Escherichia coli, also known as E. coli is a gram-negative, facultatively anaerobic, rod-shaped, coliform bacterium of the genus Escherichia that is commonly found in the lower intestine of warm-blooded organisms (endotherms). Most E. coli strains are harmless, but some serotypes can cause severe food poisoning in their hosts, and are occasionally responsible for product recalls due to food contamination. The harmless strains are part of the normal flora of the gut and can benefit their hosts by producing vitamin K2, and preventing colonization of the intestine with pathogenic bacteria, having a symbiotic relationship.
E. coli is expelled into the environment within fecal matter. The bacterium grows massively in a fresh fecal matter under aerobic conditions for three days, but its numbers decline slowly afterward. E. coli and other facultative anaerobes constitute about 0.1 % of gut flora, and fecal-oral transmission is the major route through which pathogenic strains of the bacterium cause disease. Cells are able to survive outside the body for a limited amount of time, which makes them potential indicator organisms to test environmental samples for fecal contamination. A growing body of research, though, has examined environmentally persistent E. coli, which can survive for extended periods outside of a host. The bacterium can be grown and cultured easily and inexpensively in a laboratory setting and has been intensively investigated for over 60 years.
E. coli is a chemoheterotroph whose chemically defined medium must include a source of carbon and energy. E. coli is the most widely studied prokaryotic model organism, and an important species in the fields of biotechnology and microbiology, where it has served as the host organism for the majority of work with recombinant DNA. Under favorable conditions, it takes only 20 minutes to reproduce.
E. coli stains Gram-negative because its cell wall is composed of a thin peptidoglycan layer and an outer membrane. During the staining process, E. coli picks up the color of the counterstain safranin and stains pink. The outer membrane surrounding the cell wall provides a barrier to certain antibiotics such that E. coli is not damaged by penicillin. Strains that possess flagella are motile. The flagella have a peritrichous arrangement. E. coli belongs to a group of bacteria informally known as coliforms that are found in the gastrointestinal tract of warm-blooded animals. E. coli normally colonizes an infant’s gastrointestinal tract within 40 hours of birth, arriving with food or water or from the individuals handling the child. In the bowel, E. coli adheres to the mucus of the large intestine. It is the primary facultative anaerobe of the human gastrointestinal tract. (Facultative anaerobes are organisms that can grow in either the presence or absence of oxygen.) As long as these bacteria do not acquire genetic elements encoding for virulence factors, they remain benign commensals.
Escherichia coli (E. coli) bacteria normally live in the intestines of people and animals. Most E. coli are harmless and actually are an important part of a healthy human intestinal tract. However, some E. coli are pathogenic, meaning they can cause illness, either diarrhea or illness outside of the intestinal tract. The types of E. coli that can cause diarrhea can be transmitted through contaminated water or food, or through contact with animals or persons.
E. coli consists of a diverse group of bacteria. Pathogenic E. coli strains are categorized into pathotypes. Six pathotypes are associated with diarrhea and are collectively referred to as diarrheagenic E. coli.
- Shiga toxin-producing E. coli (STEC)—STEC may also be referred to as Verocytotoxin-producing E. coli (VTEC) or enterohemorrhagic E. coli (EHEC). This pathotype is the one most commonly heard about in the news in association with foodborne outbreaks.
- Enterotoxigenic E. coli (ETEC)
- Enteropathogenic E. coli (EPEC)
- Enteroaggregative E. coli (EAEC)
- Enteroinvasive E. coli (EIEC)
- Diffusely adherent E. coli (DAEC)
Escherichia coli O157:H7
Escherichia coli O157:H7 is the predominant serotype of Escherichia that form one group of EEC. This group of EEC is termed enterohemorrhagic E.coli or EHEC. Escherichia coli (or E. coli) is the most infecting organism in the family of Gram-negative bacteria known as Enterobacteriaceae.
This bacteria (E.coli) was discovered in the human colon in 1885 by German bacteriologist Theodor Escherich. And named after him in 1911. Dr. Escherich also showed that certain strains of the bacterium were responsible for infant diarrhea and gastroenteritis. Escherichia coli O157:H7 was first recognized as a pathogen in 1982 during an investigation into an outbreak of hemorrhagic colitis associated with the consumption of hamburgers from a fast-food chain restaurant. Retrospective examination of more than three thousand E. coli cultures obtained between 1973 and 1982 found only one isolate with serotype O157:H7 and that was a case in 1975. In the ten years that followed, there was approximately thirty outbreaks recorded in the United States, in Washington of E. coli infections. Escherichia coli O157:H7 is a major health problem. It is estimated to cause infection in more than 70000 individuals a year in the United States, and the U.S centers for disease control and prevention (CDC) suggests Escherichia coli O157:H7 is responsible for the majority of “E. coli” outbreaks in the U.S. The diarrheal illness was first recognized when the CDC personnel isolated Escherichia coli O157:H7 from patients in two separate outbreaks in Oregon and Michigan. The illnesses /were associated with eating hamburgers at the restaurants of a national chain; some patients experienced hemorrhagic colitis (inflammation and bleeding of the colon). Thus, hemorrhagic coli is due to Escherichia coli O157:H7 is commonly referred to as hamburger disease. Since that time, Escherichia coli O157:H7 also has been associated with contaminated water, foods, and unpasteurized or incorrectly pasteurized (heat-treated) dairy products.
