The microorganisms in your intestines, primarily bacteria, form a “partnership” with them that starts right at birth. It takes around three years for gastrointestinal microbiota to build up, before a period of relative stability until old age when it undergoes profound changes again.
The digestive system of a newborn is quickly colonized by “simple” microbiota, originating from their mother’s vaginal and fecal bacteria. Breastfeeding provides favorable bacteria, such as bifidobacteria, but intestinal microbiota takes around three years to stabilize. This stabilization depends on numerous factors. For example, anaerobic bacteria, which don’t need oxygen to survive, are in the minority in infants but are dominant in adults. The development of these anaerobic bacteria is slower when birth is by caesarian section or premature and microbiota is less diverse than in infants born vaginally at full-term. The diversity of microbiota also depends on the early childhood environment: less diversity has been observed in Westernized countries, which may be linked to overactive hygiene. Gastrointestinal microbiota is enhanced over the years, but it always retains the imprint of the early childhood profile. Less microbial diversity can lead to potentially long-term illness.
A loss of diversity in the elderly
In adulthood, few factors can radically change the dominant microbiota aside from hormonal variations during puberty, pregnancy, and menopause. But it is in old-age that variations become more pronounced. Microbiota is impoverished by physiological changes, such as a reduction in immunity, a less varied diet, taking numerous medications including antibiotics, and especially lifestyle, when people become less independent. In fact, elderly people living in retirement homes have more impoverished microbiota than those living at home and continuing with their usual diet. This profound change in the equilibrium of the microbiota can lead to illness, such as intestinal inflammation and increased sensitivity to infection.