The human microbiome contains trillions of microorganisms . These organisms vary from person to person like fingerprints, and their composition depends on both host and environmental factors, of which diet plays a crucial role . Knowledge of the human microbiome is possible thanks to the introduction of new DNA sequencing methods, which have been developed over the last decade (Human Microbiome Project). This is when the notion of dysbiosis, which is not quite correct, was coined, i.e. disruption of the normal human microbiota. In the absence of standards for the composition and function of the microbiome, dysbiosis is a conventional term describing the differences in the composition and function of the microbiome between a healthy population and a population affected by, for example, a disease; despite its imperfections, this definition is quite suitable for describing changes in the microbiome in the case of various diseases, including cancer.

The microbiome can influence the development and course of cancer through direct oncogenic effects, pro-inflammatory effects on mucous membranes, generation of metabolic abnormalities, modulation of the immune response and efficacy of anticancer treatment. Both tumour tissue and neighbouring tissues contain their own microbiome, and the same applies to other tissues and body fluids, which, through the microbiome and its metabolites, antigens, etc., can influence tumour development, progression and response to treatment. The gut microbiome being an important regulator of the immune response. It can also influence tumours and their treatment in distant organs. Due to the link between the microbiome and cancer, the potential of its modification in oncological treatment is of great interest to researchers and clinicians.

The aim of this paper is to present the current state of knowledge of one of the most popular methods of modifying the microbiome - probiotics, which are commonly used by oncology patients. The safety aspects of the use of probiotics and current meta-analyses on this group of products are mainly discussed.