Scientific Classification of Escherichia coli O157:H7
- Domain —————Bacteria
- Phylum —————Proteobacteria
- Class —————–Gammaproteobacteria
- Family —————Enterobacteriaceae
- Genus —————-Escherichia
- Species —————coli
Epidemiology of Escherichia coli O157:H7
Escherichia coli O157:H7 was first recognized as a pathogen in 1982 during an outbreak investigation of hemorrhagic colitis. Escherichia coli O157:H7 infections can lead to hemolytic uremic syndrome (HUS), characterized by hemolytic anemia, thrombocytopenia, and renal injury. Still, it was not until 1993, after a large multi-state Escherichia coli O157:H7 outbreak linked to undercooked ground beef patties sold from a fast-food restaurant chain, that Escherichia coli O157:H7 became broadly recognized as an important and threatening pathogen. Clinical laboratories began examining more stool specimens for Escherichia coli O157:H7. In 1994, Escherichia coli O157:H7 became a nationally notifiable infection, and by 2000, reporting was mandatory in 48 states, leading to an estimated 2168 hospitalization and 61 deaths annually, and it’s an important cause of acute renal failures in children.
Although reported outbreaks account for only a minority of Escherichia coli O157:H7 cases, outbreak investigations contribute greatly to understanding Escherichia coli O157:H7 epidemiology by identifying transmission routes vehicles, and mechanisms of contamination. Outbreak findings oblige regulatory and public health agencies and industry to evaluate prevention and control measures so similar outbreaks can be prevented, and knowledge of transmission routes and vehicles allows consumers to be educated and reducing risky behavior that can decrease their risk for infection.
Pathogenicity of Escherichia coli O157:H7
Escherichia coli O157: H7’s ability to induce injury in humans is as a result of its ability to produce numerous virulence factors such as,
Acid resistance of Escherichia coli O157:H7.
Acid resistance (AR) is the ability of bacteria to protect themselves from extremely low pH (<pH 3.0). The low pH in the stomach (pH 1.5-3.0) is one of the first host defenses against foodborne enteric pathogens. The ability to survive in the acidic environment of the stomach increases the chances of bacteria to colonize the intestines and cause infection.
Acid resistance is associated with the lowering of the infectious dose of enteric pathogens. The low infectious dose is one of the best-known characteristics of Escherichia coli O157:H7, making this bacterium highly infectious.
Escherichia coli O157:H7 can survive and persist in numerous environments such as soil, water, and food as well as in animal reservoirs. Escherichia coli O157:H7 has been shown to survive for a year in manure-treated soil and for 21 months in raw manure that had not been composted.
Escherichia coli O157:H7 can survive for a long time in the water, especially at cold temperature water, sediments contaminated with bovine feces can serve as a long term (>8months) reservoir of Escherichia coli O157:H7. To survive in varied environments. Escherichia coli O157:H7 requires the ability to adapt to variations or extreme changes in temperature, pH, and osmolarity conditions commonly encountered in nature. These environmental adaptations of Escherichia coli O157:H7 plays an important role in the persistence and dissemination of this microorganism on farms and the increasing transfer from cattle to cattle.
Production of Shiga toxin.
The bacteria can produce two types of toxins termed Shiga (stx1 and stx2), also termed vero toxins. Shiga toxin acts like the plant toxiciricin by inhibiting protein synthesis in endothelial cells and other cells. Endothelial cells line the interior surface of blood vessels and are known to be extremely sensitive to Escherichia coli O157:H7, which is cytotoxigenic to these cells. Shiga toxin-producing E.coli does not make the animals that carry it ill because cattle lack the Shiga toxin receptor, globotriaosylceramide, and therefore can be an asymptomatic carrier of the bacterium.
Possession of plasmid O157 (PO157).
