The gastrointestinal tract, in addition to being the largest endocrine organ, is a place of communication between the highest concentration of immune cells in the body, a network of 200-600 million neurons and the trillions of bacteria, fungi and viruses. The gut bacteria, composed of thousands of different microbial species and more than 15,000 kinds of bacteria for a weight equal to 1 kg, represents the first protection system of the gastrointestinal (GI) apparatus. The functions performed by the flora are many; in addition to the contribution to the establishment of the intestinal barrier, it promotes its maintenance, stimulating epithelial regeneration through the production of short chain fatty acids (SCFAs), leading to mucous membrane formation.
Recent data show the strong link between gut flora dysfunction and conditions such as obesity, allergies, autoimmune disorders, irritable bowel syndrome, inflammatory bowel disease, and psychiatric disorders. Bacteria contributes to alteration of intestinal permeability thus leading to the production and spread into the bloodstream of a potent pro-inflammatory endotoxine lipopolysaccharide (LPS). This small molecule has an important influence in the modulation of the central nervous system (CNS), increasing the activity of areas responsible for the emotionalism control such as amygdala. LPS serum levels are significantly higher in autistic patients and patients suffering from depression compared to healthy individuals. It also lead to production of inflammatory cytokines that alter the physiological brain activity. Escherichia coli
Source: CDC/National Escherichia, Shigella, Vibrio Reference Unit at CDC
Accumulating evidence suggests that the gut microbiota may also regulate brain function and behavior. Results from animal models indicate that disturbances in the composition and functionality of some microbiota members are associated with neurophysiological disorders, strengthening the idea of a microbiota–gut–brain axis and the role of microbiota as a “peacekeeper” in the brain health. Alteration of gut flora microbiota leads to the colonization by bacteria able to produce neurotoxins, contributing, at least in part, to autistic symptoms. Patients affected by depression disorder and chronic fatigue syndrome have increased numbers of Alistipes and Faecalibacterium bacteria as part of their gut flora.
Currently, researches are being made in order to clarify the correction of gut microbiota in the management of psychiatric disorder. Some studies have successfully tested minocycline, second-generation tetracycline, as a treatment for depression, on the basis of its neuroprotective activities and regulation of pro-inflammatory agents.
Parkinson's disease is the second most common neurodegenerative disorder of aging. Its pathological hallmark is neuronal inclusions termed Lewy bodie. The finding of these Lewy bodies in the intestinal enteric nerves led to the hypothesis that the intestine might be an early site of PD disease in response to an environmental toxin or pathogen. One potential mechanism for environmental toxin to gain access to neuronal tissue is compromised intestinal barrier integrity. This intestinal hyperpermeability significantly correlated with increased intestinal E. coli bacteria.4 Nowadays, gastrointestinal dysfunctions can be considered as early biomarkers since they are commonly found among confirmed patients and occur much earlier than their motor signs. According to Braak's hypothesis, the disease is thought to originate in the intestine and then spread to the brain via the vagus nerve.5
- Gut microbiota in autism and mood disorders. Francesca Mangiola, Gianluca Ianiro, Francesco Franceschi, Stefano Fagiuoli, Giovanni Gasbarrini, Antonio Gasbarrini
- Gut Microbiota: The Brain Peacekeeper. Chunlong Mu, Yuxiang Yang, and Weiyun Zhu
- Moody microbes or fecal phrenology: what do we know about the microbiota-gut-brain axis? Paul Forsythe, Wolfgang Kunze, and John Bienenstock
- Increased Intestinal Permeability Correlates with Sigmoid Mucosa alpha-Synuclein Staining and Endotoxin Exposure Markers in Early Parkinson's Disease. Christopher B. Forsyth et al. PLoS One. 2011; 6(12): e28032. via PubMed Central
- Gastrointestinal Dysfunctions in Parkinson's Disease: Symptoms and Treatments. Andrée-Anne Poirier et al. Parkinsons Dis. 2016; 2016: 6762528. via PubMed central