Escherichia coli, better known as E. coli, are Gram negative, rod-shaped bacteria belonging to the family Enterobacteriaceae. Escherichia species are also included in the group of genera referred to as the coliforms.
Not all E. coli cause disease: the species is found as part of the normal, healthy human gut flora as well as in the environment. Therefore the presence of E. coli in processed food products can indicate faecal contamination. For this reason E. coli is widely used as an ‘indicator organism’ for the presence of potentially more dangerous bacteria, such as Salmonella.
Although most strains of E. coli do not cause illness, some have been associated with human infections resulting in diarrhoea, or occasionally more severe illness. There are at least six different groups of diarrhoea-causing E. coli, but only two types are associated with foodborne disease.
1. Verocytotoxin-producing (VTEC), sometimes referred to as Shiga-like toxin-producing (STEC). This group includes a subset of serotypes often referred to as enterohaemorrhagic E. coli (EHEC). Not all VTEC are associated with human disease but those that are EHEC can cause haemorrhagic colitis (bloody diarrhoea).
2. Enteroaggregative E. coli (EAEC), causing acute and persistent diarrhoea.
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VEROCYTOTOXIN PRODUCING ESCHERICHIA COLI (VTEC)
What are VTEC?
The verocytotoxin producing Escherichia coli (VTEC) are a group of strains within the species E. coli, some of which are highly pathogenic and capable of causing potentially serious foodborne infections in humans. Of all the E. coli groups it is the VTEC that are of most concern in developed countries.
In excess of 200 VTEC have been described and some have been associated with outbreaks of severe foodborne disease in many countries. The VTEC most frequently associated with foodborne illness is the serotype Escherichia coli O157:H7. Other important VTEC serotypes that have caused foodborne infections are O26, O91, O103, O111 and O145.
What foods can be contaminated?
VTEC are usually associated with foods derived from cattle, such as beef, particularly minced/ground beef, and dairy products derived from raw milk. Although VTEC could be present on any raw beef product, minced meats are considered more of a risk because the pathogen is transferred from the surface to the center of the product during the mincing process.
Studies in the USA and the UK have found that VTEC can be present, at least occasionally, on most farms. But food surveys have found that the prevalence of the bacteria in beef and raw milk is generally low. VTEC have also been found on pork and lamb mince, fruits, vegetables and seeds and associated products. Fresh produce can be contaminated at any stage during cultivation or processing, via contaminated water supplies, or cattle manure used as a fertilizer.
How does it affect human health?
The incubation period for illness caused by VTEC can be between 1 and 14 days, although on average it is 3 – 4 days. The infective dose is thought to be very low, possibly just 10 cells. This is probably because these bacteria are unusually acid tolerant and so may survive passage through the stomach. Symptoms may be restricted to mild diarrhoea, and some individuals may not experience any symptoms at all.
But in some 50% of those infected, especially in vulnerable groups, VTEC infection can cause more serious symptoms. These include bloody diarrhoea, abdominal cramps, vomiting and very occasionally, fever. The illness typically resolves itself after 5 – 10 days, but in a small number of cases, particularly in children under 5 years of age and the elderly, VTEC infection can lead to haemolytic uraemic syndrome (HUS), potentially resulting in kidney failure. HUS in children can also result in seizures, coma and sometimes death. Around one third of individuals showing signs of VTEC infection are hospitalised and the average mortality rate from HUS caused by VTEC infections in the UK and in North America is 3 – 5 %.
How common is illness?
Fortunately, in view of the potentially serious symptoms, VTEC infections are comparatively rare. Nevertheless, in Europe between 1995 and 2002, incidence of infection more than doubled to 3.2 cases per 100,000 of the population, before leveling off. During 2008, 3,159 cases (53% O157 VTEC) were reported in 25 EU member states, resulting in an overall incidence of 0.7 cases per 100,000 of population. Ireland, the UK, and Scotland have a higher incidence than many other European countries, but the reasons for this are not known.
