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正常菌丛人类胃肠道内的细菌构成一个巨大而复杂的生态系统,一个人结肠内就有400个以上的菌种。从口腔进入胃的细菌绝大多数被胃酸杀灭,剩下的主要是革兰氏阳性需氧菌,胃内细菌浓度<10~3 CUF/ml(CUF即colony forming unit菌落形成单位)。小肠菌丛的构成介于胃和结肠之间。近端小肠的菌丛与胃内相近,但常能分离出大肠杆菌和厌氧菌。在远段回肠,厌氧菌的数量开始超过需氧菌,其中大肠杆菌恒定存在,厌氧菌,如类杆菌属、双歧杆菌属、梭状芽胞杆菌属,都有相当浓度。在回盲瓣的远侧,细菌浓度急骤上升,结肠细菌浓度高达10~(11)～10~(12)CFU/ml,细菌总重量几占粪便干重的1/3。厌氧菌达需氧菌的10~2～10~4倍,主要菌种为类杆菌属、双歧杆菌属和真杆菌属。
Normal flora Bacteria in the human gastrointestinal tract form a vast and complex ecosystem, with more than 400 species in one’s colon. The majority of bacteria that enter the stomach from the mouth are killed by gastric acid, leaving the majority of Gram-positive aerobes with a bacterial concentration of <10-3 CUF / ml in the stomach (CUF colony forming unit colony forming unit). The composition of intestinal flora is between the stomach and the colon. The proximal intestinal flora and stomach similar, but often isolated Escherichia coli and anaerobic bacteria. In the distal ileum, the number of anaerobic bacteria begins to exceed that of aerobic bacteria, of which Escherichia coli is constantly present, and anaerobic bacteria such as Bacteroides, Bifidobacterium and Clostridia have considerable concentrations. On the far side of the ileocecal valve, the bacterial concentration rose sharply. The concentration of colon bacteria was as high as 10 ~ (11) ~ 10 ~ (12) CFU / ml. The total bacteria weight accounted for 1/3 of the total dry weight of the feces. Anaerobic bacteria up to 10 ~ 2 ~ 10 ~ 4 times the aerobic bacteria, the main species Bacteroides, Bifidobacterium and Eubacterium.