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| The Bug Bible > Escherichia coli
Escherichia coli Description
E. coli are facultative anaerobic Gram-negative non-sporing rods that ferment lactose. They are catalase positive, oxidase negative and indole positive. They can grow at a temperature ranging from 7 to 46°C and can grow at a minimum water activity of 0.95. They are fairly acid tolerant and can grow at pH values ranging from 4.4 to 10. Most strains are motile however there are some non motile strains. E. coli is easily killed by adequate heating.
A number of strains of E. coli are pathogenic for humans. The sub types are separated by serotyping and the presence of specific virulence factors:
EPEC: Enteropathogenic E. coli. The cells adhere to the lining of the small intestines causing changes in the electrolyte secretion and structural damage of the small intestines.
EIEC: Enteroinvasive E. coli. Cells invade and multiply in the lower intestine causing destruction of the intestine cells and inflammation.
ETEC: Enterotoxigenic E. coli. Large numbers of E. coli are required to be ingested. They survive in the stomach and produce toxins.
EHEC: Enterohaemorrhagic E. coli and STEC Shiga toxigenic E. coli are defined as strains of E coli that produce Shiga toxins. Cells adhere to the lower intestine, grow and produce toxins.
These are the principal sub groups however other sub groups have been identified.
Characteristics of Illness
EPEC:
Time of Onset of symptoms: Onset times can range from 17 to 72 hours.
Infective dose: Unknown
Symptoms: Most EPEC infection occurs in young children where they cause watery diarrhoea, fever, vomiting and abdominal pain. Illness may be self limiting or a protracted chronic enteritis. In adults, severe diarrhoea is the predominant symptom with nausea, vomiting, abdominal cramps and low grade fever. Diarrhoea is watery with considerable mucous but without blood.
Duration of symptoms: The symptoms usually last for 6 hours to 3 days, although in some children symptoms have been known to last up to 14 days.
EIEC:
Onset of symptoms: Symptoms usually appear 8 to 24 hours after ingestion of the contaminated food.
Infective dose: It is thought that more than 106 E. coli per gram of food are required for infection.
Symptoms: Enteroinvasive E. coli. The illness caused by this strain is similar to Shigellosis. Many patients commonly experience watery diarrhoea which may be associated with an enterotoxin. Some patients develop dysentery with symptoms including chills fever, headache, muscular cramps and diarrhoea. Stools are frequently bloody.
Cells invade the epithelial cells of the small intestine and colon causing an acute inflammatory response. Changes seen in the colon are similar to those seen in mild Shigella dysentery.
Duration of symptoms: Unknown.
ETEC (Traveller's diarrhoea):
Onset of symptoms: Symptoms usually occurs 8 to 44 hours are ingestion of contaminated food.
Infective dose: In adults it is thought that between 108 to 1010 E. coli cells need to be ingested for diarrhoea to occur.
Symptoms: Illness is characterised by watery diarrhoea with fever, abdominal cramps, malaise and vomiting. In its most severe form it can bring about rice water stools with dehydration.
Pathogenicity is dependent on viable cells surviving the stomach and penetrating the mucosal cells of the small intestine producing enterotoxin in situ.
Duration of symptoms: The illness may last 3 to 19 days.
EHEC and STEC:
STEC are defined as those E coli that produce Shiga toxins and EPEC are those strains of STEC that have caused or may cause enterohaemorrhagic colitis in humans. These stains possess accessory virulence factors. The most common serotype of EHEC associated with human disease is E coli 0157
Onset of symptoms: Symptoms usually appear 3 to 4 days after ingestion of contaminated food. Illness can be caused by person to person contact.
Pathogenicity is determined by the viable cells surviving the acid conditions of the stomach, attaching to the lower intestine and producing large amounts of one or more Shiga toxins.
Infective dose: The infective dose is very low. In some outbreaks EHEC were detected in the foods at 2 cells per 25 grams of food.
Symptoms: A range of symptoms can occur including diarrhoea, severe abdominal cramps and vomiting. Other more serious symptoms include bloody diarrhoea, haemolytic uraemic syndrome (HUS), thrombotic thrombocytopenic purpura (TTP) and death.
Duration of symptoms: The illness can last for 2 to 9 days depending on the person affected. Some individuals may develop the more serious symptoms and in these cases treatment is needed.
Diagnosis of human illness
In most cases correct diagnosis would require identifying the E. coli serotype responsible from both food samples and faecal samples. For those strains that produce a toxin, the toxin can be identified.
Complications
The most severe clinical signs are normally seen in children and the elderly. EHEC strains can cause severe complications. Haemolytic uraemic syndrome (HUS), characterised by renal failure and haemolytic anaemia, can lead to permanent loss of kidney function. In North America and the UK HUS is the major cause of acute renal failure in children. In the elderly, HUS, plus two other symptoms, fever and neurologic symptoms, constitutes thrombotic thrombocytopenic purpura (TTP). This illness can have a mortality rate in the elderly as high as 50%.
Susceptible individuals
All people are susceptible to foodborne disease caused by E. coli. Outbreaks caused by EHEC strains can have a serious affect on children, the elderly and the immune-compromised.
Associated foods
The main reservoir of EIEC organisms is believed to be infected humans. There is lack of association with animals or foods of animal origin. Water contaminated with human faeces can cause further human infection.
