Background: Irritable bowel syndrome (IBS) is a common functional disorder of the gastrointestinal tract (GIT). Despite the prevalence of this disease, our understanding of the etiology remains limited. However, there is no doubt that microbial factors play a key role in the pathophysiology of IBS. The aim of the study: To investigate the microbiological composition of the intestines of patients with IBS-D. Materials and methods: We used the next-generation sequencing (NGS) method, aimed at targeted sequencing of the hypervariable region V3 of the 16S rRNA gene, which allowed us to study in detail the changes in the microbiome composition in patients. Results: An increase in the number of members of the genera Streptococcus and Haemophilus was revealed in the group of patients with IBS-D, they are mainly represented by opportunistic pathogens and are associated with the development of IBS. At the same time, there was a decrease in bacterial genera: Ruminococcaceae NK4A214, Butyricimonas, Christensenellaceae R-7, Ruminococcaceae DTU089, Coprobacter, Enterococcus and Sciscionella compared to the healthy group. Significant results were also obtained in the correlation analysis between bacteria, showing the relationship of bacteria associated with IBS. Conclusion: Genetic analysis revealed significant differences between the groups of healthy and IBS patients in the composition of the bacterial microbiome. Our results show that the composition of the gut microbiome changes in people with IBS. Thus, this study contributes to understanding the development of IBS-D in terms of microbiome changes.