The Oral Microbiota: Implications in Mucosal Health and Systemic Disease—Crosstalk with Gut and Brain

During the last ten years, the scientific community has increasingly acquired greater knowledge of the importance of oral microbiota, in general, for the physical condition of humans. Not only oral diseases, related to oral dysbiosis, are examined, but also several systemic inflammatory degenerative diseases induced by this condition. This narrative review aims to shed light on the communication mechanisms between the oral cavity and different mucosal compartments, and to explain how the changes in microorganisms may alter their balance, leading to disease. Many potential pathogenic bacteria can induce oral dysbiosis, among them Porphyromonas gingivalis and Fusobacterium nucleatum are the most explored; however, other bacterial species such as Tannerella forsythia, Treponema denticola, Aggregatibacter actinomycetemcomitans and Filifactor alocis are able to give rise to local and systemic diseases through the release of toxins. The two-way communication system between the gastrointestinal tract and the central nervous system, known as the gut–brain axis, is strongly influenced by the gut microbiota and can ultimately be studied even more broadly and in depth if we consider the influence of the oral microbiota on this axis. Taste receptors’ activity also has a significant role, being able to affect a subject’s food choice by interacting with the microbiota. Qualitative and quantitative alterations in microorganisms existing in the main mucosal compartments may easily lead the host to develop systemic degenerative inflammatory diseases.