Composition and Distribution of the Intestinal Microflora

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 .

Metabolic Activities

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

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.

Bacterial Diarrheas

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.

Viral Diarrheas

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

Parasitic Diarrheas

Some protozoa (especially Entamoeba histolytica and Giardia lamblia) as well as some intestinal helminths can cause diarrheal disease.

Clinical Diagnosis

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).

The Enterohepatic Circulation

Enzymes produced by intestinal bacteria play a central role in the enterohepatic circulation. Substances that undergo enterohepatic circulation are metabolized in the liver, excreted in the bile, and passed into the intestinal lumen, where they are reabsorbed across the intestinal mucosa and returned to the liver via the portal circulation. The enterohepatic circulation generally involves compounds that are conjugated in the liver to a polar group such as glucuronic acid, sulfate, taurine, glycine, or glutathione. Conjugation increases the solubility of the metabolite in bile, but the conjugated compounds are poorly absorbed by the intestinal mucosa. Enzymes produced by intestinal bacteria—such as ß-glucuronidase, sulfatase, and various glycosidases—deconjugate these compounds, releasing the parent compounds which are readily absorbed across the intestinal wall. Many endogenous compounds undergo enterohepatic circulation, including bilirubin, bile acids, cholesterol, estrogens, and metabolites of vitamin D. In addition, many drugs that are excreted by the liver, including digitalis, diethylstilbestrol, morphine, colchicine, rifampin, and chloramphenicol, enter this pathway.

Antibiotics block the enterohepatic circulation by suppressing the intestinal flora and thereby reducing the levels of deconjugating enzymes. If an antibiotic is given to a patient who is also taking a drug that undergoes enterohepatic circulation, the resulting depression of the enterohepatic circulation will increase the fecal excretion of the drug and thereby lower its plasma level and half life. For example, the blood levels and half life of the estrogen in birth control pills decrease when antibiotics are administered.

The Microflora and Nutrition

Enzymes produced by intestinal bacteria are important in the metabolism of several vitamins. The intestinal microflora synthesizes vitamin K, which is a necessary cofactor in the production of prothrombin and other blood clotting factors. Treatment with antibiotics, particularly in individuals eating a diet low in vitamin K, can result in low plasma prothrombin levels and a tendency to bleed. Intestinal bacteria also synthesize biotin, vitamin B₁₂, folic acid, and thiamine.

The intestinal flora is capable of fermenting indigestible carbohydrates (dietary fiber) to short-chain fatty acids such as acetate, propionate, and butyrate. The major source of such fermentable carbohydrate in the human colon is plant cell wall polysaccharides such as pectins, cellulose, and hemicellulose. The acids produced from these fiber substrates by bacteria can be an important energy source for the host.

Some people are deficient in intestinal lactase, the mucosal enzyme responsible for hydrolyzing the disaccharide lactose in milk. In these individuals, lactose is not adequately digested and absorbed in the intestine. Lactose that reaches the large bowel undergoes vigorous bacterial fermentation. The result can be distention, flatus, and diarrhea.

Protective Activities of the Flora

Like other complex ecosystems, the intestinal microflora is relatively stable over time, maintaining roughly constant numbers and types of bacteria in each area of the bowel. The stability of normal flora both discourages infection by exogenous pathogens and prevents overgrowth of potentially pathogenic members. New organisms that enter the system in contaminated food or water generally are suppressed by the established flora. This suppression is related to production by members of the resident flora of antimicrobial substances such as bacteriocins or short-chain fatty acids, which inhibit the growth of alien microorganisms. Antibiotics that kill off part of the intestinal flora can upset its balance and may open the door to infection or pathologic overgrowth.

The pathogenesis of Salmonella food poisoning illustrates this phenomenon. Normal individuals are quite resistant to Salmonella, and a large oral inoculum is required to initiate infection. If the intestinal flora is suppressed by antibiotics, however, the individual becomes much more susceptible and can be infected by a relatively small inoculum.

Diseases Caused by Overgrowth of Potential Pathogens

The normal intestinal flora includes small populations of organisms that cause disease if they overgrow. For example, overgrowth of Clostridium difficile produces severe inflammation of the colon with diarrhea (pseudomembranous colitis). Administration of antibiotics initiates the process by suppressing the normal flora.

Peritonitis

Bacteria from the intestinal flora are the prime cause of infection in the peritoneal cavity when the normal barriers of the intestinal wall are violated. The intestinal wall can be perforated by trauma (knife wounds, gunshot wounds, blunt trauma), by disease (appendicitis, penetrating intestinal cancers), or by surgical procedures. Once the mucosal barrier is breached, bacteria penetrate through the intestinal wall into the normally sterile peritoneal cavity and its surrounding structures. Poor circulation, reduced oxygen supply, and dead tissue in the vicinity of the perforation promote the formation of an abscess and particularly favor the growth of anaerobic bacteria. Cultures of a peritoneal abscess generally yield several types of bacteria from the intestinal microflora, particularly species of Bacteroides, Clostridium, and Peptostreptococcus and E coli.

