The present article investigates the relationship between the microbiota of the oral cavity and oral mucositis induced by systemic antitumor therapy in patients with malignant neoplasms. This article highlights modern ideas about the composition of the normal microbiota of the oral cavity and its changes during chemotherapeutic treatment.
 It has been shown that the normal oral microbiota includes representatives of such genera as Actinomyces, Corynebacterium, Fusobacterium, Leptotrichia, Neisseria, Prevotella, Streptococcus, and Veillonella. Moreover, it is emphasized that despite the existence of modern molecular genetic techniques and development of an oral microbiota database, determining the role of individual taxonomic units in oral homeostasis remains challenging.
 Existing studies of changes in the qualitative and quantitative composition of oral microflora against the background of drug antitumor therapy have demonstrated that treatment is associated with significant changes in the microbiological landscape of the oral cavity. An increase was noted in the number of Gram-negative anaerobic opportunistic flora and a decrease in the representation of protective commensal flora. It has been demonstrated that the components of a bacterial cell can modulate the local reactions of a macroorganism through a system of Toll-like receptors while acting in different directions.
 Several unresolved fundamental issues related to the role of oral microbiota in the pathogenesis of oral mucositis are highlighted.