Neurodegenerative Diseases and the Human Microbiota

Recent years have seen an explosion of interest in the relationship between the microbiota – the trillions of microorganisms that inhabit the human body – and the host, both in health and disease. The microbiota comprises a complex community or ecosystem of microorganisms that colonize external tissues, such as skin, hair and nails, and internal tissues including the intestine and the vaginal and conjunctiva mucosa. The microorganisms that make up the human microbiota include viruses, bacteria, archaea, fungi and, to a lesser extent, protozoa. The largest number of microorganisms are found in the digestive tract, with the majority in the oral and nasopharyngeal cavity and in the intestine. The composition of the microbiota varies greatly between individuals and also varies in the same person depending on age, on diet and lifestyle, and even on the state of the person’s health. Not surprisingly, the microbiota influences many physiological functions beyond food metabolism, and can affect both the immune system and the nervous system. The microbiota composition may be imbalanced (dysbiosis) in patients with different pathologies, and can play an important role in the severity of different clinical symptoms. In this book, we introduce a new concept in the field of human microbiota in health and disease. The fundamental idea is that both fungi and bacteria can be found in some internal tissues that should be "sterile", which can lead to polymicrobial infections. In the absence of symptoms, these infections may go unnoticed over many years or decades. However, if the number of fungi and/or bacteria in a given tissue increases due to, for example, stress, changes in diet, lifestyle, etc., clinical symptoms may begin to emerge, which can vary greatly depending on the affected tissue. This diversity in symptoms will also depend on the specific species of fungi and/or bacteria that affect each individual. The internal microbiota may be the root cause of numerous diseases in humans whose etiology is still unknown. Among th