Foodborne diseases / Escherichia coli

Escherichia coli

Escherichia coli is a part of the natural flora of the animal intestine and hence its presence in foods can be used as an index of faecal contamination. A small minority of E. coli strains may cause illness in humans and other animals. In particular, E. coli serotype O157:H7 is recognised as an important foodborne pathogen.

1. Bacteriological characteristics

E. coli is a facultatively anaerobic, Gram-negative, rod shaped bacterium belonging to the family Enterobacteriaceae.
Pathogenic strains have been divided into several classes, namely, (1) enteropathogenic E. coli (EPEC), which causes diarrhoea; (2) enterotoxigenic E. coli (ETEC), which produce specific toxins and cause infant diarrhoea in developing countries and traveller’s diarrhoea; (3) enteroinvasive E. coli (EIEC), which is capable of invading the epithelium of the large intestine and (4) enterohaemorrhagic E. coli (EHEC), which causes haemorrhagic diarrhoea. In some circumstances this may be followed by the haemolytic uraemic syndrome (HUS), which can lead to kidney failure. The strains produce cytotoxins that are active against cultured verocells.

2. Disease in humans

The majority of the pathogenic strains of E. coli cause diarrhoea following the ingestion of contaminated food or water containing as few as 100 cells. Other syndromes include HUS and thrombocytopenic purpura. Illnesses caused by E. coli have probably increased in recent years with E. coli O157:H7 being the most important pathogenic serotype.

3. Sources of infection and epidemiology

Pathogenic strains occur among the normal microflora of the gastrointestinal tract of animals and can be transferred to carcass meat during dressing. As a result, the organisms have been found on carcasses and in processed meat such as minced or ground beef. E. coli can survive freezing, and frozen products such as beefburgers may represent a hazard to the consumer if cooked inadequately. Beef meat is an important source of E. coli O157:H7 infection for humans while pig and poultry meat is not, although this serotype is capable of colonising the intestinal tract of chickens following oral inoculation. Additionally, food handlers may contaminate meat and meat products during processing. Certain serotypes may cause disease in poultry, which sometimes leads to an increased use of antibiotics. Therefore in control programs the screening of strains for antibiotic susceptibility has been included.

4. Growth properties in relation to meat and meat products

E. coli O157:H7 can grow at temperatures between 7-46°C with the optimum being 35-40°C. The organism can also grow at pH 4.5, and in the presence of 6.5% NaCl, when other factors are favourable. It fails to grow when the NaCl concentration is 8.5%.

5. Isolation

Various selective broths and agars are available for isolating E. coli in general. The latter contain bile salts, a fermentable carbohydrate, such as lactose, and a pH indicator. Alternatively, fluorogenic substrates to detect β-glucuronidase activity have been used as differential agents. Routine isolation media do not distinguish between commensal and pathogenic strains. Several media have been developed to facilitate the isolation of E. coli O157:H7, which has the unusual, although not specific, trait of being unable to ferment sorbitol. When specific pathogens are sought immunological (e.g. ELISA and immunomagnetic separation) and DNA based )e.g. gene probes) techniques can be used.
Incubation of media at 44°C selects for E. coli although this temperature should be reduced to 42°C for E. coli O157:H7 since the organism is more temperature sensitive. The toxins of pathogenic strains can be identified at specialist laboratories to confirm the identity of the organisms.

6. Control measures in slaughterhouses and cutting plants

It is not possible to rear animals that are free from E. coli and so the only control measures that can be used in the slaughterhouse are those general measures adopted to minimise faecal contamination of the carcass and cross-contamination during further handling of meat.