Chapter 22

Microbial Diseases of the Gastrointestinal Tract

Structures of the digestive system

•          Divided into two parts:

–        The gastrointestinal tract (GI tract)

•          The pathway from the mouth to the anus

–        Accessory digestive organs

•          Organs involved in grinding food or providing digestive secretions

–        Salivary glands

–        Liver

–        Gallbladder

–        Pancreas

•          The function of GI tract

–        digests food

–        absorbs nutrients and water into the blood

–        and eliminates waste

•          Components of the gastrointestinal tract

–        Mouth-site where food is moistened and chewed

–        Esophagus-tube leading to the stomach

–        Stomach-secretes HCl, enzyme:  pepsin

–        Small intestine- digestion and absorption of nutrients

–        Large intestine (colon)-completes absorption of nutrients: water, steroids and minerals

–        Rectum and anus eliminate waste

Normal Microbiota of the Digestive System

•          Tongue, teeth, small intestine, colon, rectum

–        Heavily colonized with various bacteria

-      The largest number found in the colon

–        Oral cavity contains up to 700 different bacterial species

•          Esophagus, stomach, duodenum

–        “almost sterile” – how can that be?

–        HCl in stomach; peristalsis and the rapid transport of food helps prevent colonization by microbes

MICROBIAL DISEASES OF THE DIGESTIVE SYSTEM

Dental Caries (Tooth Decay)

•          Accumulation of microbes on the surface of the teeth is called dental plaque -biofilm.

The plaque is made up of streptococci (Streptococcus mutans) and filamentous bacteria Actinomyces

•          Bacteria hydrolyze the sucrose into glucose and then use glucose to synthesize dextran (gummy polysaccharide)

•          Inside the plaque, bacteria ferment the fructose into lactic acid. Lactic acid damages the enamel of the tooth

•          Bacteria invade the dentin (calcified tissue of a tooth)

•          Bacterial population inside the dentin is composed of Gr+ bacilli and filamentous bacteria (about 300 bacterial species are identified)

•          If not treated, the tooth decay advances to pulp and roots (contain blood vessels and nerves)

•          The infection can spread to the neighboring soft tissue

•          Prevention of dental caries - reducing the amount of sucrose in the diet and, proper brushing and flossing

Periodontal Disease

•          Initial stage – gingivitis

–        Symptoms: swelling, bleeding of gingiva, formation of pockets

•          Advanced stage – periodontitis

–        Symptoms: tissue destruction, formation of deep pockets, build up of calculus (deposits of CaCO3), loosening of teeth, bone loss

•          Causative agent: polymicrobial biofilm

Mumps

•          Causative agent: Mumps virus - paramyxovirus

•          Disease of parotid (salivary) gland (located bellow the ears)

•          The virus is acquired by respiratory rout

•          Symptoms: swelling of parotid gland, fever, pains during swallowing

•          Complications: inflammation of testis (possible sterility), meningitis, inflammation of ovaries

•          Available vaccine (MMR).

Virulence factors:

•          The virus induces the infected cell to fuse with the adjacent cell – a large multinucleate cell is formed (syncytia)

•          The virus particles freely pass from cell to another – evading the antibodies

Bacterial Infections

Gastric Ulcer

•          Lesions in the mucus membrane of the stomach Caused by Helicobacter pylori

•          The bacterium can survive in the acidic environment in the stomach by producing urease that neutralizes the acid               

•          H. pylori causes destruction of the protective mucus layer that leads to erosion of the wall

•          Symptomes: Abdominal burning pain

Diagnosing Helicobacter

•          Visually by endoscopy – a flexible tube is inserted through the mouth into the stomach

•          ELISA test – blood is tested for the presence of antibodies against Helicobacter

Acute Diarrhea

•          A frequent loose or liquid bowel movement

•          An average person experiences 1.2 to 1.9 cases of diarrhea per year

•          More frequent in children

•          In tropical countries children experience more than 10 episodes of diarrhea per year

•          Cause: Food contaminated with bacteria and/or their toxins

Salmonellosis

•          Caused by Salmonella – Gr-, facultative anaerobic rods, there are about 2000 serotyps

•          Bacteria invade the intestinal mucous membrane. From there they can enter the lymphatic system.

•          Incubation time is 12-36h but it depends on the number of cells ingested.

•          Recovery - after few days, typically self-limited infection

•          Antibiotics not effective

•          Source of infection: poultry, eggs, egg products

Typhoid fever

•          Caused by Salmonella typhi

•          Bacteria multiply in phagocytic cells; can be found in blood, urine feces.

