Abstract: The mechanisms leading to neurological and neurodegenerative diseases are not completely known, and new, more effective, therapeutic treatments are necessary for most neurological pathologies. The treatment of neurological and neurodegenerative diseases is complicated due to the blood–brain barrier, which makes it difficult for drugs to access the brain areas in which they must act to improve the pathology. A tool that can help to overcome this difficulty is the use of extracellular vesicles, which can easily cross the blood–brain barrier. The extracellular vesicles are considered a main way of communication between the brain and the rest of the body, with important implications for the physiopathology and therapy of neurological diseases. In recent years, the involvement of microbiota in many neurological pathologies, as well as its possible therapeutic role, has also become evident. A key mediator in the pathologic and beneficial effects of microbiota seems to be the bacterial extracellular vesicles. There is an important communication between the brain and the intestinal microbiota (the gut–brain axis), by which the microbiota influences brain function, impacts on mental health, and plays a role in different neurological and neurodegenerative diseases. The identification of the mechanisms involved in this gut–brain axis is essential to understanding the mechanisms of neurological pathologies and to developing more effective treatments for these diseases. Bacterial extracellular vesicles would play a relevant role in these processes. This review compiles the recent information and evidence on the role of bacterial extracellular vesicles in brain pathologies and on the therapeutic utility of bacterial extracellular vesicles in neurological and neurodegenerative diseases. One advantage of bacterial extracellular vesicles compared to extracellular vesicles derived from other cell types, such as stem cells, is that bacterial extracellular vesicles are generally easier to produce and modify. Bacterial extracellular vesicles may be easily modified to target a specific pathology and/ or to enhance its therapeutic efficacy. Although the studies are still scarce, they open a wide field of possibilities for future studies, which will lead to a deeper understanding of the role of microbiota and bacterial extracellular vesicles in neurological pathologies and the underlying mechanisms, as well as to the development of new treatments based on the use of bacterial extracellular vesicles in neurological diseases.

Key words: bacteria, bacterial extracellular vesicles, gut–brain axis, inflammation, microbiota, neuroinflammation, neurological diseases, neurotransmission, pathogenic, probiotic, therapeutic treatment