A plasmid is an extrachromosomal DNA that is capable of replicating independently of the chromosome plasmids are mobile elements that provide various host beneficial traits, such as resistance to antibiotics and heavy metals. Escherichia coli O157:H7 contains a highly conserved plasmid, named PO157. These plasmid-encoded genes are required for full pathogenesis in many enteropathogenic bacteria, including Shigella, yersinia, salmonella, and E.coli sp. pO157 has been required for the expression of fimbriae and adhesion of epithelial cells. The possession of pili (fimbriae) of these organisms provides an adhesive receptor that is specific for human intestinal cells.
Mode of Transmission
Habitat: Escherichia coli O157:H7 has been found more in the intestine of some animals like beef cattle, dairy cattle, pigs, sheep, and goats. Animals are merely the reservoir of the bacteria.
The most frequent route of transmission for Escherichia coli O157:H7 infection are:
Escherichia coli O157:H7 in food
E.coli can get into the meat during processing. If the infected meat is not cooked to 160°F (71°C), the bacteria can survive and infect you when you eat the meat. This is the most common way people in the United States become infected with E. coli.
Other foods that can be infected with Escherichia coli O157:H7 include:
- Raw milk or dairy products. Bacteria can spread from a cow’s udders to its milk. Check the labels on dairy products to make sure they contain the word “pasteurized.” This means the food has been heated to destroy bacteria.
- Raw fruits and vegetables, such as lettuce, alfalfa sprouts, or unpasteurized apple cider or other unpasteurized juices that have come in contact with infected animal feces.
Escherichia coli O157:H7 in water
Human or animal feces infected with E. coli sometimes get into lakes, pools, and water supplies. People can become infected when a contaminated city or town water supply has not been properly treated with chlorine or when people accidentally swallow contaminated water while swimming in a lake, pool, or irrigation canal.
Escherichia coli O157:H7 from person-to-person contact
The bacteria can also spread from one person to another, usually when an infected person does not wash his or her hands well after a bowel movement. E. coli can spread from an infected person’s hands to other people or to objects. Particularly in child day-care.
Infections have been documented from people visiting the petting zoo, dairy farms, or campgrounds where cattle have previously grazed.
Cattle are the natural reservoir of Escherichia coli O157:H7. Between 1 % and 50 % of healthy cattle carry and shed Escherichia coli O157:H7 in the feces of any given time. Contaminated ground beef is the most common vehicle for Escherichia coli O157:H7 outbreaks. Beef products may become contaminated during slaughter, and the process of grinding beef may transfer pathogens from the surface of the meat to the interior. Therefore, if ground beef is completely cooked, the bacteria can survive.
In addition, there are a variety of contaminated food vehicles other than ground beef that has been linked to each O157:H7 incidences, including unpasteurized milk, drinking water, and salami, beef jerky and fresh produce such as lettuce, radish sprouts, fresh spinach and apple cider.
Symptom of Escherichia coli O157:H7
The symptoms of Escherichia coli O157:H7 is divided into two which are;
- The initial symptom infection.
- The later symptom infection.
Initial symptoms of Escherichia coli O157:H7 infection
The initial symptoms of Escherichia coli O157:H7 infection usually appear about three to five days (though occasionally in as few as one day or as many as ten days) after a person ingests the bacteria; the symptoms include
- stomach cramps, and
- Diarrhea that often is bloody.
The person may have a mild fever of about 100 to 101F (37.7 to 38.3 oC). These symptoms can be seen in infected children and adults.
Later symptoms of Escherichia coli O157:H7 infection
The majority of people (especially normal adults) that are infected resolve the infection without antibiotics in about five to seven days.
However, some people (about 10 % of people infected, especially children under the age of 5 and the elderly) develop more severe signs and symptoms, and these people usually require hospitalization and aggressive treatment. These patients develop the usual symptoms listed above but do not resolve the infection. They develop symptoms that last longer (at least a week) and, if not treated promptly, the infection may lead to disability or death.
These symptoms or complications fall into three main categories;
Hemorrhagic (bloody) diarrhea:
Hemorrhagic (bloody) diarrhea symptoms are an increased amount of blood in the diarrheal stool that does not seem to resolve and is usually accompanied by severe abdominal pain. Although this may resolve within a week, some individuals can develop anemia and dehydration that can cause death.
Haemolytic-uremic syndrome (HUS):
Hemolytic-uremic syndrome symptoms include pallor (due to anemia), fever, bruising or nosebleeds (due to destruction of blood platelets that are needed for blood to clot), fatigue, shortness of breath, swelling of the body, especially hands and feet, jaundice, and reduced flow of urine may be seen. HUS symptoms usually develop about 7 to 10 days after initial diarrhea begins. HUS is the most common cause of kidney failure in children; children under ten years old are the most likely to develop HUS. E. coli O157:H7 produces toxins that damage the kidneys and destroys platelets that can lead to kidney failure, excessive bleeding, seizures, or death.
