Relevance. The relevance of the work is determined by the fact that the efforts of doctors seeking to minimize damage that occur against the background of infectious load and deformation of periodontal tissues do not lead to a decrease in the prevalence and intensity of periodontitis. At present, it is not known how the kinetics of microorganisms increases to the semen, allowing it to overcome the pressure of the gingival fluid that is filtered through periodontal fibers.Purpose. Since infection plays the main role in the occurrence and development of periodontitis, the aim of the work is to search for hydrodynamic mechanisms that complement the etiopathogenesis of periodontitis and explain the difficulties of its treatment.Materials and methods. The article discusses the existence of biota in film – static and plankton – dynamic forms. As a result of a review of literature data, it is proved that the transition of biota from one form to another is determined by a wide range of factors, the most relevant of which is the quality of the environment. Staying in a biota habitat optimal for life, it transforms into a planktonic form of existence, which allows it to colonize the deeper sections of the periodontium. Colonization of surfaces is possible by diffusion, i.e. leveling the concentration of microorganisms in available volumes of biological fluids. This aspect of the etiopathogenesis of periodontal diseases is called “hydrodynamic”. Separation of the hydrodynamic aspect allows us to consider the pressure gradient in the volume of the gingival sulcus as a factor determining the direction of filtration of biological fluids.Results. It was not possible to identify well known hydrodynamic mechanisms, that explain the possibility of microorganisms reaching the deep parts of the periodontium.Conclusion. The article presents arguments showing that biological laws linking the quality of the environment with the dynamics of increasing population numbers do not work in the area of the periodontal sulcus, or their action is blocked by laws of a different nature. Discussing the planktonic (dynamic) form of the existence of biota, based on the laws of hydrodynamics, taking into account the type of tooth movement, it becomes possible to substantiate the hydrodynamic mechanism of reaching the deep departments of periodontal biota, to clarify preventive and therapeutic measures aimed at reducing the incidence of periodontal diseases.