Gasrtointestinal bacterial infections

Gastroentritis*Is the most common form of acute

gastrointestinal infection, causing diarrhea with or without vomiting.

*Groups who are at risk of GE are children in developing, elderly ,travellers to developing countries , homosexual men ,and infants in day care facicilities.

MechanismsBacteria can cause diarrhea in three different ways which are:

1.Mucosal adherence : this is by effacement of intestinal mucosa and presented as moderate watery diarrhea.Typical example is Enteropathogenic E.coli (EPEC).

2.Mucosal invasion : by penetration and destruction of mucosa and the typical clinical presentaion is dysentry.Examples are Shigella ,Campylobacter , and Enteroinvasive E.coli (EIEC).

3.Toxin production

*Enterotoxins : this occurs by fluid secretion without mucosal damage with the resultant profuse watery diarrhea.Examples are Vibrio cholera ,Salmonella spp.,Campylobacter spp.,Entertoxogenic E.coli (ETEC) ,Bacillus cereus ,Staphylococcus aureus producing enterotoxin B,Clostridium perfrigens type A.

*Cytotoxin : through damage to mucosa and presented as dysentry.Typical examples are Salmonella spp. ,Campylobacter spp., and Enterohaemorrhagic E.coli (EHEC).

Clinical syndromesBacterial gastroentritis can bedivided on clinical grounds into two broad syndromes:

1.Watery diarrhea ( usually due to enterotoxins, or adherence ).

2.Dysentry ( usually due to mucosalinvasion and damage ).

N.B…..some pathogens such as Campylobacter jejuni there may be overlap between the two syndromes.

SalmonellaGastroentritis can be caused by many of the numerous serotypes of salmonella ( all of which are members of a single species , S.choleraesuis ), but the most commonly implicated are S.enteritidis and S.thyphimurium.These organisms , which are found all over the world, are commensals in the bowels of the livestock and they are transmitted to man in contaminated foodstuffs.

*Salmonella can affect both large and small bowel, and induce diarrhea both by production of enterotoxins and by epithelial invasion.

*The typical symptoms commence abruptly 12-48 hours after infection and consist of nausea ,cramping abdominal pain , diarrhea and sometimes fever.

*The diarrhea can vary from profuse and watery to a bloody dysentery syndrome.Spontaneous resolution usually occurs in 3-6 daqys, although the organism may persist in the faecesfor several weeks.

*Bacteraemia occurs in 1-4% of cases and is more common in elderly and the immunosuppressed .Occasionally bacteraemia is complicated by metastatic infection , especially atheroma on vascular endothelium,with devastating consequences.

*In healthy adults salmonella gastroenteritis is usually minor illness ,but young children and the elderly are at risk of significant dehydration.

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*Specific diagnosis is made by culturing of the organism from blood or faeces , but management is usually empirical. Antibiotic therapy ( ciprofloxacin 500 mg twice daily ) may decrease the duration and severity of symptoms , but is rarely warranted.

Campylobacter jejuni

*C.jejuni is also zoonotic infection , existing as a bowel commensal in many species of livestock.It is found world-wide , but is a common cause of childhood gastroenteritis in developing countries.Adults in these countries may be tolerant of the organism , excreting it asymptomatically.

*Like salmonella, campylobacter can affect large and small bowel and can cause a wide variety of symptoms.

*The incubation period is usually 2-4 days , after which there is an abrupt onset of nausea , diarrhea and abdominal cramps.The diarrhea is usually profuse and watery, but an invasive haemorrhagic colitis is sometimes seen.

*Bacteraemia is very rare , and infection is usually self – limiting in 3-5 days.Diagnosis is made from stool cultures.If symptoms are severe, azithromycin 500 mg daily is the drug of choice .

Shigella

*Sigellae are enteroinvasive bacteria which cause classical bacillary dysentery.The principal species causing gastroenteritis are S.dysenteriae ,S.flexeneri and S.sonnei ,which are found with varying prevalence in different partes of the world.

*All cause a similar syndrome , as a result of damage to intestinal mucosa.Some strains of S.dysenteriae also secrete a cytotoxin affecting vascular endothelium.

*Although shigellae are found world-wide , transmission is strongly associated with poor hygiene.The organism is spread from person to person , and only small numbers need to be ingeste3d to cause illness (< 200, compared to 10 for campylobacter and >10 for salmonella).

*Symptoms start 24-48 hours after ingestion and typically consist of frequent small-volume stools containing blood and mucus.

*Dehydration is not as significant as in the secretory diarrhoeas , but systemic symptoms and intestinal complications are worse.

*The illness is usually self –limiting in 7-10 days, but in children in developing countries the mortality may be as high as 20%.

*Antibiotic treatment decreases the severity and the duration of diarrhea , and possibly reduces the risk of further transmission.

*Resistance to antibiotics is widespread

:in some areas amoxicillin or co-trimoxazole may still be effective , but in many places nalidixic acid or ciprofloxacin is needed.

Enterohaemorrhgic E.coli(EHEC) EHEC(usually serotype O157:H7 ,and also known as verotoxin- producing E.coli, or VTEC) is a well recognized cause of gastroenteritis in man.

*EHEC secrete a toxin (Shiga-like toxin 1) which affects vascular endothelial cells in the gut and in the kidney.

*After an incubation period of 12-48 hours it causes diarrhea( frequently bloody) , associated with abdominal pain and nausea.

*Some days after the onset of symptoms the patient may develop thrombotic thrombocytopenic purpura (TTP) , or haemolytic uremic syndrome (HUS).This is more common in children , and may lead to permanent renal damage or death.Treatment is mainly supportive : there is evidence that antibiotic therapy might precipitate HUS by causing toxin release

Enterotoxigenic E.coli(ETEC)

*ETEC produce both heat-labile and heat-stable enterotoxins which stimulate secretion of fluid into the intestinal lumen.The result is watery diarrhea of varying intensity , which usually resolves within a few days.Transmission is normally from person to person via contaminated food.The organism is common in developing countries , and is a major cause of traveller ‘s diarrhea.

Vibrio

*Cholera, due to Vibrio cholera is the prototypic pure enterotoxogenic diarrhea

*Vibrio parahaemolyticus causes acute watery diarrhea after eating raw fish or shellfish that has been kept for several hours without refrigeration.Explosive diarrhea, abdominal cramps and vomiting occur with a fever in 50%.It is a self-limiting , lasting up to 10 days.

Yersinosis*Yersinia enterocolitica is a zoonosis of a variety

of domestic and wild mammals.Human disease can arise either via contaminated food products ,e.g. pork , or from direct animal products.

*Yersinia enterocolitica can cause a range of gastroenteric symptoms including watery diarrhea , dysentery , and mesenteric adenitis.The illness is usually self – limiting , but ciprofloxacin may shorten the duration.

Staphylococcus aureus

*Some strains of Staph. aureus can produce a heat- stable toxin( enterotoxin B) which causes massive se3cretion of fluid into intestinal lumen.

*It is a common cause of food – borne gastroenteritis, outbreaks usually occurring as a result of poor food hygiene.

*Because the toxin is preformed in the contaminated food, onset of symptoms is rapid , often within 2-4 hours of consumption

There is violent vomiting , followed within hours by profuse watery diarrhoea.The symptoms have usually subsided within 24 hours.

Bacillus cereus

B.cereus produces two toxins .One produces watery diarrhoea up to 12 hours after ingesting the organism.The other toxin is preformed in food and causes severe vomiting ,e.g ‘fried rice poisoning’