Humans appear also to be the major reservoir of ETEC strains and they are often detected in the faeces of asymptomatic human carriers in developing countries. ETEC strains have been isolated from foods in some surveys however it was not determined whether the isolates carried the colonisation factors required for human infection.
EPEC strains have been isolated from a number of animals however, while showing some similarities with human EPEC strains they also show some significant differences. It appears likely that humans are the principal if not the sole reservoir of EPEC strains that infect humans.
Where foods have been implicated as a vehicle for infection by these strains it is thought that human faeces was the primary source.
EHEC and STEC
The intestinal tract of ruminants particularly cattle and sheep is a major reservoir of EHEC. The organisms may also occur in the faeces of other ruminants; buffalo, goats and deer. Some serotypes have been found in pigs, wild birds, horses, dogs and cats.
Fruits and vegetables can be contaminated by the direct use of animal manures as fertiliser, dust from animal enclosures and contaminated water. The organisms can adhere to the surfaces, particularly cut or damaged surfaces, of vegetables and fruits. The organism can enter internal tissues through damaged surfaces and once attached inside the tissue cannot be readily removed by washing. It has been found within seeds used for the production of sprouts.
Many foods have been associated with foodborne disease outbreaks caused by EHEC strains, including hamburgers, fermented meats (salami), cold cooked meats, raw milk, vegetables, sprouts, salads, unpasteurised apple juice and water.
Most outbreaks are caused by poor food hygiene practices and the consumption of contaminated raw foods.
Food analysis
Some E. coli strains can be isolated by standard microbiological tests however additional sophisticated tests are required to determine the potential pathogenic characteristics. Unless a food is implicated in an outbreak analysis for EIEC, ETEC and EPEC strains is not a standard procedure.
The EIEC group are often less reactive biochemically than other E coli and are frequently anaerogenic and either late lactose fermenters or do not ferment lactose.
There are tests available for the EHEC strain 0157.
Outbreaks
EPEC: EPEC strains have been responsible for many outbreaks of infantile diarrhoea, although less frequently in recent years. In adults, outbreaks have occurred due to contaminated water in Europe and North America, and in cold meat and meat pies in the UK.
EIEC: Many cases of foodborne disease caused by EIEC strains are seen to occur in warmer months. Outbreaks have occurred due to contaminated water and soft cheeses.
ETEC: ETEC strains are a major cause of infant diarrhoea in developing countries, and are often a cause of traveller's diarrhoea. They are rarely a problem in developed countries, although occasional outbreaks can occur due to contaminated food and water. A number of outbreaks caused by ETEC strains have occurred on cruise ships.
EHEC: An outbreak caused by E. coli O111, occurred in Australia in 1995. 23 people (mainly children) were diagnosed with HUS, with one fatality. The source of the outbreak was traced to contaminated smallgoods. In 1996 E. coli O157 was responsible for an outbreak which left 6 people ill on the Gold Coast. All persons affected had bloody diarrhoea. It was thought that cross-contamination from raw food to cooked food was the principle cause of the outbreak. Many outbreaks caused by EHEC have occurred overseas, mainly due by the strain E. coli O157.
- In the USA in 1993, up to 500 became ill with the main symptom being bloody diarrhoea. The source of the outbreak was traced to rare cooked hamburger patties. A similar outbreak occurred in the USA in 1996 with 10 people having symptoms consisting of bloody diarrhoea and abdominal cramps. Again in 1997 frozen hamburger patties were implicated in an outbreak which affected at least 15 people. This outbreak resulted in the recall of 25 million pounds of patties.
- Two outbreaks in the USA caused by E. coli O157 occurred with unpasteurised apple cider/juice being implicated food. Approximately 74 people were affected with one fatality. The source of the outbreak was traced to practices on the farm, where apples that had dropped onto the ground became contaminated and were then used to manufacture the juice/cider. Since no kill step was including in the manufacture, the E. coli survived.
- In Japan in 1996, 9500 people became ill after eating contaminated radish sprouts. 12 people died.
- In Scotland in 1996 400 people became ill after consuming contaminated cold and cooked meats. Symptoms included diarrhoea, vomiting and HUS. There were 21 fatalities. Poor practices in the butcher shop were thought to be responsible for the contamination.
Prevention
· Obtain food from safe sources ensure as far as possible that unprocessed animal manures are not used on fruits and vegetables.
· Store perishable foods either chilled or frozen.
· In the refrigerator store cooked foods above raw foods.
· Wash vegetables and fruit well and do not use vegetables and fruits showing obvious tissue damage.
· Cook mince meat to at least an internal temperature of 75ºC for 2 minutes.
· If cooked food is not eaten immediately cool rapidly to 5ºC and do not hold food in the danger zone (5°C to 60°C).
· Prevent cross contamination from raw to cooked foods.
· Ensure that cleaning equipment like sponges and dishcloths are disinfected.
· Wipe up meat juices with disposable paper towels.
· Wash your hands with soap after visiting the toilet, handling raw food and storing raw foods.
Further Information
FDA Bad Bug Book
Literature:
Hocking, A.D. et al. (2003). Foodborne Microorganisms of Public Health Significance. 6th ed. North Sydney. AIFST NSW Branch Food Microbiology Group.
Doyle, M.P. (1989). Foodborne Bacterial Pathogens. Marcel Dekker; New York.
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