Gut Reactions

Natural History, Feb, 1999 by Sherwood L. Gorbach

A one-cell-thick wall stands between us and disaster.

Although not widely recognized--and certainly not the topic of polite conversation--our intestinal tract, like that of other mammals, is the habitat for an amazingly rich population of microorganisms. While the intestine of an embryo in the uterus is sterile, newborns acquire their bowel flora during passage through the birth canal. As the diet of the infant changes from milk-based fare to a more varied diet, the mix of microbes changes, assuming its adult character by the second year of life.

This microflora--mostly bacteria--is usually harmless and occasionally helpful, yet always menacing as a potential source of infection. What separates us from invasion by the potential pathogens in our bowel is a fragile barrier just one cell thick: the intestinal lining. For most of our lives, we generally coexist peacefully with our microflora. But if the intestinal wall is breached by certain species of bacteria (these are a minority of the bowel's total population), a spreading, life-threatening infection develops, known since ancient times as sepsis.

The microbial residents of the stomach and most of those in the small intestine come from the mouth, arriving with saliva and food. These microbes--including streptococci, lactobacilli, and a few fungal species--are relatively sparse, generally less than 10,000 microorganisms per milliliter of intestinal contents. In the lower small intestine (the ileum), some additional species from the large intestine (or colon) migrate up, adding to the resident microbes' numbers and complexity. The most dramatic change, however, occurs in the colon itself, which is separated from the small intestine by a sphincterlike structure known as the ileocecal valve.

The colon is replete with microorganisms, mostly bacteria, with total numbers in the range of 100,000,000,000,000. Their dense concentration there approaches the theoretical limit of what can fit into the given space. More than five hundred different species of bacteria regularly reside in the colon of every living person. Functioning in virtual obscurity from its host, the flora does its daily work of fermentation and metabolism. The colon is a dark and dank place, virtually devoid of atmospheric oxygen, and its bacteria are mostly strict anaerobes--that is, they are adapted to live without oxygen and in fact would perish in its presence. The anaerobes outnumber the other bacteria, such as the well-known Escherichia coli, commonly called E. coli, by a factor of 1,000 to 1. Hence, the colon is a one-liter anaerobic fermentation vessel that receives nutrients--undigested foodstuffs such as grains, complex carbohydrates, and cellulose--via the regular propulsive motions of the small intestine. This is the good side of the Dr. Jekyll/Mr. Hyde life of our colonic bacteria. Their ability to digest nutrients that have escaped absorption in the small intestine (because they could not be broken down by the enzymes there) provides us with important sources of energy, proteins, and vitamins that would otherwise be lost in fecal evacuations.

The sinister side of the flora is displayed when bacteria manage to penetrate the intestinal wall, causing local destruction and then widespread infection. Perhaps the most common ways bacteria escape involve penetration of the bowel by a knife or bullet wound, or a crush injury, as in an automobile accident. The breach permits the microflora to spread to the organs in the peritoneal cavity and the surrounding muscle and fat. Damage to the intestinal wall can also be caused by a disease process in the intestine itself--for example, a perforated appendix or a colon cancer that has eroded the bowel wall. In each case, the end result is an extensive, spreading infection of the abdominal lining that is known as peritonitis. The free-ranging bacteria released from the bowel enter the bloodstream and produce sepsis, with its accompanying shock syndrome (fever, falling blood pressure, and the resultant compromise of vital organs, such as the liver and kidneys). Since multiple species of bacteria are involved, broad-spectrum antibiotics must be used to control this complex infection, along with judicious surgery to repair the bowel wall.

The intestinal wall can also be damaged by intrinsically virulent bacteria that we consume in contaminated or spoiled food or beverages. Salmonella and Yersinia preferentially invade the lower ileum, while Shigella, Campylobacter and E. coli 0157:H7 seek out the large bowel. In the early stages, only the pathogenic bacteria themselves penetrate the bowel wall, but once this damage has occurred, other members of the resident flora may join the invasion and cause more extensive disease. In most cases involving a specific food-poisoning bacterium, a targeted antibiotic is used, and the infection clears up rather promptly. If other members of the microbial flora are also involved, however, the antibiotic coverage must be expanded to include broad-spectrum drugs to treat what has become a systemic, life-threatening infection.