vertebrate System Studies

Probiotics, which have been shown to have health benefits when consumed, have drawn a lot of interest in recent years because of their ability to treat and avoid a variety of diseases. While the exact mechanism of action of probiotics is not yet fully understood, growing evidence suggests that they act through various pathways to regulate immune function, improve gut barrier function, modulate the gut-brain axis and neurological complications [134].

Based on the results of both the animal and human studies, the consumption of probiotics has a significant beneficial effect on AD Table 1. We can conclude that most of the study is based on Bifidobacterium and Lactobacillus and as an outcome this study reveals that probiotics can improve memory dysfunction and cognitive dysfunction in similar to neurodegenerative diseases [87,98]. With respect to AD, the CNS’s inflammatory reaction to damage or infection is called neuroinflammation, which is accompanied by an accumulation of glial cells. Activated microglia and astrocytes generate pro-inflammatory cytokines like IL6, IL8, and IL10, and these cytokines directly cause neuronal injury [135]. According to studies, probiotics can restore chronic inflammation, the function of clearing abnormal proteins, and synaptic dysfunction. Neurodegeneration and brain loss are caused by all of these occurrences. To understand and solve the puzzle of how probiotics were beneficial in AD, there have primarily been four clinical trials, which are mentioned in Table 2, below. Regarding PD, a new clinical trial of probiotic capsules (containing Lactobacillus acidophilus, Bifidobacterium bifidum, Lactobacillus reuteri, and Lactobacillus fermentum) demonstrates the same effects as the MDS-UPDRS [136]. Another 2019 research demonstrates that the major pro- and anti-inflammatory cytokines, as well as ROS, are produced by Lactobacillus and Bifidobacterium genus when peripheral blood mononuclear cells (PBMCs) isolated from people with Parkinson’s disease (PD) are compared to healthy participants [137]. In relation to other ND Lacticaseibacillus rhamnosus HA-114 can improve the energy balance and cholesterol homeostasis in ND animals [131]. In relation to neurodegenerative disease, only 10% of the research concentrates on Streptococcus and Clostridium species. So on, we give a summary of the literature in this systematic review by using data that was extracted from different sources.