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Basic concepts / Intestinal microflora
Intestinal microflora
In patients with achlorhydria or relatively high gastric pH as a result of therapy (e.g., with H2 antagonists), counts of microorganisms in the stomach may reach as many as 106 -107/ml or even higher. Therefore, individuals with reduced or absent gastric acid have a high incidence of bacterial colonization in the upper small bowel and are more susceptible to bacterial diarrhoeal disease(1).
On the contrary, the normal acid pH of the stomach markedly protects against infection from enteric pathogens, e.g., Vibrio cholerae , responsible of cholera.
As intestinal content pH becomes alkaline, the resident flora gradually increases.]
Bile has antibacterial properties. In addition, the forward propulsive motility (peristalsis) is also a key element in suppressing the upper bowel flora. In the adult duodenum, there are 103-106 bacteria per gram of content; in the jejunum and ileum, 104-107 bacteria per gram of contents; and in the cecum and transverse colon, 108-1010 bacteria per gram. In the sigmoid colon and rectum, there are approximately about 1011 bacteria per gram of stool, constituting 10-30% of the faecal mass.
The colonic microflora is a diverse ecosystem made up of more than 1000 bacterial species. Its composition is dependent upon a number of factors including age, antibiotic use and diseases. The dominant population consists of strict anaerobic bacteria: Bacteroides, Bifidobacterium, Eubacterium and Peptostreptoccocus. Bacteroides (gram-negative) is the most numerous of these four microorganisms, followed closely by Bifidobacterium (gram-positive).
The intestinal microflora develops in stages throughout the life-span as a result of diet, the host health status and the environmental conditions.
At birth, the intestine is sterile, but microorganisms are soon introduced with food.
In breast-fed children, the intestine contains large numbers of lactic acid Streptococci, Lactobacilli and Bifidobacteria. These aerobic and anaerobic, gram-positive, non-motile organisms (e.g., Bifidobacterium species) produce acid from carbohydrates and tolerate pH 5.0.
In bottle-fed children, a more mixed flora coexists in the bowel, and Lactobacilli are less prominent.
As food habits develop toward the adult pattern, the bowel flora changes again.
Diet itself also has a strong influence on both the relative composition and activity of the colonic microflora.]
Gastric acid in the stomach kills most of the organisms that are swallowed. The stomach acidity keeps the number of microorganisms at a minimum, and the stomach normally has a sparse flora consisting typically of =<103cfu/g.
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