Modern approaches to the essence and assessment of gut dysbiosis. Review

An imbalance, dysfunction, or disruption of the gut microbiota (GM) is increasingly recognized as an indicator of certain disease or poor health. Due to the complexity and individual variation in microbial communities, there is no gold standard for determining the presence or degree of this imbalance or disruption of the gut microbiota, although many studies refer to it as gut dysbiosis. The problem with the definition of intestinal dysbiosis is caused by the lack of a clear definition of a healthy intestinal microbiota. Several indices have been defined and applied to qualify the term «gut dysbiosis». According to the results of an extensive literature search, known indices of dysbiosis (ID) can be methodologically grouped into five categories: large‑scale profiling of bacterial markers, relevant taxon‑based methods, environmental classification, random prediction, combined α/β‑diversity. These methods for identifying and quantifying gut dysbiosis successfully capture differences between GM associated with specific conditions, diseases, or interventions, as well as that present in healthy individuals or at baseline (before intervention). Such indices can help to characterize diseases and adverse conditions, predict treatment outcomes, and provide information different from conventional alpha and beta estimates of microbial diversity. It should be kept in mind that dysbiosis is not a well‑defined condition and that indices of dysbiosis differ in methodological and clinical contexts, developed for different cohorts and to describe a variety of conditions. Current basic methods and principles related to the assessment of GM in a clinical context are highlighted, including the current use of dysbiosis indices, their calculation and indicators in the context of specific diseases and conditions. Dysbiosis indices are not separate measurements and should be interpreted in the context of clinical data. It should be remembered that the GM that characterizes a particular disease or intervention often changes due to disturbances in factors such as diet, medication, oxygen availability, or immune responses. In this case, ID is used as a diagnostic marker, but not necessarily as a predictor of the disease. However, the value of dysbiosis indices as simple tools to describe the differences between different gut microbial communities is important. 
 