which snuggle unmolested in its depths. Furthermore, some vessels give the bacteria a slower ride through the acid. Small amounts of carbohydrates may pass through the stomach in thirty minutes or less. A high-fat meal may take several hours to move on.
With the exception of organisms such as Helicobacter pylori, which causes ulcers and is probably transmitted through contaminated food and water, bacteria rarely find the stomach sufficiently hospitable to take up residence there. Toxins are a different matter. They may irritate the stomach directly or may slip off to one’s neural vomit center, where they stimulate a relaxation of the cardiac sphincter and esophagus, with simultaneous contraction of the abdominal muscles and viscera. Throwing up is an apt description; one might imagine a small but powerful baseball player hiding in one’s depths.
Toxins formed by the bacteria Staphylococcus aureus (which hang around many people’s noses) and Bacillus cereus (which prefer rice fields) may attack in this way. Metals such as copper, zinc, tin, and cadmium, if ingested in high doses, may do something similar. We might ingest such metals through leaching from a container or because our crops were grown in industrially contaminated soils.
Now that your food has survived a trip through the stomach, your gastronomic happiness rests in the hands of the small intestine, a 23-foot-long glistening, slithery tube. Although many toxins that can damage the stomach also attack this part of the gut, it is here that infectious agents such as bacteria and viruses first enter the fray. To describe this place appropriately, one must stoop to the cryptic and bilious language of darkness and melodrama. Enzymes are secreted by the pancreas and by cells in the crypts lining the long, serpentine halls. Bile from the liver enters the first part of the small intestine, the duodenum, and makes the fat soluble. Food that is broken down is ingested through the absorptive cells clustered on the folds, called villi, which, for bacteria, is the plural of villa, a great place to luxuriate.
Farther along, in the lower part of the ileum, most of the fluids and electrolytes mentioned earlier are absorbed. Few bacteria live naturally in the upper small intestine. Buffeted by peristaltic waves and rained upon with bile salts, they tend to move on in search of more hospitable environments. As well, the normal bacterial burghers of the lower small intestine fill up the available living space and effectively exclude, in most cases, all but the most aggressive newcomers. This phenomenon has been called competitive exclusion and is the rationale behind eating friendly bacteria, say, from yogurt, to prevent the pathogenic, disease-causing ones from moving into your neighborhood.
The small intestine is the central starting place for most foodborne illnesses. Nausea and vomiting, as in the stomach, still occur but are less common the farther into the bowels the intruders descend, at which point abdominal cramps and diarrhea predominate.
One group of bacteria, the Yersinia (related to the organism that causes human plague), has two members, Y. enterocolitica and Y. pseudotuberculosis, that are associated with food. In 1978, an intern from the Centers for Disease Control in Atlanta, Georgia, was going over records of appendectomies in Oneida County, New York. (This is the sort of sport in which an epidemiologist might engage during his or her spare time.) He found a definite peak in September 1976, and many of the sick kids came from a couple of small villages. His curiosity aroused, he dug deeper and found that in September and October of that year over two hundred children were sick with abdominal illness; thirty-six of them were hospitalized, and sixteen had appendectomies. This epidemic of apparent appendicitis was eventually traced back to chocolate milk provided in the school cafeteria. The milk was probably contaminated when the chocolate was mixed into the pasteurized milk in open vats. The ability to mimic appendicitis by infecting the tissues around the appendix is a particular trademark of this organism. People have succumbed to yersiniosis after drinking contaminated water or milk or eating a wide variety of foods, including tofu, chocolate syrup, raw pork, and—to put an old twist on an old saw—yellow snow.
While we are talking about what foodborne diseases do in the human gut, I must add something else: THERE IS NO SUCH DISEASE AS THE STOMACH FLU.
There. I have said it. When some viruses, with names like noroviruses and rotaviruses, invade the small intestine, they can cause diarrhea. Most of these viruses come from other peoples’ butts. They get into water and into food, and they travel on handshakes. Not a nice thought. The flu vaccine won’t protect you. Wash your hands.
Bacteria or viruses that manage to cause disease in the intestine often have special antigens, or proteins. Like specialized claws, these enable them to cling to the cell walls. Bacteria such as Bacillus cereus, Clostridium perfringens, and various strains of toxin-producing E. coli will attach themselves to the intestinal walls. There they multiply and produce toxins (called enterotoxins) that mimic the body’s own secret code words (adenyl cyclase, for instance, will open a few doors), stimulating secretion of fluids into the intestines. Diarrhea and dehydration are the result.
Other bacteria or toxins such as Salmonella, if ingested in large numbers, may simply wreak wanton destruction on the cells lining the intestinal villi, even without attachment. In these cases, the food cannot be absorbed. Or the bacteria may themselves slip past the guards, invade the blood circulation, and knock the whole body back with a generalized illness (septicemia).
In some special cases, such as infant botulism, the normal, well-behaved bacterial citizens of the intestine have not yet moved in to set up shop, leaving their ecological niche wide open for opportunists. In other cases, antibiotics may be used, and like indiscriminate gut developers, they lay waste to particular ecological niches, where the pathogens can then multiply.
Parasites such as the tapeworms Taenia saginata and Taenia solium will attach themselves to the intestinal wall and feed, resulting in a chronic intestinal upset.
Many foodborne illnesses result from the absorption of toxins or viruses from the intestine into the blood; they do not cause disease in the gut itself but slip past the body’s many defenses into the circuitous channels of blood to attack, say the liver (hepatitis A) or the nervous system (paralytic shellfish poisoning). Others, like the parasites Toxoplasma and Trichinella, enter the bloodstream and, like imperialistic Europeans, invade all parts of your body, starting small communities in the warm and nutritious New World of your flesh. The most serious foodborne toxins attack your nervous system, resulting in symptoms ranging from tingling, numbness, and vertigo to fatal paralysis.
In some cases, disease is caused not so much by the invader itself as by the immune response of the body. Hepatitis A doesn’t usually cause serious disease in children, for instance, many of whom pick up the virus in day-care centers; children’s immune systems are not well developed, and the viruses move in without causing major damage. The parents of kids from daycare centers are the ones at risk of getting very sick; when they get infected from their kids, their bodies attack the infected liver cells and they get jaundice.
With food allergies, the intruder may seem small and inoffensive, but the body engages in a vitriolic and self-destructive war against it. There are a variety of ways to get food allergies. If you expose enough people to a food intensively, over a long period, then some of them will get allergies. That is in part why peanut allergies occur in cultures where people eat lots of peanuts and milk allergies where people drink lots of milk. Foods that are described as “nonallergenic” in North America, and that are eaten by people with food allergies, may be common causes of allergies in other parts of the world.
Some food allergies may be acquired when your body is exposed to food molecules in unexpected ways. Let’s say that you suffer a bout of bacterial diarrhea after eating undercooked shrimp. During your illness, your gut may be damaged, and some rather large shrimp molecules may barge their way across the wall from the lumen of the intestine (which, remember, is technically outside your body) into the inner sanctum of your blood and tissues. Your body, seeing these large, predigested shrimp molecules as foreigners, in the same general