In the USA there are an estimated 110,000 EHEC cases annually, resulting in the hospitalisation of approximately 3,200 people. During 2009 there were 723 VTEC (63% O157 VTEC) cases reported in 10 US states, giving an overall incidence of roughly 1.5 cases per 100,000 of the population.
Outbreaks
VTEC outbreaks, particularly those caused by E. coli O157:H7, have frequently been associated with undercooked minced (ground) beef products such as hamburgers – it has been dubbed ‘hamburger disease’. However, VTEC outbreaks have also been caused by a wide variety of other foods such as cooked meats, raw milk, cheese, yoghurt, mayonnaise, unfermented apple cider, melon, salad leaves such as lettuce and spinach, parsley, coleslaw, venison jerky, salami, frozen pepperoni pizza, prepackaged cookie dough, in-shell hazelnuts (filberts), raw shelled walnuts and alfalfa sprouts. Contaminated water sources are also a common source of VTEC outbreaks.
The largest recorded non-O157 VTEC foodborne outbreak occurred during 2011 in Germany, with additional cases amongst individuals who had travelled to Germany but returned to other countries before becoming ill, and a related, though much smaller outbreak in France. Approximately 3,900 people developed illness during the outbreak, with at least 800 cases of HUS and 48 deaths being recorded. The outbreak strain was identified as E. coli O104:H4 and unusually, was found to possess a number of features more typical of enteroaggregative E. coli, together with the capacity to produce Shiga (vero) toxin, which combination may have made this strain highly infective. The profile of the outbreak victims was also unusual, with the majority of HUS cases being healthy adults aged 20 years or more. The outbreak was associated with the consumption of fenugreek sprouts grown from seeds imported from Egypt.
Where does it come from?
The main infection reservoir for O157 VTEC is cattle, which, together with other ruminants such as sheep, seem to be carriers of VTEC without suffering illness. Other animals have also been found to excrete VTEC, including pigs, goats, deer, horses, dogs, cats, rats, seagulls, pigeons, and geese. E. coli O157 has also been isolated from houseflies. A number of outbreaks have been associated with direct contact with infected animals in petting zoos.
Contamination of water supplies with animal faeces has led to outbreaks linked to drinking water and wells, as well as from recreational waters. Soil manured with animal faeces, or in fields where animals have been grazing, can be contaminated with VTEC and contamination may be transferred to crops. O157 VTEC has been found to survive for 150 days in soil and for 90 days in cattle faeces.
How is it affected by environmental factors?
Temperature
VTEC can grow over the temperature range 7 – 46ºC (although some sources suggest possibly up to 50ºC) with an optimum of 37 ºC. Some isolates of E. coli O157:H7 have been reported to grow in raw milk at 8ºC. E. coli O157:H7 grows poorly at 44 – 45ºC, so that traditional methods to detect E. coli in food may not pick it up. VTEC also survive well at chilled and freezing temperatures.
VTECs are not heat resistant organisms, but resistance can vary in different foods. D-values are affected by factors such as levels of salt, carbohydrate, protein and fat, as well as the water activity and the pH of the foodstuff. For E. coli O157, D57ºC values of 5 minutes, and D63ºC values of 0.5 minutes have been reported in meat. Research indicates that the thermal resistance of non-O157 VTEC is relatively similar.
VTEC present on the surface of foods are likely to be inactivated rapidly during cooking, but cells at the center of ground meat products and rolled meat joints will only be inactivated if the center of the product is heated sufficiently. Advice has been given in the USA and the UK on the cooking of hamburgers (meat patties, beef burgers) to ensure safety. In the USA, the advice is that they should reach an internal temperature of 71ºC throughout, and in the UK it is recommended that they be cooked to 70 ºC for 2 minutes, or the equivalent, in all parts of every burger.
pH
VTEC are unusual amongst E. coli because they are relatively acid tolerant. The minimum pH for the growth of E. coli O157 under otherwise optimum conditions is reported as 4.0 – 4.4, although the minimum value is affected by the type of acid, with acetic and lactic acids being more inhibitory than hydrochloric acid. The organism is able to survive acid conditions (down to 3.6) and has been reported to survive for two months at 4ºC at a pH of 4.5.
