Microbiology of the Gastrointestinal Tract

7/30/2019 Microbiology of the Gastrointestinal Tract

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Microbiology of the Gastrointestinal

Tract

Microbiology departmentFK UHT


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Composition and Distribution of the

Intestinal Microflora

The intestinal microflora is a complexecosystem containing over 400 bacterialspecies. facultative anaerobes. The flora is

sparse in the stomach and upper intestine,but luxuriant in the lower bowel.

Bacteria occur both in the lumen andattached to the mucosa, but do not normally

penetrate the bowel wall .

The flora also plays a role in fiber digestionand synthesizes certain vitamins.


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The intestinal microflora prevent infection byinterfering with pathogens.

The flora includes low populations of potentially

pathogenic organisms such as Clostridiumdifficile. Antibiotics that disturb the balance of thenormal flora can support both infection byexogenous pathogens and overgrowth byendogenous pathogens.

If the bowel wall is break, enteric bacteria canescape into the peritoneum and cause peritonitisand abscesses.


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Protective Activities of the Flora

the intestinal microflora maintaining roughly

constant numbers and types of bacteria in each

area of the bowel.

The stability of normal flora both discourages

infection by exogenous pathogens and prevents

overgrowth of potentially pathogenic members.


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Antibiotics that kill off part of the intestinal flora

can disturb its balance and may open the door to

infection or pathologic overgrowth.

Normal individuals are quite resistant to

Salmonella, and a large oral inoculum is required

to initiate infection. If the intestinal flora is

suppressed by antibiotics, however, the individual

becomes much more susceptible and can beinfected by a relatively small inoculum.


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Bacterial Diarrheas

Enterotoxin-Mediated Diarrheal

Diseases

by Vibrio cholerae and ETEc

1. there is intestinal fluid loss that is related to theaction of an enterotoxin on the small bowelepithelial cells.

2. the organism itself does not invade the mucosalsurface; it colonizes the upper small bowel,adhering to the epithelial cells and elaborating theenterotoxin. The mucosal architecture remains

intact with no evidence of cellular destruction.Bacteremia does not occur.

3. the fecal effluent is watery and oftenvoluminous, so that the diarrhea can result inclinical dehydration. The fluid originates in the

upper small bowel, where the enterotoxin is mostactive.


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Gastrointestinal Disease Caused by

Invasive Bacteria

invasive bacteria exert their main impact on thehost by causing gross destruction of the epithelialarchitecture; histologic findings include mucosal

ulceration and an inflammatory reaction in thelamina propria.

The principal pathogens in this group areSalmonella, Shigella, Campylobacter, invasive Ecoli, and Yersinia. The enteric viruses also invade

intestinal epithelial cells, but the extent of mucosaldestruction is considerably less than that caused byinvasive bacterial pathogens.


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Salmonella species are a common cause of food

poisoning.

The main site of attack is the lower ileum, where

the salmonellae cause mucosal ulceration. They

rapidly make their way through the epithelial

surface into the lamina propria and enter the

lymphatics and bloodstream.

Two virulence factors are associated with intestinal

infection: one responsible formucosal invasion, and

the othercausing secretion of fluid and electrolytes

into the bowel.


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Shigella an invasive diarrheal disease of the lower bowel in

which the stool contains an inflammatory exudatecomposed of PMN cells.

invade the epithelium of the colon and causesuperficial ulceration.

Two virulence factors :

initial penetration of the mucosal surface bydestroying the brush border; the bacteria aresubsequently engulfed by invagination of the

plasma membrane.

virulence factor allows the organism to

multiply within the mucosal tissue. Mucosalulceration results, accompanied by an intenseinflammatory response in the lamina propria.

bacteremia are uncommon.


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Viral Diarrheas

The rotaviruses are a very important cause ofinfantile diarrhea. Adults may be infected virus,

but clinical disease appears almost exclusively inchildren younger than 2 years.

Calicivirus, can produce gastroenteritis in all agegroups. The initial lesion forms in the proximalsmall bowel. The mucosal architecture is damaged,with shortening of the villi and hyperplasia of thecrypts. An inflammatory exudate then appears inthe lamina propria.


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Parasitic Diarrheas

Several species of protozoa and helminths can

cause diarrheal disease, although exposure to

enteric parasites is more common in tropical anddeveloping countries.

Some of the more common causes of parasitic

diarrhea are Entamoeba histolytica, Giardia

lamblia, Strongyloides stercoralis, and the

intestinal flukes.


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Microbiology of the Gastrointestinal Tract