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The intestinal microflora is a complex ecosystem containing over 400 bacterial species. Anaerobes outnumber facultative anaerobes. The flora is sparse in the stomach and upper intestine, but luxuriant in the lower bowel. Bacteria occur both in the lumen and attached to the mucosa, but do not normally penetrate the bowel wall .
Intestinal bacteria are a crucial component of the enterohepatic circulation in which metabolites that are conjugated in the liver and excreted in the bile are deconjugated in the intestine by bacterial enzymes, then absorbed across the mucosa and returned to the liver in the portal circulation. Many drugs and endogenous compounds undergo enterohepatic circulation. Antibiotics that suppress the flora can alter the fecal excretion and hence the blood levels of these compounds. The flora also plays a role in fiber digestion and synthesizes certain vitamins.
The intestinal microflora may prevent infection by interfering with pathogens. The flora includes low populations of potentially pathogenic organisms such as Clostridium difficile. Antibiotics that upset the balance of the normal flora can favor both infection by exogenous pathogens and overgrowth by endogenous pathogens. If the bowel wall is breached, enteric bacteria can escape into the peritoneum and cause peritonitis and abscesses.
Enterotoxin-Mediated Diarrheas: Enterotoxigenic bacteria, such as Vibrio cholerae and enterotoxigenic Escherichia coli strains, colonize the upper bowel and cause watery diarrhea by producing an enterotoxin that stimulates mucosal cells to secrete fluid via an increase in intracellular AMP.
Invasive Diarrheas: Invasive bacteria, such as Shigella and Campylobacter, penetrate the intestinal mucosa. A bloody, mucoid diarrheal stool with inflammatory exudate is produced.
Rotavirus and Calicivirus (formerly Norwalk virus) are major causes of diarrheal disease. Rotavirus diarrhea affects mostly young children; Calicivirus causes disease in all age groups
Some protozoa (especially Entamoeba histolytica and Giardia lamblia) as well as some intestinal helminths can cause diarrheal disease.
In general, enterotoxigenic bacteria and viruses affect the upper bowel, causing watery diarrhea and periumbilical pain. The invasive bacteria act primarily in the colon (Shigella and Campylobacter) or lower ileum (Salmonella). The stool in these diseases may contain blood. Colitis is marked by painful straining at stool (tenesmus).
Composition and Distribution of the Microflora
Metabolic Activites of the Microflora
The Intestinal Microflora and Infection
