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Chapter: Microbiology

Bacterial infections of thegastrointestinal tract

Many organisms cause gastrointestinal infections. The most im-portant ones are organisms belonging to the genera Salmonella, Shi-gella, and Vibrio and certain types of Escherichia coli. These are ac-quired by oral route.


       Many organisms cause gastrointestinal infections. The most im-portant ones are organisms belonging to the genera Salmonella, Shi-gella, and Vibrio and certain types of Escherichia coli. These are ac-quired by oral route.

Acute gastroenteritis due to Shigella

Organisms belonging to the genus Shigella produce bacillary dysentery. They are gram negative bacilli, nonmotile organisms. There are four species, Shigella dysenteriae, Sh.flexneri, Sh.boydii, and Sh.sonnei



The infection is limited to the gastrointestinal tract. Blood stream invasion is rare. Infection is communicable. Invasion of the mucosal epithelium leads to the formation of micro-abscesses in the wall of theEsch.coli large intestine and ileum which finally results in necrosis of the mucous membrane, ulceration, bleeding and formation of pseudo membrane on the ulcer area. This consists of fibrin, leukocytes, cell debris, ne-crotic mucous membrane and bacteria.

Toxins of Shigella

Endotoxin: Upon autolysis, the organisms release LPS which causes the irritation of the bowel wall.


Shigella dysenteriae produces exotoxin which is a heat la-bile toxin that affects the gut and CNS. It acts as enterotoxin and in-creases the local concentration of cyclic adenosine mono phosphate (AMP) and results in intense and prolonged hyper secretion of water and chlorides and inhibits the re-absorption of so-dium. The gut lumen is distended with fluid, hyper motility and causes diarrhea. As a neurotoxin it acts on CNS and causes meningismus and coma mainly in children.

Shigellae produce an early non bloody, voluminous diarrhea and later dysentery with blood, mucous, and pus in stools.

Laboratory  diagnosis

Fresh stool, mucus flek and rectal swabs are collected for cul-ture. Large number of leucocytes, RBC may be seen microscopically. Selective media like Salmonella shigella agar, Deoxycholate citrate agar are used. The organisms are non lactose fermenters and are identified by biochemical test and with specific antisera. Ciprofloxacin is used for the treatment nowadays.

Acute gastroenteritis due to Escherichia coli

Generally E.coli is a normal flora of the gut of man. But it may sometimes cause gastrointestinal disease. It may range from mild, self-limiting diarrhea to hemorrhagic colitis. Such stains fall into five groups with specific serotypes and with different Pathogenic mechanisms

1.  EnteropathogenicEsch.coli(EPEC) : These cause infantileenteritis in children


2.      Enterotoxigenic Esch.coli (ETEC) : These cause community acquired diarrhoeal disease in areas of poor hygiene. It isresponsible for traveller’s diarrhea.


3.           Enteroinvasive Esch.coli (EIEC): It causes an illness similar to Shigella dysentery in all age groups


4.      Verotoxin producing Esch.coli (VTEC): It causes haemorrhagic colitis and haemolytic uremic syndrome


5.     Enteroaggrgative Esch.coli (EAggEC). It causes chronic diarrhoeal disease.


Contaminated food and water supplies are the most important vehicles of these organisms. Infantile enteritis in hospitals are transmit-ted through patient to patients, through the hands of attendants and contaminated feeds. VTEC infections are acquired through meat , un-pasteurized milk and direct contact with animals.


Esch.coli organisms are grown and their pathogenic character-istics are identified by appropriate tests.


Avoid exposure to infectious agents. Fluid and electrolyte im-balance must be corrected early. Strict hygiene is essential in hospitals. Food borne infections should be avoided by processing and handling cooked meat products separately from raw materials.

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