Lower Alimentary System Infections

ADVANCE ORGANIZER

Amongst the most common and the most severe infections of humans are gastrointestinal infections of the alimentary tract. Many organisms have adapted so that they can get through the stomach intact. Some especially insidious parasites even use the acid condition of the stomach to signal their exit from protective cysts. Transmission of diseases of this sort is often by the fecal-to -oral route. That is, the next host gets the disease from eating food or drinking water or putting contaminated items such as fingers into their mouth. This is sometimes complicated by their being human carriers of these enteric infections that transmit disease for several years. Diarrhea resulting from these diseases is responsible for millions of deaths worldwide each year. Food poisoning can be caused either by infectious microorganisms consumed with food or by toxic products remaining after microbial growth in food.

READING ASSIGNMENT: CHAPTER 24

ANATOMY AND PHYSIOLOGY
The chief function of the lower alimentary system is breakdown of food to subunits of macromolecules, absorbing nutrients and recycling fluids. The liver produces bile which is inhibitory for many bacteria and helps with digestion of fats and vitamins. Bile causes the brown color of feces after it is acted on by intestinal bacteria. The pancreas produces insulin and alkaline fluid containing digestive enzymes. The small intestine secretes digestive juices and absorbs nutrients. The large intestine helps recycle body water. It contains large numbers of microorganisms. Many of these are opportunistic pathogens.

NORMAL FLORA
The small intestine has few microorganisms. The large intestine has lots. In fact, microorganisms make up about 1/3 of the weight of feces. Most common are Lactobacillus and Bacteroides, both anaerobes. The microbes in the digestive tract are responsible for the synthesis of vitamins, degradation of indigestible substances, production of cholesterol, chemical alteration of medications and unfortunately, carcinogen production. As with other normal flora they can competitively inhibit the growth of pathogens.

BACTERIAL DISEASES OF THE DIGESTIVE TRACT

Cholera is a severe form of diarrhea caused by a toxin of Vibrio cholerae. It is a curved, facultative anaerobic gram negative rod The exotoxin acts on the epithelium of the small intestine causing an outpouring of fluid without producing damage to the intestinal cells. Cholera toxin catalyzes the transfer of an adenosine diphosphate ribose unit to an arginine residue of the stimulatory G protein [Gs]. This modification is called ADP-ribosylation. It blocks the hydrolysis of GTP to GDP bound to the alpha subunit of Gs. This locks the Gs in the active state. Adenylate cyclase becomes persistently active and cyclic AMP levels rise. The consequent activation by phosphorylation of chloride channels in the intestinal epithelial cells causes the efflux of Na+ and Cl- and water.
Shigella , a non-motile, lactose non-fermenting enterobacteria can invade the colon epithelium. This is a cause of dysentery. Only some strains make Shiga toxin.
Gastroenteritis is often caused by salmonellae of animal origin. The organisms often enter the intestinal tract with food–often (dare I say, usually) poultry and eggs although wild and domestic animals are also sources of human infection. These lactose non-fermenting, citrate-utilizing, H25 producing enterobacteria come in 2000 serotypes. Many of them exhibit phase variation.
Gastroenteritis can be caused by some strains of E. coli, a lactose fermenting enterobacterium. It is those strains of E. coli that have been transformed with virulence factors. This means that the plasmids and the virulence can be transferred to other entericorganisms.There are five main groups of pathogenic E. coli:
ETEC [enterotoxigenic E coli]make toxins that cause excessive water and electrolyte secretion (another form of traveler’s diarrhea). The toxin is very much like cholera toxin in its action and structure.
EPEC [enteropathegenic E. coli] cause alteration of intestinal cell structure and are a common cause of nursery epidemics, Stick to the brushy edge of lining cells and destroy projections of the brush border of the intestine.
EIEC [enteroinvasive E. coli] invade intestinal epithelium and can cause dysentery. They have a plasmid based invasion factor the penetrates the gut epithelium and casues infection underneath.
EHEC [enterohemorrhagic E. coli] possess a “Shiga-like” toxin and can cause the hemolytic-uremic syndrome. Also called verotxoin because it kills Vero cells in tissue culture.
EaggEC [enteroaggregative E. coli] These are named for their ability to stick in clumps to gut surfaces. Important in persistant infantile diarrhea.

Diarrhea, when caused by bacteria, is most often caused by Campylobacter jejuni. a curved gram negative bacterium. Like salmonellae, it usually originates from domestic animals. It can, like the shigellae and certain strains of E. coli cause dysentery with fever, headache and shock.

Typhoid fever is caused by Salmonella typhi which is a serological type of S.choleraesuis. Symptoms include high fever, headache and abdominal pain. If left untreated, the death rate is high. Human carriers maintain the disease It is pathogenic ONLY for humans. There is a new oral attenuated vaccine to help to prevent this disease.

Food poisoning caused by toxic (by-) products of microbial growth in food.
Staphylococcal food poisoning is caused by certain strains of Staphylococcus aureus is commonest. The toxin is heat stable.
Food poisoning can be caused by
Clostridium perfringens (anaerobic, encapsulated often found in meat, toxin is released in the intestine when sporulation occurs)

and
Bacillus cereus (aerobic Gram positive, spore forming rod often found in rice.

Both are gram positive rods.Botulism occurs in three forms: food, intestinal and wound. It is characterized by paralysis and commonly results in death. It is caused by Clostridium botulinum, an anaerobic, Gram positive spore forming rod.
FUNGI CAUSED POISONING

Ergot poisoning can be caused by the growth of certain fungi on grain.
Aflatoxins are produced by certain Aspergillus. The result is liver damage and certain cancers.

VIRAL INFECTIONS OF THE LOWER ALIMENTARY TRACT

Hepatitis

Hepatitis A is spread by fecal contamination of hands, food, or water. Most cases are mild and may be asymptomatic. Antibody to the virus is widespread in the population.
Hepatitis B is spread mainly by blood and blood products, sexual intercourse and from mother to infant. It is fairly severe. Chronic infection is rather frequent and can lead to liver cancer. Carriers are common and can have infectious virus in their blood streams without knowing it.
Hepatitis C cases most cases of transfusion associated hepatitis.
Hepatitis delta virus requires Hepatitis B to replicate–since it is a defective RNA virus.
Hepatitis E is an RNA virus transmitted by the fecal-oral route and can cause fatalities, especially amongst pregnant women.
Also togaviruses and flaviviruses
Viral gastroenteritis
Rotoviruses and Norwalk viruses are the most important causes of viral gastroenteritis. Rotoviruses mainly affect young children and can cause traveler’s diarrhea. Norwalk viruses cause about half of the TOTAL U.S. gastroenteritis outbreaks.

PROTOZOAN DISEASES OF THE LOWER ALIMENTARY SYSTEM

Giardiasis is caused by Giardia lamblia. This flagellated protozoan with an adhesive disk is usually transmitted by drinking water contaminated by feces of humans and wild animals. It has a worldwide distribution and is a common cause of traveler’s diarrhea. This is the most common water-borne illness.
Entamoeba histolytic a is a major cause of dysentery. Amebiasis is frequently chronic and causes intestinal ulceration. The infection can spread to the liver.