HUMAN BIOLOGY

210 Chapter 11

11.10 infections in the Digestive system

Because it opens at both ends to the outside world, the

GI tract is a convenient portal into the body for bacteria,

viruses, and other pathogens that contaminate foods,

water, and hands.

Diarrhea, or watery feces, is a common effect of an

intestinal infection. Diarrhea can develop when an irritant

(such as a bacterial toxin) causes the lining of the small

intestine to secrete more water and salts than the large

intestine can absorb. It can also develop when infections,

stress, or other factors speed up peristalsis in the small

intestine, so that there isn’t time for enough water to be

absorbed. Section 3.12 mentioned the dangerous bacterial

disease cholera. Much more common is diarrhea caused

by a rotavirus (Figure 11.18A). Public health authorities

estimate that by age 2, most children have had at least one

rotavirus infection. Diarrhea is a cause for concern in small

children because they easily become dehydrated, losing

water and salts that nerve and muscle cells need to function

properly. Figure 11.18B shows the protozoan Giardia intes-

tinalis, which causes giardiasis. It forms cysts that enter

water or food in contaminated feces. Symptoms include

explosive diarrhea and “rotten egg” belches.

Several harmful strains of E. coli bacteria infect the GI

tract (Figure 11.18C). One of them, called O157:H7, normally

lives in the intestines of cattle. If a person eats ground beef

or some other food that is contaminated with this microbe,

it can cause a dangerous form of diarrhea that is compli-

cated by anemia. A few cases have led to kidney failure

and death.

Bacteria that cause dental caries—tooth decay—

flourish on food residues in the mouth, especially sugars

(Figure 11.19A). Daily brushing and flossing are the best

way to avoid a bacterial infection of the gums, which can

lead to gingivitis (jin-juh-vy-tus). This inflammation can

spread to the periodontal membrane that helps anchor each

tooth in the jaw. Untreated periodontal disease can slowly

destroy a tooth’s bony socket, which can lead to loss of the

tooth and other complications.

A peptic ulcer is an open sore in the lining of the

stomach or small intestine. Most are caused by the

bacterium Helicobacter pylori (Figure 11.19B). It produces

a toxin that inflames the stomach lining and causes

damage that allows hydrogen ions and pepsins to dif-

fuse into the lining—and that does further damage.

Antibiotics can cure peptic ulcers caused by H. pylori.

They don’t help with the 20 percent of ulcers related to

factors such as chronic stress, smoking, and overuse of

aspirin and alcohol. H. pylori also is responsible for some

cases of gastritis (an inflammation of the GI tract) and

stomach cancer.

If you have ever had a case of “food poisoning,” your

stomach or intestines have been colonized by bacteria such

as Salmonella, which can contaminate meat, poultry, and

eggs (Figure 11.20).

Figure 11.18 intestinal

infections can cause

diarrhea. a Rotavirus

particles isolated from a

diarrhea sample. B Giardia

intestinalis. c E. coli 15 7: H 7.

(A: CDC/Byron Skinner; B: Dr. Stan

Erlandsen, University of Minnesota;

C: Courtesy of Dr. Michael S. Donnenberg)

B c

Figure 11.20 Food and water may also harbor bacteria.

Shown at left is a colored image of Salmonella bacteria (gold)

on a stainless steel surface, such as that of a kitchen knife.

(Left: Dr. Tony Brain/Science Source; Right: © Cengage Learning/Gary Head)

Figure 11.19 Various bacteria

infect the mouth and stomach.

a Bacteria on a human tooth.

B Helicobacter pylori, the bacterium

that causes most peptic ulcers.

a

B

A: Stanley Flegler/Visuals Unlimited, Inc.; B: Peter Hawtin, University of Southampton/Science Source

a

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