Defining reference values for the gut microbiota in a Southern European population.

Introduction: The clinical implementation of gut microbiota analysis requires the definition of reliable reference values derived from standardized and certified methodologies applied to a population representative of the intended clinical setting. Methods: In this study, 250 fecal samples were analyzed using a CE-certified 16S rRNA bacterial profiling assay for taxonomic characterization across multiple bacterial levels. Sequencing reads were quality-filtered and aligned against the RDP database (release 11, update 5); only sequences with ≥80% alignment coverage and ≥97% similarity were retained. Results: The resulting taxonomic distributions were first compared with data previously obtained from a similar population, revealing only minor differences. As an additional validation, comparative analyses were performed with data from a clinical study on fecal microbiota transplantation (FMT). Donor profiles were highly consistent with those obtained in the present study, whereas pre-transplant samples showed marked deviations from the reference ranges; post-transplant profiles progressively converged toward them. Discussion: Although the limited sample size precludes accurate assessment of rare taxa (<0.1% relative abundance), the use of a single Next-Generation Sequencing (NGS) platform and the focus on a Southern European population characterized by the Mediterranean diet allowed the establishment of the first set of gut microbiota reference values derived from a certified