•          Incubation period is up to 3 weeks.

•          Symptoms: Headache, high fever and diarrhea (after 2nd week)

•        Susceptibility: Travel in areas with poor sanitation

•          Patients can become chronic carriers of the diseases. They shed bacteria from their gall bladder

•          Treatment: Antimicrobial drugs

-      Vaccine available - not used routinely

E. coli O157:H7

•          Considered as an emerging pathogen – first recognized in 1980

•          Symptoms: gastroenteritis, bloody diarrhea, anemia, kidney damage, blindness, seizure and stroke

•          Virulence factors

–        Shiga toxin - gene introduced through phage-mediated transfer

–        Type III secretion system (set of proteins)

•          Intimin enables E. coli to attach to the host cell

•          insert bacterial products into the host cell

•          Transmission: ingestion of contaminated and undercooked beef, lettuce, and vegetables

•          Treatment: supportive therapy, antibiotics of no use

Cholera

•          Causative agent: Vibrio cholerae, slightly curved rod with a single polar flagellum.

•          The bacterium grows on the epithelial cells in the small intestine (does not enter the host cell)

•          Produces enterotoxin - causes excessive secretion of water and electrolytes.

•          Symptoms: severe diarrhea with sudden loss of fluids, shock, collapse, and even death.

•          The blood becomes very viscous.

•          Treatment: replacing lost fluid and electrolytes, tetracycline

•          Mortality rate:

–        Untreated – 50%

–        Treated – 1%

Staphylococcal food poisoning

•          Caused by Staphylococcus aureus - lives in nasal cavity

•          Food provides favorable environment for multiplication of bacteria (if left at room temperature)

•          S. aureus can outgrow other competing bacteria (tolerates higher osmotic pressure, higher temperatures, low moisture level).

•          Foods such as custards, sauces, processed meats – contaminated and left unrefrigerated

•          Toxins are released. Taste and smell of the food is not altered

•          Further cooking can eliminate  bacteria but not the toxin

•          The toxin damages the tissues

•          Symptoms of intoxication: abdominal cramps, vomiting, and diarrhea

•          The disease is self-limiting, recovery within 24h

Protozoan Infection

Cryptosporidiosis

•          Cryptosporidium hominis (infects humans)

•          Contracted by ingestion of the oocysts; drinking or recreational water

•          Water contamination – animal waste

•          Oocysts develop into sporozoits – embed in the epithelial cells of the intestine

•          Symptoms: diarrhea

•          Treatment:  usually not requited in the case of healthy individuals; antidiarrheal agent

Acute Diarrhea with Vomiting
-Food Poisoning

•          The symptoms are violent and incubation period is very short 1-6 h

•          Diseases caused by ingestion of a preformed toxin (bacterial or some other)

Giardiasis

•          Causative agent Giardia lamblia

•          A flagellated protozoan

•          Infection via contaminated water

•          Attaches to intestinal wall, interferes with food absorption

•          Symptoms: nausea, intestinal gas, weakness, abdominal cramps, diarrhea 

•          Healthy carriers shed cysts in their feces

•          Prevention: avoid consuming untreated water (backpackers in wilderness)

•          Treatment: Metronidazole

Viral Infections

Hepatitis

•          Hepatitis is an inflammation of the liver caused by an infection with viruses

•          There are five known viruses that can cause hepatitis: (Hepatitis A, B, C, D, E)

•          Symptoms include jaundice, abdominal pain, fatigue, nausea, vomiting, appetite loss

Hepatitis A

•          Causative agent: Single stranded RNA virus, no envelope

•          Infection is acquired via oral rout (fecal contamination of food, water, and oysters)

•          In 50% of cases symptoms are subclinical

•          Symptoms: anorexia, nausea, fever, abdominal discomfort, fever, and chills, and jaundice

•          Vaccine is available

Hepatitis B

•          Causative agent: Hepatitis B virus (HBV), double stranded DNA virus with the envelope.

•          Transmission: break in the skin - body fluids (blood, saliva, breast milk, and semen)

•          Multiplies exclusively in the liver

•          Symptoms: loss of appetite, fever, joint pains, and jaundice.

•          10% of patients become carries (reservoirs of the disease). They are more likely to get liver cancer.

•          Vaccination recommended for health care professionals and other high-risk groups.

•          Vaccine is obtained by genetic engineering.

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