1. The alimentary tract is the passageway from the mouth to the anus. It is a major route for pathogens to enter the body.

2. The alimentary system is composed of the mouth and teeth, saliva-producing glands, esophagus, stomach, small intestine, liver, pancreas, and large intestine.

24.2 Normal Flora

The Mouth

1. The species of bacteria that inhabit the mouth colonize different locations depending on their ability to attach to specific receptors.

2. Dental plaque consists of enormous quantities of bacteria of various species attached to teeth or each other. (Figure 24.3)

3. The presence of teeth allows for colonization by anaerobic bacteria.


1. The normal fasting stomach is devoid of microorganisms.

2. Microorganisms make up about one-third of the weight of feces.

3. The biochemical activities of microorganisms in the large intestine include synthesis of vitamins, degradation of indigestible substances, competitive inhibition of pathogens, chemical alteration of medications, and production of carcinogens.

Upper Alimentary System Infections

24.3 Bacterial Diseases of the Upper Alimentary System

Tooth Decay (Dental Caries) (Table 24.1)

1. Dental caries is caused mainly by Streptococcus mutans involved in formation of extracellular glucans from dietary sucrose.

2. Penetration of the calcium phosphate tooth structure depends on acid production by cariogenic dental plaque. S. mutans is not inhibited by acid and stores fermentable intracellular polysaccharide. (Figure 24.4)

3. Control of dental caries depends mainly on supplying fluoride and restricting dietary sucrose. Dental sealants fill the pits and crevices in children's teeth.

Periodontal Disease (Table 24.1)

1. Periodontal disease is caused by an inflammatory response to the plaque bacteria at the gum line; it is mainly responsible for tooth loss in older people. (Figure 24.5)

Trench Mouth (Table 24.1)

1. Trench mouth, or acute necrotizing ulcerative gingivitis (ANUG), can occur at any age in association with poor mouth care. (Figure 24.6)

Helicobacter pylori Gastritis (Table 24.2)

1. H. pylori predisposes the stomach and the uppermost part of the duodenum to peptic ulcers. (Figures 24.7, 24.8)

2. Treatment with antimicrobial medications can cure the infection and prevent peptic ulcer recurrence.

24.4 Viral Diseases of the Upper Alimentary System

Herpes Simplex (Table 24.3, Figure 24.9)

1. Herpes simplex is caused by an enveloped DNA virus. Infected cells show intranuclear inclusion bodies.

2. HSV persists as a latent infection inside sensory nerves; active disease occurs when the body is stressed.

Mumps (Table 24.4, Figure 24.10)

1. Mumps is caused by an enveloped RNA virus that infects not only the parotid glands, but the meninges, testicles, and other body tissues.

2. Mumps virus generally causes more severe disease in persons beyond the age of puberty; it can be prevented using an attenuated vaccine. (Figure 24.11)

Lower Alimentary System Infections

24.5 Bacterial Diseases of the Lower Alimentary System

Cholera (Table 24.5)

1. Cholera is a severe form of diarrhea caused by a toxin of Vibrio cholerae that acts on the small intestinal epithelium.

Shigellosis (Table 24.6)

1. Shigellosis is caused by species of Shigella, common causes of dysentery because they invade the epithelium of the large intestine. (Figure 24.14)

2. One Shigella species produces Shiga toxin, which causes the hemolytic uremic syndrome.

Escherichia coli Gastroenteritis (Tables 24.7, 24.8)

1. Virulence factors often depend on plasmids, which can transfer virulence to other enteric bacteria.

2. Some E. coli strains, such as O157:H7, originating from wild and domestic animals, can cause the hemolytic uremic syndrome.

Salmonellosis (Table 24.9)

1. Strains of Salmonella that originate from animals usually cause gastroenteritis. The organisms are often foodborne, commonly with eggs and poultry.

2. Typhoid fever is caused by Salmonella Typhi, which only infects humans. The disease is characterized by high fever, headache, and abdominal pain. Untreated, it has a high mortality rate. An oral attenuated vaccine helps prevent the disease.

Campylobacteriosis (Table 24.10)

1. Campylobacter jejuni is the most common bacterial cause of diarrhea in the United States; it usually originates from domestic animals. (Figure 24.15)

24.6 Viral Infections of the Lower Alimentary System

Rotaviral Gastroenteritis (Table 24.11)

1. Rotaviral gastroenteritis is the main diarrheal illness of infants and young children, but also can involve adults, as in traveler's diarrhea.

2. Rotaviruses are segmented RNA viruses of the reovirus family. (Figure 24.16)

Norwalk Virus Gastroenteritis (Table 24.11)

1. Norwalk virus gastroenteritis accounts for almost half the cases of viral gastroenteritis in the United States.

2. Norwalk viruses are small RNA viruses of the calcivirus family. (Figure 24.17)

Hepatitis A (Table 24.12, Figures 24.18 and 24.21)

1. Hepatitis A is usually mild or asymptomatic in children; some cases are prolonged, with weakness, fatigue, and jaundice.

2. Hepatitis A virus (HAV) is a picornavirus spread by fecal contamination of hands, food, or water.

3. An injection of gamma globulin gives temporary protection from the disease; an inactivated vaccine is available actively to immunize against the disease.

Hepatitis B (Table 24.12, Figure 24.21)

1. Hepatitis B virus (HBV) is a hepadnavirus spread by blood, blood products, semen, and from mother to baby. (Figures 24.19, 24.20)

2. Asymptomatic carriers are common and can unknowingly transmit the disease.

3. Chronic infection is common and can lead to scarring of the liver (cirrhosis) and liver cancer.

Hepatitis C (Table 24.12, Figure 24.21)

1. Hepatitis C virus (HCV) is a flavivirus transmitted mainly by blood; approximately 60% of cases are acquired from needle sharing by injecting drug abusers.

Review Questions 631

2. Hepatitis C is asymptomatic in over 60% of acute infections; more than 80% of infections become chronic.

24.7 Protozoan Diseases of the Lower Alimentary System

Giardiasis (Table 24.13)

1. Transmission of Giardia lamblia, a mastigophoran, is usually by drinking water contaminated by feces. It is a common cause of traveler's diarrhea. (Figure 24.22)

2. Its cysts survive in chlorinated water and must be removed by filtration.

Cryptosporidiosis (Table 24.14)

1. The life cycle of Cryptosporidium parvum, a coccidium, a member of the Apicomplexa, takes place in the small intestinal epithelium.

2. Its oocysts are infectious, resist chlorination, and are too small to be removed by most filters. (Figure 24.23)

3. It is a cause of many water and foodborne epidemics, and traveler's diarrhea.

Cyclosporiasis (Table 24.15)

1. Transmission of Cyclospora cayetanensis, a coccidium, is fecal-oral, via water or produce such as berries; it causes traveler's diarrhea.

2. Oocysts are not infectious when passed in feces, and thus no person-to-person spread; no hosts other than humans are known.

Amebiasis (Table 24.16)

1. Entamoeba histolytica, a member of the Sarcodina group of protozoa, is an important cause of dysentery; often chronic; infection can spread to the liver and other organs. (Figure 24.24)

2. The organism exists in ameba and cyst forms; the quadrinucleate cyst is infectious.

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