Fig. 2.

Every time a human baby is born, a rich and dynamic microbial ecosystem develops from a sterile environment. During the first year of life, until weaning, the gut ecosystem is prevalently colonized by opportunistic microorganisms to which a baby is exposed in its environment (Palmer et al. 2007). The earliest colonizers are often aerobes such as Staphylococcus, Streptococcus and Enterobacteria, followed by anaerobic later colonizers such as Eubacteria and Clostridia. After these earlier stages, it is generally though that the microbiota of breast-fed infants is largely dominated by Bifidobacterium. After weaning, the developmental changes in the gut mucosa and in the intestinal IS, together the introduction of a solid diet, drive to the transition to a resilience adult-like profile of the human gut microbiota, characterised by a remarkable microbial biodiversity. The ageing of the gut microbiota starts after a subject-specific ‘threshold age’ which depends on individual characteristics such as diet, country and, eventually, frailty. In any case, changes of diet, lifestyle and the immunosenescence of the intestinal IS dramatically impact the microbial ecology of the human GI tract. Symmetrically to what happens in the early stage of our life, the aged-type microbiota shows a low microbial biodiversity, and it is characterised by an increase in opportunistic environmental facultative aerobes Staphylococcus, Streptococcus, Enterobacteriaceae, as well as a reduction in anaerobes, such as Clostridium clusters IV and XIVa and Bacteroidetes. However, differently from the infant-type microbiota, the aged type is characterised by a low abundance of Bifidobacterium