What is E. coli?
E. coli are pathogenic strains and are a primary cause of food-borne illness mostly in developing countries.
Among the species of the Enterobacteriaceae family and Gram-negative bacteria, E. coli causes more infection and is a well-established agent responsible for food poisoning.
E. coli is a common microflora present in the intestinal tract of humans and animals and is commensal bacteria that live harmlessly.
These commensal bacteria are mostly non-pathogenic strains and do not cause any infection to the host cell.
Pathogenic E. coli strains have a virulence mechanism and cause severe diseases such as gastroenteritis, dysentery, hemolytic uremic syndrome (HUS), urinary tract infection (UTI), septicemia, pneumonia, and meningitis.
Diarrheagenic E. coli are characterized into six categories:
- Enterotoxigenic E. coli (ETEC)
- Enteropathogenic E. coli (EPEC)
- Enterohemorrhagic E. coli (EHEC)
- Enteroinvasive E. coli (EIEC)
- Enteroaggregative E. coli (EAEC)
- Diffusely adhering E. coli (DAEC)
These virotypes are categorized on the basis of pathogenic features with virulence factors and their mechanism with host cells.
Each virotypes are linked to different diseases and toxin production.
Shiga-toxin possessing E. coli strains (STEC) are the independent strains of E. coli that do not accompany virulence factors but are responsible for causing hemorrhagic colitis.
E. coli is widely used as a model organism to study the physiology, metabolism and genetic structure of the bacteria however, in recent years the increasing number of E. coli outbreaks has been a major concern.
Characteristics of E. coli
- Gram-negative bacteria
- Non-spore forming
- Aerobes or facultative anaerobes
- Range of growth temperature 4 – 45°C
- Can survive at pH 3.6
- Can survive in fermentation and drying process
Sources of contamination of E. coli Food Poisoning
- E. coli are the normal intestinal microflora of warm-blooded animals and are transmitted via the fecal-oral route.
- Cattle, sheep, goats, giraffes, camels, dogs, cats and swine are the major reservoir of Shiga-toxin E. coli (STEC).
- Humans are infected through contaminated water, fruits, vegetables, meat (especially ground beef) and fish (finfish and shellfish).
- Fruits and vegetables are frequently contaminated from the contaminated water and soil and also from the untreated manures which are used as biofertilizers for the plants.
- Contamination of pathogen occurs frequently during slaughtering of meat through fecal contact, cross-contamination, unhygienic processing of food products.
- Direct human to animal contact or human to human contact can also transmit the disease.
Epidemiology of E. coli Food Poisoning
- From the epidemiological data, the overall mortality of E. coli infection is less than 1% but if the patient suffering from hemolytic uremic syndrome (HUS) then they might be at risk.
- In recent years, the outbreak has been reported in other countries such as Australia, Argentina, Denmark, Germany, Italy, Sweden and South Africa.
- In 1996, a huge outbreak occurred in Sakai city of Japan where more than 7000 people were affected.
- Similarly in England and Wales, 381 people were infected by VTEC 0157 of which 59 people developed HUS, 14 people died and the remaining patients were hospitalized.
- In 1982, STEC caused two outbreaks which were reported as the first outbreak caused by a human pathogen.
- This outbreak was due to the consumption of undercooked ground beef which caused hemorrhagic colitis.
- Since then other serotypes of STEC is associated with many foodborne outbreaks.
- There is a characteristic seasonal distribution of pathogens in North America and the UK, mostly occurring in the summer and early autumn where preschool children are the likely age group that is infected.
- Foods that were associated with most outbreaks are ground beef, raw milk, yogurt, cheese, cured and fermented meat products, non-fermented apple cider, raw vegetables and salads.
- Secondary E. coli infection is common in households, hospitals, age care homes, nursery schools, bathing waters and swimming pools.
Intestinal pathogenic E. coli (Pathogenesis and symptoms)
- Enterotoxigenic E. coli affects especially children and adults in developing countries as it produces either heat-stable enterotoxins or heat-labile enterotoxins.
- Symptoms include fever, abdominal cramp, nausea and watery diarrhea similar to that of Vibrio cholerae but are not severe.
- This enterotoxigenic pathogen also affects travelers as they might get exposed to contaminated water and food while traveling therefore, ETEC is also known as traveler’s diarrhea.
- ETEC attaches to the epithelial cells with the help of fimbriae or fibrils and produces heat-stable or heat-labile enterotoxins.
