Microbiology, 2021

986 24 • Digestive System Infections
Infections and intoxications of the lower GI tract often produce symptoms such as nausea, vomiting, diarrhea,
aches, and fever. In some cases, vomiting and diarrhea may cause severe dehydration and other complications
that can become serious or fatal. Various clinical terms are used to describe gastrointestinal symptoms. For
example, gastritis is an inflammation of the stomach lining that results in swelling and enteritis refers to
inflammation of the intestinal mucosa. When the inflammation involves both the stomach lining and the
intestinal lining, the condition is called gastroenteritis. Inflammation of the liver is called hepatitis.
Inflammation of the colon, called colitis, commonly occurs in cases of food intoxication. Because an inflamed
colon does not reabsorb water as effectively as it normally does, stools become watery, causing diarrhea.
Damage to the epithelial cells of the colon can also cause bleeding and excess mucus to appear in watery
stools, a condition called dysentery.
CHECK YOUR UNDERSTANDING
• List possible causes and signs and symptoms of food poisoning.
24.2 Microbial Diseases of the Mouth and Oral Cavity
Learning Objectives
By the end of this section, you will be able to:
• Explain the role of microbial activity in diseases of the mouth and oral cavity
• Compare the major characteristics of specific oral diseases and infections
Despite the presence of saliva and the mechanical forces of chewing and eating, some microbes thrive in the
mouth. These microbes can cause damage to the teeth and can cause infections that have the potential to
spread beyond the mouth and sometimes throughout the body.
Dental Caries
Cavities of the teeth, known clinically as dental caries, are microbial lesions that cause damage to the teeth.
Over time, the lesion can grow through the outer enamel layer to infect the underlying dentin or even the
innermost pulp. If dental caries are not treated, the infection can become an abscess that spreads to the
deeper tissues of the teeth, near the roots, or to the bloodstream.
Tooth decay results from the metabolic activity of microbes that live on the teeth. A layer of proteins and
carbohydrates forms when clean teeth come into contact with saliva. Microbes are attracted to this food source
and form a biofilm called plaque. The most important cariogenic species in these biofilms is Streptococcus
mutans. When sucrose, a disaccharide sugar from food, is broken down by bacteria in the mouth, glucose and
fructose are produced. The glucose is used to make dextran, which is part of the extracellular matrix of the
biofilm. Fructose is fermented, producing organic acids such as lactic acid. These acids dissolve the minerals
of the tooth, including enamel, even though it is the hardest material in the body. The acids work even more
quickly on exposed dentin (Figure 24.7). Over time, the plaque biofilm can become thick and eventually calcify.
When a heavy plaque deposit becomes hardened in this way, it is called tartar or dental calculus (Figure 24.8).
These substantial plaque biofilms can include a variety of bacterial species, including Streptococcus and
Actinomyces species.
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