As the body's largest digestive and detoxifying organ, gut contains hundreds of millions of gut microbes. As we get older, most people lose not only time, but also the "good bacteria" that accompany us along the way. Infancy -- Residents Moving in, Starting to Occupy Places The time when microbes appear in the human body can be traced back to the fetal period as researchers found the presence of microorganisms in the placenta, amniotic fluid and meconium of the fetus . The baby's gut interacts with external bacteria for the first time after birth. Facultative anaerobic bacteria such as enterobacter, escherichia coli, lactobacillus and streptococcus settle down first. After 2-3 days of birth, anaerobic bacteria such as bifidobacterium, bacteroides and clostridium settle. Bifidobacterium is the dominant microflora in human intestine at this time. The mother's gut flora determines the type of gut flora of the first few months of the baby's. In 6-month-old babies, the intestinal flora may change significantly after taking complementary foods, with bacteroides, clostridium and anaerobic bacteria increasing rapidly, and bifidobacteria decreasing. A decrease in the number of bifidobacteria or an increase in the number of non-beneficial flora may lead to an increased risk of allergies, infections and other diseases in infants . Juvenile Period -- Residents Change. Greed Of Eating "Forced Away" Bifidobacteria? Intestinal microorganisms can be divided into four categories: thick-walled bacteria, bacteria, actinomycetes and proteobacteria. Intestinal microecology reach a relatively stable state at the age of 2-5 years, but the intestinal flora between individuals is highly dynamic and specific, and would change continuously over time. New changes will occur in the intestinal microbiota of young people aged 12-18. As adolescents grow and develop, the number of microorganisms of bacteroides would increase in their bodies. The number of microorganisms of the thick-walled bacteria would reduce. The number of enterobacteria, enterococci would increase. The number of bifidobacteria and lactobacilli would further reduce. There are no significant changes in other flora, including clostridium. During this period, lactobacilli and bifidobacteria would reduce but enterobacteria and enterococci would increase. Enterobacter and enterococcus, which are unstable factors in the human body, can not only decompose food energy, but also have certain pathogenicity. Experts speculate that the decrease of bifidobacteria and lactobacillus may be related to the fact that infants and children's diet is dominated by dairy products, while adolescents eat more growth-related diets and fatty foods . As age change, the intestinal environment becomes unsuitable for these two florae. Adulthood -- Residents Settle Down. Good Flora Showing "First Tide of Weakness" 2In the adulthood of 20-60 years old, human growth and development are basically completed. Microorganisms in the intestine also tend to be stable. The general trend is that the number of bacteroides is increasing year by year. And with the increase of age, the body's colonization resistance decreases year by year, becoming more vulnerable to pathogens . A study compared intestinal microbial changes in people aged 20-40 and 40-60 years old. The results showed that the main metabolites of intestinal microecological structures in the two age groups were similar. However, after taking dietary fiber, the 40-60 age group was more likely to have a feeling of fullness, which was presumed to be the reason for the decrease of acetic acid content and the increase of propionic acid and butyric acid. It can be speculated that the decline of intestinal flora function may be much earlier than the degradation of the flora structure. Grey-Headed Era Good Bacteria Move Away, Bad Bacteria Settle Down The overall number of intestinal flora in the elderly reduce significantly. Bacteroides, lactobacilli, and bifidobacteria, which have a positive effect on health, are decreasing. The genus Ruminococcus which can decompose cellulose will be almost wiped out. The reduction of "good bacteria" makes the intestines a "paradise" for neutral bacteria and certain pathogenic bacteria. Studies have shown that the number of spoilage bacteria such as streptococcus, enterobacter, and staphylococcus in the elderly increase significantly. This is part of the reason for the decline in the health of the elderly. The difference and continuity of intestinal flora structure and composition are synchronized with age. Therefore, both children and adults should supplement probiotics properly to maintain intestinal microecological balance and protect health. References:  Chen Junkui, Liu Wei, Wang Xin, et al. Study on Intestinal Flora and Metabolic Difference in Different Stages of Adults [J]. Journal of Gastroenterology and Hepatology, 2019 (3): 276-281.  Zhang Ting. Role of Probiotics in Children's Growth and Development [J]. Chinese Journal of Child Health, 2018 (8): 816-818.  Liu Ping , Dong Lina , Han Yi , et al. Characteristics of Intestinal Flora in Adolescents [J]. Chinese Remedies & Clinics, 2015, 15 (12): 1685-1688  Guo Feixiang. Analysis of Intestinal Flora of Different Age Groups in Longevity Area of Bama, Guangxi [D]. 2015.