Thrombotic thrombocytopenic purpura (TTP):
Thrombotic thrombocytopenic purpura is caused by the loss of platelets; however, the symptoms that occur are somewhat different and occur mainly in the elderly. The symptoms are fever, weakness, easy, rapid or “spontaneous” bruising, renal failure, and mental impairment that can rapidly progress to organ failures and death. Until the 1980’s, TTP was considered a fatal disease, but since the 1980’s, plasma exchange and infusion techniques have reduced the death rate in TTP patients to about 10 %. For most people (about 90%), the E. coli infection clears, and a good outcome or prognosis occurs. However, if any of the above-mentioned complications happen, the prognosis may range from good to poor.
The Prognosis for Someone Infected With Escherichia coli O157:H7
People with E. coli O157:H7 infection usually (about 90 %) have a self-limited disease, and the outcome is excellent. However, the prognosis worsens with the development of complication(s). Good hydration lessens the chances of complications and improves the outcome. Individuals who develop hemorrhagic diarrhea and are treated promptly have better outcomes with reduced hospitalization. Complications such as HUS and TTP have a wide range of prognosis from good to poor, depending on the overall health of the individual and how quickly the complications are diagnosed and treated. For example, some individuals can recover completely, but others may require IV fluids, plasma exchange, plasma infusion, or dialysis and may have an end-organ failure (usually kidney failure) and neurologic problems. A few (about 10%) of TTP patients will die. Although infrequent, relatively healthy children and adults have died from E. coli O157:H7 infections due to dehydration.
Diagnosis of Escherichia coli O157:H7 infection
The diagnosis is usually made by an accurate history, physical exam, and analysis of the fecal sample. A presumptive diagnosis is made if the patient’s history indicates an association with persons, foods, or fluids known to contain E. coli O157:H7. Such a presumptive diagnosis is often made during outbreaks of the disease. Culture of E. coli O157:H7 from a fecal specimen on selective media (sorbitol-MacConkey agar) gives a definitive diagnosis of the infection when clear-appearing colonies react with O157 antiserum. Other tests include PCR and immunofluorescence tests. The CDC has recommended that all patients being evaluated for community-acquired diarrhea should have their stool samples analyzed by immunologic test systems that detect all types of Shiga toxins as this test will likely detect almost all bacteria that produce Shiga toxins, especially the E. coli O157:H7 strains.
The CDC suggests that this test is even better than bacterial culture techniques, but recommend that both culture and immunologic tests should be done at the same time.
Treatment of Escherichia coli O157:H7
Patients, especially healthy adults, often require no treatment for Escherichia coli O157:H7 since many infections are self-limited. However, the use of antibiotics for acute diarrheal illness has not proven useful. In fact, some studies have shown that antibiotics may increase the chances of developing HUS or might lead to more severe disease. This increase in severity (as a result of antibiotic), is thought to occur because of antibiotic damages the bacteria, causing them to release even more toxin. Most investigators suggest antibiotic use only if a patient is septic, that is there is evidence that the bacterium has spread to parts of the body other than the intestines.
However, the main treatment is hydration in the form of either oral or intravenous hydration. This can be through the replacement of fluids and electrolytes to treat or prevent dehydration. Infection with Escherichia coli O157:H7 should be treated by a physician, especially in children and the elderly. HUS and TTP require complex supportive cases in the hospital. Patients with kidney failure may need dialysis.
Prevention of Escherichia coli O157:H7
There usually are several outbreaks of food poisoning every year due to E. coli. A recent outbreak occurred in a restaurant chain located in two states (Washington and Oregon – the chain Chipolte, decided to temporarily shut down 43 locations because of about 22 people developing the infection. This is Chipolte’s third outbreak in one year. Companies need to follow strict guidelines for food preparation. The following guidelines on preventing E. coli O157:H7 are recommended by the CDC.
- Wash hands thoroughly after using the bathroom or changing diapers and before preparing or eating food. Wash hands after contact with animals or their environments (at farms, petting zoos, fairs, even your own pets in your own yard).
- Cook meats thoroughly. Ground beef and meat that has been needle-tenderized should be cooked to a temperature of at least 160 F/70 C. It’s best to use a thermometer, as meat color is not a very reliable indicator of “doneness.”
- Avoid raw milk, unpasteurized dairy products, and unpasteurized juices (like fresh apple cider).
- Avoid swallowing water when swimming or playing in lakes, ponds, streams, swimming pools, and backyard “kiddie” pools.
- Prevent cross-contamination in food preparation areas by thoroughly washing hands, counters, cutting boards, and utensils after they come in contact with raw meat.
- Drink only treated and pasteurized fluids.
- Hamburgers ordered in a restaurant should be cooked through completely, so that no pink hamburger meat is visible inside.
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