Escherichia coli O157:H7
Escherichia coli O157:H7
Escherichia coli is a Gram-negative bacterium (a bacterium that has a cell wall that contains two membranes sandwiching a thin, but strong supporting layer) that normally inhibits the intestinal tracts of humans and other warm-blooded animals.
There are hundreds of different types (strains) of E. coli that differ from one another only slightly in their composition. Most of these strains are harmless and many are beneficial, as they can manufacture some vitamins that are needed for proper functioning of the body. Strain O157:H7 is an exception; in contrast to many of the other strains, E. coli O157:H7 does not normally reside in the human intestinal tract of humans. It normally lives in the intestinal tract of cattle. While harmless in the cattle, it can be dangerous to people. Ingesting food or water that is contaminated with O157:H7—typically by exposure of the food or water to cattle feces or handling by someone whose hands are soiled—can produce a severe, even life-threatening infection.
The descriptor O157 is a code that refers to a structure called lipopolysaccharide that is located on the outer surface of the bacterium. Different configurations of lipopolysaccharide are possible, which can affect the disease-causing ability of the bacterium. The other descriptor, H7, refers to a form of the bacteria's locomotive structure called the flagellum.
Since its first description in the early 1980s, the illness due to E. coli O157:H7 has sickened thousands, and over one thousand people have died as a result of the infection that can destroy intestinal and kidney cells.
E. coli O157:H7 is one of four types of the bacterium that can infect the gastrointestinal tract, causing a disease called gastroenteritis. Additionally, O157:H7 is described as being enterohemorrhagic—this means it is able to destroy the cells lining the intestinal tract, which causes copious bleeding.
The severe intestinal damage that occurs during an infection by E. coli O157:H7 is due to the production of powerful toxins. These toxins, which are called verotoxin and shiga-like toxin, are similar to the destructive toxin produced by another disease-causing bacterium, Shigella dysenteriae.
WORDS TO KNOW
GRAM-NEGATIVE BACTERIUM: Any type of bacterium that is identified and classified by its property of not retaining crystal-violet dye during Gram's method of staining.
STRAIN: A subclass or a specific genetic variation of an organism.
TOXIN: A poison that is produced by a living organism.
Indeed, the similarity of the toxins in the two different bacteria reflects how strain O157:H7 came into existence. The strain was discovered in Argentina in 1977. Studies of the sequences of the genetic material in O157:H7 and S. dysenteriae support the idea that, likely in the intestinal tract of a cow, a typical E. coli acquired genetic material from a neighboring S. dysenteriae. The acquired genetic material included the gene that coded for the destructive Shigella toxins. The genetically altered E. coli, O157:H7, now was capable of producing the toxins.
In 1982, strain O157:H7 was first identified as a cause of illness, when an outbreak of severe diarrhea in several states in the United States was found to be due to undercooked hamburgers. The disease became known as “hamburger disease.” This is unfortunate, since subsequently it became clear that other foods including various kinds of produce, fruits, unpasteurized juices and milk, and cheese products can be contaminated with strain O157:H7. Produce and fruits can become contaminated when sprayed with sewage-containing water during their growth. If the food is not washed prior to eating, the bacteria can be ingested. In 2006, for example, a multi-state illness outbreak due to O157:H7 was traced to organically grown lettuce. Some consumers had eaten the lettuce without first washing it.
The U.S. Centers for Disease Control and Prevention (CDC) estimates that up to 85% of all O157:H7 infections are food-borne infections.
When cattle are slaughtered, intestinal contents can splatter on the carcass. In whole cuts of meat such as a T-bone steak, the bacterial-contaminated surface can be made safe to eat by proper cooking of the cut of meat. However, when surface-contaminated meat is ground up, bacteria including O157:H7 can be distributed throughout the meat. The only way to kill all these bacteria is to adequately cook the meat. This is why undercooked meat can still contain living O157:H7 that are capable of causing the infection.
O157:H7 can also contaminate drinking water. This occurs when O157:H7-containing feces mixes with the drinking water. If the water is not properly treated to remove or kill the bacteria, drinking the water can sicken a person. For example, in the summer of 2000, one of the wells in the community of Walkerton, Ontario, Canada was contaminated by storm run-off from a cattle field. Improper treatment of the water caused thousands of people to become ill and seven people died. Some of the survivors had permanent kidney damage due to the destruction of the kidney caused by the O157:H7 toxins.
In a few instances in the U.S. and Canada, O157:H7 infection has been traced to childrens’ petting zoos; stroking fur that is soiled by feces can be dangerous if the child puts the hand in their mouth.
The toxins are so destructive because they not only damage the host cells they contact, but, because they shut down the manufacture of host cell proteins, they prevent repair of the damage. The toxins can damage the cells because the bacteria bind very tightly to the cells. In fact, the association of O157:H7 with a host cell causes the cell to change its shape, forming a pedestal on which the bacterium anchors. This strong association enables the bacteria to remain in position and establish the infection.
An early symptom of the infection, which occurs as the intestinal cells become damaged, is watery diarrhea. Destruction of intestinal cells causes the diarrhea to become bloody. A person can also experience nausea and vomiting. The fluid loss and pain can be debilitating and intake of fluids is important to prevent more serious problems. In most people, these symptoms fade within several weeks, as the body's immune system is able to successfully deal with the infection. People whose immune systems are immature or malfunctioning can develop a more widespread infection. The kidney damage that can occur can be so extensive that the kidney stops functioning. This occurs in 10–15% of those who contract the infection. The infection can also affect the pancreas, brain, and other organs; this assault can be overwhelming and can cause death.
Approximately 10–15% of those infected with strain O157:H7 develop hemolytic uremic syndrome. The syndrome is the leading cause of sudden-onset kidney failure in children in the world. As well, the elderly can develop a condition known as thrombocytopenic purpura, which consists of fever and nerve damage. In the elderly, this complication of E. coli O157:H7 infection can kill almost half of those who become infected.