Enterotoxin-Mediated Diarrheal Diseases

Several enterotoxin-producing bacteria cause diarrheal diseases (Table 95-1). The diarrheal disease caused by Vibrio cholerae and enterotoxigenic strains of E coli has three main characteristics. First, there is intestinal fluid loss that is related to the action of an enterotoxin on the small bowel epithelial cells. Second, the organism itself does not invade the mucosal surface; rather, it colonizes the upper small bowel, adhering to the epithelial cells and elaborating the enterotoxin. The mucosal architecture remains intact with no evidence of cellular destruction. Bacteremia does not occur. Third, the fecal effluent is watery and often voluminous, so that the diarrhea can result in clinical dehydration. The fluid originates in the upper small bowel. where the enterotoxin is most active.

Table 95-1

Toxin-Producing Bacteria Associated With Diarrheal Disease.

Cholera

The paradigm of the enterotoxigenic diarrheal diseases is cholera (see Ch. 24), in which stool volume can exceed 1 L/h, with daily fecal outputs of 15 to 20 L if the patient is kept hydrated. Cholera is caused by V cholerae, which is usually ingested in contaminated water. Vibrios that survive passage through the stomach colonize the surface of the small intestine, proliferate, and elaborate the enterotoxin. Cholera toxin acts via adenylate cyclase to stimulate secretion of water and electrolytes from the epithelial cells into the lumen of the gut. The duodenum and upper jejunum are more sensitive to the toxin than the ileum is. The colon is relatively insensitive to the toxin and may still absorb water and electrolytes normally. Thus, cholera is an “overflow diarrhea,” in which the large volumes of fluid produced in the upper intestine overwhelm the resorptive capacity of the lower bowel.

Cholera stool is described as resembling rice water—a clear fluid flecked with mucus—and is isotonic with plasma. Microscopy reveals no inflammatory cells in the fecal effluent; all that can be seen are small numbers of shed mucosal cells.

Enterotoxigenic E coli Diarrhea

Certain strains of E coli cause diarrheal disease by elaborating enterotoxins (see Ch. 25). These strains produce two types of enterotoxin. One, called heat-labile toxin, is similar in structure and in its mechanism of action to cholera toxin. The other, called heat-stable toxin, appears to act via guanylate cyclase. Enterotoxigenic E coli strains are the most common cause of travelers' diarrhea

Other Diarrhea-Causing Toxins

Many strains of Shigella produce an enterotoxin, called Shiga toxin, that causes secretion of fluid from the small intestine (see Ch. 22). Shiga toxin has a destructive, cytotoxic effect on the small-bowel epithelium, causing gross injury to the bowel surface. It does not activate adenylate cyclase. E coli 0157:H7, the organism associated with consumption of undercooked chopped meat, also produces a Shiga-like toxin; it causes bloody diarrhea and colitis. An organism that produces a different type of cytotoxin is Vibrio parahaemolyticus, a bacterium associated with seafood. Food-poisoning strains of Staphylococcus aureus and Clostridium perfringens both produce enterotoxins that are cytotoxic. The staphylococcal enterotoxin also has a direct effect on the vomiting center in the brain.

Gastrointestinal Disease Caused by Invasive Bacteria

Unlike the enterotoxigenic organisms, invasive bacteria exert their main impact on the host by causing gross destruction of the epithelial architecture; histologic findings include mucosal ulceration and an inflammatory reaction in the lamina propria. The principal pathogens in this group are Salmonella, Shigella, Campylobacter, invasive E coli, and Yersinia. The enteric viruses also invade intestinal epithelial cells, but the extent of mucosal destruction is considerably less than that caused by invasive bacterial pathogens.

Salmonella Enteritis

Salmonella species are a common cause of food poisoning. The main site of attack is the lower ileum, where the salmonellae cause mucosal ulceration. They rapidly make their way through the epithelial surface into the lamina propria and enter the lymphatics and bloodstream. At least two virulence factors are associated with intestinal infection: one responsible for mucosal invasion, and the other causing secretion of fluid and electrolytes into the bowel.

Shigella Dysentery

Shigella organisms cause bacillary dysentery, an invasive diarrheal disease of the lower bowel in which the stool contains an inflammatory exudate composed of polymorphonuclear leukocytes. The bacilli invade the epithelium of the colon and cause superficial ulceration. This invasive process depends on the presence of two virulence factors. The first mediates the initial penetration of the mucosal surface by destroying the brush border; the bacteria are subsequently engulfed by invagination of the plasma membrane. The second virulence factor allows the organism to multiply within the mucosal tissue. Mucosal ulceration results, accompanied by an intense inflammatory response in the lamina propria. The infection is usually restricted to the mucosa; lymph node involvement and bacteremia are uncommon.

Fluid Production in Invasive Diarrheal Diseases

The mechanism(s) by which the fluid that causes watery diarrhea is produced in the invasive diarrheal diseases is under debate. Three mechanisms have been proposed. First, Shigella and possibly Salmonella strains apparently produce an enterotoxin that stimulates the mucosa to secrete water and electrolytes. Second, there is evidence that invasive organisms stimulate prostaglandin synthesis at the site of inflammation and that the prostaglandins induce fluid secretion. In experimental animals, fluid secretion can be blocked by prostaglandin inhibitors such as indomethacin and aspirin. Third, some evidence suggests that damage to the colonic epithelium causes diarrhea by prevention of normal resorption of fluid.