- Heat-labile enterotoxin resembles cholera toxin and permanently activates adenylate cyclase which increases the level of CAMP resulting in loss of electrolyte and watery diarrhea.
- EPEC is mostly pathogenic to infants and young children causing diarrhea mainly in developing and tropical countries with poor sanitation.
- This disease has a high potential fatality rate among infants and neonates.
- It also affects farm animals, dogs, cats, pigs and rabbits.
- Symptoms include diarrhea, abdominal cramp, vomiting, headache, fever and chills.
- EPEC do not produce toxins like ETEC but some strains of EPEC invades tissue cells and produce Shiga toxin.
- Pathogenesis of EPEC involves three stages:
- The first stage is an attachment of EPEC to the host cell with the help of fimbriae, the second stage includes signal transduction where the cell attaches and forms lesions destructing the microvilli and forms cup-like cytoskeletal protein.
- Then the attached EPEC alters the cytoskeleton of the host cell in the third stage that affects the mitochondrial function and increases membrane permeability resulting in loss of nutrients and ions from the body.
- Enterohemorrhagic E. coli causes bloody diarrhea and is the main pathogen responsible for causing hemorrhagic colitis outbreaks.
- Patients suffering from hemorrhagic colitis are likely to get infected by hemolytic uremic syndrome (HUS) which is a life-threatening disease.
- EHEC also produces Shiga toxin that destroys Vero cells therefore, it is also known as verotoxin-producing E. coli (VTEC).
- The first step of EHEC pathogenesis is the colonization in the intestinal cells and its attachment and effacement are similar to that of EPEC.
- It damages the blood vessels of the colon and causes bloody diarrhea with abdominal cramps and vomiting.
- Other complications such as acute renal failure, microangiopathic hemolytic anemia, thrombocytopenia and hemolytic uremic syndrome (HUS) may develop.
- Enteroinvasive E. coli invades into epithelial cells and spreads cell to cell.
- EIEC is related to Shigella species genetically and pathogenetically causing watery diarrhea and dysentery.
- Abdominal cramps, profuse watery diarrhea and fever are the common symptoms but some may develop dysentery and bloody mucoid diarrhea.
- EIEC invades the epithelial cells with the help of pINV which is a factor encoded by genes on a plasmid.
- pINV causes enterotoxicity of EIEC and disrupts the barrier function of the epithelial cells.
- The symptoms of EAEC are similar to that of ETEC that causes persistent diarrhea in children for more than 14 days.
- Watery mucoid diarrhea, mild fever but no vomiting are the common symptoms of EAEC infection.
- EAEC adheres to the monolayer of cultured cells with a characteristic “stacked-brick” pattern.
- Aggregative Adherence Fimbriae I (AAF/I) helps in adherence to Hep-2 cells and erythrocyte hemagglutination.
- EAEC also produces cytotoxin that damages the mucosa membrane of the intestine destabilizing the host cell cytoskeletal structure.
6. Diffusely Adhering E. coli (DAEC)
- DAEC has its unique diffuse attachment pattern mediated by fimbrial adhesins and invasins encoded by bacterial chromosomes or plasmids.
- DAEC causes watery diarrhea but is not found in a stool sample of healthy adults.
- Some studies have suggested that diarrhea associated with DAEC could be age-dependent.
Laboratory Diagnosis of E. coli
- E. coli are grown on nutrient media which includes vitamins, minerals and nitrogens to aid growth.
- Lysogeny or Luria broth, Terrific broth and Super optimal broth are mostly used growth medium that promotes fast growth.
- Other commonly used media are MacConkey agar, Eosin methylene blue agar, Sorbitol MacConkey agar and cystine-lactose electrolyte-deficient agar.
- PCR-based assays are used to detect the target genes of the pathogens and are highly sensitive.
- Multiplex PCR, real-time PCR are used in the laboratory to detect and identify pathogens.
Treatment and Control of E. coli Food Poisoning
- E. coli infection is self-limiting but in severe cases, antibiotic therapy is recommended otherwise fluid therapy and bed rest resolves the illness.
- Ciprofloxacin, norfloxacin and ofloxacin are choices of antibiotics for ETEC infection.
- Control measures include proper selection, preparation and storage of food products.
- Proper refrigeration temperature, cooking and heating at an appropriate temperature and personal hygiene must be maintained by food handlers to avoid cross-contamination.
- Travelers are advised to maintain hygiene and avoid potential food and water, drink boiled water and properly cooked food to prevent infection.
- Slaughtering animals must be done only after inspecting the animal and testing the presence of EHEC.
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