Water activity
The minimum reported water activity for the growth of VTEC is 0.95. Salt (Sodium chloride) at 8.5 % inhibits the growth of E. coli O157 and growth is retarded at 2.5 %. VTEC are very resistant to desiccation and are able to survive many drying and fermentation processes. Outbreaks have been associated with salami type meat products.
Atmosphere
VTEC are facultative anaerobes (able to grow with or without the presence of oxygen). Modified atmosphere packaging has little effect, although it is reported that growth of the bacteria is inhibited on meat packaged under 100 % CO2.
Chemicals
VTEC are not notably resistant to preservatives and sanitizers typically used in the food industry. Organic acids (acetic and lactic acid) are used in the US to decontaminated beef carcasses.
How can it be controlled?
The control of VTEC starts on the farm with good agricultural practice. This can help reduce the shedding of E. coli O157 from cattle. Good agricultural practices are extremely important for the production of fresh fruits, salads and vegetables. It is very important to minimize the potential for faecal contamination of all food commodities.
For food processors
It is safe to assume that raw products of bovine origin (such as fresh meat and raw milk) are potentially contaminated with VTEC and to treat them accordingly using a HACCP approach. Good hygienic practices should be implemented when handling beef carcasses and the controlled use of chilled temperatures will prevent the growth of VTEC in these products. The possible survival of VTEC should also be considered during the development of products such as bovine milk cheeses and fermented meat products. There are published guidelines for producers of such foods, but the use of unpasteurised milk is best avoided.
US regulations require abattoirs and meat processing establishments to implement a step to eliminate E. coli O157:H7 and this can include decontamination. Non-intact raw beef products (as well as intact raw beef products intended to be processed into non-intact raw beef products) found positive for E. coli O157:H7 are considered ‘adulterated’ and are recalled.
It is important to ensure that heat processes (where appropriate) are designed to inactivate any VTEC. Cross contamination between raw and processed product must be avoided.
For retailers, caterers and consumers
Consumers and caterers should be advised of the risks associated with raw meat products, in particular those made from minced/ground meat, and that all beef products need to be thoroughly cooked. Advice has been given on the required internal cooking temperature for burgers (see above). In the USA consumers are advised that checking the colour of meat patties or burgers (brown as opposed to pink or red) is not a reliable indication that the product has reached a safe temperature and that they should use a thermometer to check that the required temperature has been reached.
Consumers should be advised to avoid unpasteurised dairy products, juice or cider, and to wash fruit and vegetables well (although washing may not remove all contamination). Vulnerable groups (the very young, elderly and the immunocompromised) should be advised not to eat raw or lightly cooked seed sprouts.
Are there rules and regulations?
EU regulations have some general requirements for E. coli as an indicator of faecal contamination in some products. These do not apply specifically to VTEC, but the presence of E. coli in any product that will not receive a heat treatment prior to consumption is unacceptable.
In 2009 the UK Health Protection Agency (HPA) published guidelines for assessing the microbiological safety of ready-to-eat foods placed on the market (see link below). These state that the detection of E. coli O157 and other VTEC in these products is “unsatisfactory: potentially injurious to health and/or unfit for human consumption.”
The US Food Code (2005) requires food to be safe and unadulterated and product that will not be heated prior to being consumed would need VTEC to be absent to conform to this requirement. In addition the USDA Food Safety and Inspection Service (FSIS) considers E. coli O157:H7 and E. coli O26 as adulterants in non-intact raw beef products (ground, minced or chopped), as well as intact raw beef products intended to be processed into non-intact raw beef products.
ENTEROAGGREGATIVE ESCHERICHIA COLI
What are enteroaggregative Escherichia coli?
Enteroaggregative Escherichia coli (EAEC) were first described in 1987 as causing persistent diarrhoea in a child in Peru. These E. coli strains are called aggregative because of their ability to stick to epithelial cells in a characteristic ‘stacked-brick’ pattern. EAEC produce toxins (including a cytotoxin) but do not invade cells. EAEC serotypes include O3, O44, O86, O111 and O127.
