Probiotics and Microbiota-Derived Molecules for Inflammation, Neuroinflammation and Mood Disorders

The gut microbiota plays an important role in inflammation, neuroinflammation and the function of neuronal cells. Probiotics have received increasing attention for their extensive clinical applications and beneficial health effects on various clinical ailments, including mood disorders. Probiotics can affect neural activity and host health by regulating the microbiota–gut–brain axis.

This Special Issue includes articles discussing the role of probiotics and gut-microbiota-derived molecules in inflammation and neural disorders.

The review by Conte et al. [1] summarizes and argues the role that the gut microbiota plays in the development, function and behavior of the human brain. Studies have suggested a causal link between intestinal microbiota alteration and neurodegenerative/neuroinflammatory diseases. Neurological and psychiatric disorders have been found to be associated with changes in the gut microbiota. The microbiota–gut–brain axis can have a significant impact on different neural disorders. Dysregulation of this axis, through a complex bidiretional communication system, is associated with the pathogenesis of several neurodegenerative diseases, which are discussed by the authors in the review. Furthermore, the role of mast cells in neuroimmune communication is discussed. Dysbiosis of the intestinal microbiota is responsible for the development of local and systemic inflammation, with damage to the intestinal epithelial membrane, invasion of bacteria and viruses within the brain parenchyma and, finally, neuroinflammation and dysfunction of neuronal cells. Microbial products can affect host cell pathways. The intake of beneficial probiotic bacterial strains could represent a valid therapeutic approach to be adopted in combination with classical therapies.

The review by Traina and Cocchi [2] focuses on some factors underlying depressive states and anxiety disorders. The authors discussed how a silent inflammatory state, as well as a depressive condition, lead to alterations in the levels of various mediators, including arachidonic acid, released by astrocytes and microglia which, in turn, communicate with mast cells. A dysregulation in the composition of the intestinal microbiota could initiate or even exacerbate intestinal disorders and influence a depressive state; the inflammatory condition involves mast cells and platelets, both cells sensitive to systemic inflammatory change. The authors suggest that mast cells and platelets could be excellent markers and tools for therapeutic interventions in the treatment of depression.

In their commentary [3], the authors focus on the role of membrane fluidity in the depressive state. A differential lipid fluidity of the membrane represents a crucial node of serotonin signaling. The mobility of the membrane modifies the accessibility of serotonin to its receptors, decreasing in case of increased fluidity. The gut microbiota would affect depressive behavior by affecting the accessibility of brain tryptophan and the serotonin system. The concomitance of these two phenomena would reduce the availability of serotonin and, therefore, its mood-regulating function. In this context, the microbiota–gut–brain axis could play a decisive role.

In their article, Mondo et al. [4] report the results of a study analyzing the gut microbiota of the German Shepherd dog breed and humans. They found that the German Shepherd’s gut microbiota has a different composition than other dog breeds and, in particular, shows a significant resemblance to that of depressed humans. The comparative analysis on the intestinal microbiota of the German Shepherd and the man suffering from depression showed a close similarity in the bacterial composition that led the authors to define a bacterial geography and to confirm the molecular affinity between the German Shepherd and the man suffering from depression.

In their article, Lin et al. [5] presented a study on the benefits of oral use of Lactobacillus in human bacterial vaginosis. The authors conduct a randomized, double-blind study using two formulas of Lactobacillus in patients with bacterial vaginosis. Strains of Lactobacillus were selected based on their activity to prevent the in vitro growth and adhesion of pathogenic bacteria of bacterial vaginosis to cervical epithelial cells. Through a detailed investigation, the authors show that the oral intake of two combined formulas of Lactobacillus in subjects with bacterial vaginosis has shown beneficial effects in the improvement of clinical symptoms.