Intestinal Flora

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INTESTINAL FLORA. "The entire world is covered with a layer of feces. Granted it is thicker in some places than in others, but a solid layer, nevertheless. . . ." This is how a wizened professor used to begin his clinical parasitology course for microbiology graduate students. Naturally all the students laughed, only to learn during the ensuing months that the statement is profoundly true. The lesson also bears truth in that fecal microorganisms arise from within the bowels of the digestive tract. The human intestine is home to an invisible and remarkable biosphere of living organisms dedicated to preserving its own existence. Humans serve as host to these billions and trillions of organisms that function in effect as a single living entity.

In fair exchange, during our lifespan, our gut flora provides us with health, protects us from disease, and serves as the major labor force to digest almost everything we eatfrom artichokes to zebu.

The population of microbes that inhabit our intestines is made up of hundreds of different species of bacteria and other organisms. By far the vast majority of these are anaerobic, which means they do not multiply in the presence of oxygen. Since few if any anaerobes are defined as classic pathogens, by and large these anaerobes are only of interest to culinarians because they function to metabolize and break down what we eat and make it available to be absorbed and used as fuel and energy for our own body. Other species that require air (oxygen) to live are more commonly known by the general public, and a few have even achieved stardom, mainly because of their predilection to cause disease. Thus, certain bacteria such as staphylococcus, E. coli, salmonella, shigella, enterobacter, and others have become well recognized, if not fearedalmost to the point of being a phobiaby some people, especially in the United States. Most people seem to be confused by too many overstated, highly publicized warnings, along with too many recommendations from too many different sources.

A list of the scientific names of all the different microorganisms that inhabit the human gut, sometimes described as autochthonous flora, would be very lengthy. Some understanding of science is required, however, in order to appreciate the very complex relationship we have with the microbial world living inside and on our bodies, which in turn help maintain the delicate balance between health and disease. Most important is the fact that each living human being has a rather steadfast and distinct microbial profile. This profile is almost as identifiable as a fingerprint.

When foreign bacteria are introduced to our profile, the ecosystem reacts rather quickly to disallow these species to proliferate. Accordingly, disease is not a normal finding; rather, we, for most of our existence, maintain ourselves in a general state of good health.

Some remarkable studies have demonstrated that even when our intestinal microbiologic profile is disrupted to the point of causing disease, for instance, in the case of traveler's diarrhea, the body mounts a tremendous effort to return itself to its normal healthful state, and in doing so somehow the original microbial profile returns. What we eat and how old we are does play a major role in the overall state of our live-in microbial population, and on occasion shifts of our profile do occur. For example, this happens when the microbes adjust to accommodate the various types of food we ingest. So, if we eat a diet of all starches, those species of organisms which thrive on starches will increase their relative numbers in relation to the frequency and amount of starch which we consume. When one considers diseases that are due to intestinal microbiota, it is also important to know what "pathologic bolus" means. Simply put, this phrase refers to the minimal number of pathogenic organisms needed to be ingested in order to cause a specific disease. It is usually expressed as numbers in powers of ten per gram or milliliter of menstruum. This number ranges from very large to very small, depending on the specific organism and disease. So, while certain diseases require huge numbers of bacteria in order to initiate illness, others require very few bacteria.

For example, not all salmonella species are pathogenic, and those that are generally require that a large pathologic bolus be ingested to produce illness. On the other hand, most species of shigella are intrinsically "pathogenic," and it takes only a small number to cause symptoms. It has been found, in this example, that salmonellae generally are susceptible to destruction by acidity of the stomach, and few survive to enter the intestines where the actual infection takes place. Shigellae, on the other hand, are able to withstand the acidity of our gastric juices and arrive in the intestine viable and ready to set up (unwanted) residence and cause disease.

Other major factors also play heavily in defining health and disease. Paramount to this struggle are the status of our overall nutritional habits and secondly the maintenance of the immune system and understanding how it functions to protect us from invading organisms.

In general, well-fed people are less likely to catch infectious diseases of any sort, and with some exceptions, the incidence and severity of dysenterylike diseases are also lessened if we follow a well-balanced diet. The amazing increase in the average height of individuals after the introduction of better food and balanced diets in certain Third World populations, which occurred in one single generation, is testimony to the tremendous impact diet plays in this regard. Individuals who suffer from underlying diseases and conditions that compromise the normal function of our immune system are much more vulnerable to life-threatening diseases caused by bacteria, viruses, and other organisms than are average citizens. Likewise, those who have lost protection due to impairment or destruction of the skin are much more likely to become ill. The causative agents in these cases are produced not only by recognized pathogens, but also come from normally benign species of so-called normal (or commensal) flora. Almost 90 percent of mortalities in burn patients are due to infection and sepsis. Furthermore, most of these deadly infections are produced by the patient's own intrinsic internal microorganisms. Notable for this discussion is that most foodborne diseases are indeed usually attributed to microbial species of the enteric type"enteric" meaning those normally found in the digestive tract. When they are allowed to proliferate in nutrient-rich unrefrigerated foods such as potato salad, bacterial and viral "food poisoning" is likely, and outbreaks continue to afflict even the most civilized nations.

The most common cause of both direct and crosscontamination of foods, which ultimately can lead to such outbreaks, is unofficially labeled in medical vernacular the "fecal/hand/mouth" routenot a polite description, but accurate. Furthermore, although refrigeration and proper storageand to a lesser degree chemical disinfectionare important, human hands remain the most villainous of vectors of food-borne disease. More important, frequent handwashing with regular soap and hot water remains the absolute best means of prevention.

See also Digestion; Eating: Anatomy and Physiology of Eating; Health and Disease.


Blank, Fritz. "Food on the Move: Travelers' Diarrhea: The Science of 'Montezuma's Revenge.' " Proceedings of the Oxford Symposium on Food and Cookery. Oxford, U.K., 1996.

Schaedler, Russell W., and René Dubos. "The Ecology of the Digestive Tract." Proceedings of the Cholera Research Symposium. Washington, D.C.: U.S. Department of Health, Education and Welfare, 1965.

Dubos, René, and Russell W. Schaedler. "Some Biological Effects of the Digestive Flora." American Journal of the Medical Sciences 244, no. 3 (September 1962).

Fritz Blank

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intestinal flora Bacteria and other micro‐organisms that are normally present in the gastro‐intestinal tract.

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intestinal flora (in-test-in-ăl) pl. n. bacteria normally present in the intestinal tract. Some are responsible for the synthesis of vitamin K.