EAEC is considered as an emerging pathogen and is the second most common cause of traveller’s diarrhoea. In a number of countries EAEC have been identified as the cause of large outbreaks of diarrhoea and several of these are thought to have been caused by contaminated food.
What foods can be contaminated?
EAEC is spread mainly via the faecal-oral route; both contaminated water and food can be vehicles for the bacteria.
Data is limited on the occurrence and frequency of the organism in foods, but one study found EAEC in 44 % of table sauces sampled in Guadalajara, Mexico. A study of street foods in Ghana isolated EAEC from macaroni, rice, shito (hot pepper sauce) and tomato stew, while a study in Brazil isolated EAEC from commercially produced ice.
How does it affect human health?
EAEC infections are more common in developing countries and illnesses caused by the organism are typically found in young children, although EAEC strains can also cause infection in adults. Undernourished children and the immunocompromised are particularly vulnerable and EAEC infections have been linked to persistent diarrhoea in HIV-affected individuals.
Recent studies have linked the seemingly symptomless carriage of EAEC with malnutrition and growth retardation and EAEC has also been implicated in the development of irritable bowel syndrome, although this has yet to be confirmed.
The infective dose is high and the incubation time variable. Symptoms are typically prolonged, lasting in many cases for at least 14 days, and include persistent watery diarrhoea (occasionally bloody) without fever, possibly with vomiting, leading to dehydration. The illness is generally mild, but deaths have been reported.
How common is illness?
Analysis of published data suggests that EAEC was the cause of acute and persistent diarrhoea in a median of 15% of children in developing countries, and in 4% of children in developed countries.
Outbreaks
EAEC infections can occur sporadically or as outbreaks. Since EAEC was first described in 1987 a number of outbreaks linked to food have been well documented. In the UK EAEC outbreaks were linked with restaurant meals and large catered events, and in Italy two consecutive EAEC outbreaks were associated with the consumption of unpasteurised cheese (although infected food handlers could not be ruled out). In Japan centrally prepared school lunches were linked to a large outbreak (2,697 children at 16 schools) of severe diarrhoea. Although the organism was not isolated from any implicated foods, EAEC was isolated from 10% of cases.
Where does it come from?
EAEC are found in human and animal faeces and it is thought that transmission usually occurs through contaminated food or water. Food handlers are also thought to be important reservoirs for EAEC and a study in Kenya in 2003-2004 isolated the organism from 2.1% of participants. Although less common, person-to-person transmission may also occur. The environmental reservoir for EAEC is not known, but it is isolated from environmental samples.
How is it affected by environmental factors?
Little is known about the growth or survival of EAEC in foods.
Temperature
Under otherwise optimal conditions studies indicate that EAEC grow well at both 37 and 41.5°C. A study also found that EAEC survived well in bottled spring and mineral water (at least 60 days), at 4, 10 and 23°C. Higher numbers survived at 10 and 23°C compared to those in samples stored at 4°C.
There is no specific information on the heat resistance of EAEC; however it is likely to be similar to other types of E. coli. EAEC should therefore be inactivated by typical heat processes used for the pasteurisation of food products.
How can it be controlled?
Until more is known about the reservoirs for EAEC, measures to prevent food from becoming contaminated with the organism should focus on avoiding the use of contaminated water to irrigate, wash and prepare foods as well as good hygienic practices for the preparation and storage of foodstuffs.
One of the risk factors in contracting EAEC infection is travel to developing countries. To avoid contracting enteric diseases, the US Centers for Disease Control and Prevention (CDC) advises travellers to “boil it, cook it, peel it or forget it”.
Are there rules and regulations?
There is no specific legislation regarding EAEC in foods, but there is general legislation and guidance referring to all groups of Escherichia coli.
To assess the hygiene status of a ready-to-eat food product and for shellfish, legislation and guidance can refer to levels of general E. coli (excluding VTEC). In 2009 the UK Health Protection Agency (HPA) published guidelines for assessing the microbiological safety of ready-to-eat foods placed on the market (see link below). These state that a satisfactory level of E. coli in these products is less than 20 cells/g (Note: this level is not applicable for E. coli O157 and other VTEC).
