A properly functioning gut ecosystem plays an important role in keeping healthy. Normal quantitative and qualitative bacterial composition stimulates the human immune system, protects against pathogenic microorganisms and contributes to maintaining the gastrointestinal barrier. Possible factors of perturbation in homeostasis such as drugs, stress or inappropriate diet may cause excessive development of potentially pathogenic bacteria, involved in carcinogenesis. Current knowledge in the field of gut microbiota impact and its metabolic activity are described in the present work.