Updated project metadata. The link between human gut microbiota (a complex group of microorganisms including not only bacteria but also fungi, viruses, etc.,) and the physiological state is nowadays unquestionable. Metaproteomic has emerged as a useful technique to characterize this microbial community, not just taxonomically, but also focusing on specific biological processes carried out by gut microbiota that may have an effect in the host health or pathological state. Cystic fibrosis is a genetic disease in which the microbiota of the respiratory tract determines the patient's survival and differences in composition of gut microbiota of cystic fibrosis patients respect to healthy infants have been reported. In order to characterize this host-microbiota inter-relation, we carried out the metaproteomic study of 30 stool samples from infants with cystic fibrosis.