IN CONTEXT: PERSONAL RESPONSIBILITY AND PREVENTION
The U.S. Department of Agriculture's Food Safety and Inspection Service and Centers for Disease Control and Prevention (CDC) recommend to avoid E. coli O157:H7 infection that people:
- Cook all ground beef and hamburger thoroughly. Because ground beef can turn brown before disease-causing bacteria are killed, use a digital instant-read meat thermometer to ensure thorough cooking. Ground beef should be cooked until a thermometer inserted into several parts of the patty, including the thickest part, reads at least 160° F (71.1° C). Persons who cook ground beef without using a thermometer can decrease their risk of illness by not eating ground beef patties that are still pink in the middle.
- If you are served an undercooked hamburger or other ground beef product in a restaurant, send it back for further cooking. You may want to ask for a new bun and a clean plate, too.
- Avoid spreading harmful bacteria in your kitchen. Keep raw meat separate from ready-to-eat foods. Wash hands, counters, and utensils with hot soapy water after they touch raw meat. Never place cooked hamburgers or ground beef on the unwashed plate that held raw patties. Wash meat thermometers in between tests of patties that require further cooking.
- Drink only pasteurized milk, juice, or cider. Commercial juice with an extended shelf-life that is sold at room temperature (e.g. juice in cardboard boxes, vacuum sealed juice in glass containers) has been pasteurized, although this is generally not indicated on the label. Juice concentrates are also heated sufficiently to kill pathogens.
- Wash fruits and vegetables under running water, especially those that will not be cooked. Be aware that bacteria are sticky, so even thorough washing may not remove all contamination. Remove the outer leaves of leafy vegetables. Children under 5 years of age, immunocompromised persons, and the elderly should avoid eating alfalfa sprouts until their safety can be assured. Persons at high risk of complications from foodborne illness may choose to consume cooked vegetables and peeled fruits.
- Drink municipal water that has been treated with chlorine or another effective disinfectant.
- Avoid swallowing lake or pool water while swimming. (For more information, see the CDC Healthy Swimming website.)
- Make sure that persons with diarrhea, especially children, wash their hands carefully with soap after bowel movements to reduce the risk of spreading infection, and that persons wash hands after changing soiled diapers. Anyone with a diarrheal illness should avoid swimming in public pools or lakes, sharing baths with others, and preparing food for others.
SOURCE: Centers for Disease Control and Prevention (CDC)
E. coli O157:H7 is worldwide in distribution and occurrence. The prevalence of the illness is higher in countries where agriculture is more prominent and where standards of infection control in food sources are not as stringent as other countries.
There is no evidence that race or gender makes any difference in the susceptibility to infection. However, those with immune systems that are relatively inefficient can be at increased risk; this includes children, the elderly, and those whose immune systems have been impaired by surgery or during the course of caring for another illness.
Treatment of E. coli O157:H7 infection is supportive, including blood replacement and kidney dialysis in persons with hemolytic uremic syndrome.
E. coli O157:H7 infections can be lessened by properly preparing food (such as by adequate cooking until the center of a hamburger is no longer red), washing preparation surfaces that have been in contact with raw ground meat, and handwashing. O157:H7 is readily killed by heat; boiling drinking water will kill the bacteria and destroy the toxins.
The CDC has estimated that the illness afflicts over 70,000 Americans each year. Of these, over 2,000 require hospitalization and approximately 60 people die. For those who become infected, the best that can be expected is a bout of severe diarrhea. Fortunately, for many, recovery is complete and the misery of the infection becomes a memory. For others, the infection can damage the kidney or completely destroy kidney function. For the latter, dialysis or a kidney transplant becomes a fact of life.
Aside from these human costs, the economic consequences of O157:H7 are important. The costs of medical care and lost productivity related to O157:H7 exceed $400 million annually in the United States.
E. coli O157:H7 highlights the necessity of proper hygiene, particularly proper handwashing after using the bathroom. Many food-borne cases of the illness could be prevented if food preparation was accomplished with clean hands. Furthermore, the infection can be easily prevented by cooking ground meat thoroughly. Since the initial outbreak in 1982, many restaurants no longer serve hamburgers that are not cooked to an internal temperature of 160°F (71.1°C), or are not considered “well done.”
In 2007, a Canadian bio-pharmaceutical company announced the successful development and testing of a vaccine for cattle. The vaccine operates by blocking the formation of the bacteria to the intestinal epithelial cells. The stranded bacteria are washed out of the intestinal tract. By vaccination of cattle herds, the reservoir of O157:H7 could gradually be eliminated, and outbreaks from beef would be a thing of the past. In the meantime, food safety scientists are studying other methods to decrease contamination of meat on the farm and in the slaughterhouse, and encourage the use of irradiation to keep the ground beef supply safe.
IN CONTEXT: REAL-WORLD FACTORS IN REPORTING DISEASE
Public health inspectors and scientists use variations of DNA fingerprinting on bacteria to determine the source of an E. coli infection. By comparing samples from patients exposed and potential sources, investigators can often identify a common source of an outbreak.
There are always delays between infection and source identification, typically two to three weeks. The Centers for Disease Control and Prevention (CDC) publishes the following the timeline of identification procedures so that, in part, the number of cases possible during an outbreak may be more accurately estimated:
- Incubation time: The time from eating the contaminated food to the beginning of symptoms. For E. coli O157, this is typically 3-4 days.
- Time to treatment: The time from the first symptom until the person seeks medical care, when a diarrhea sample is collected for laboratory testing. This time lag may be 1 to 5 days.
- Time to diagnosis: The time from when a person gives a sample to when E. coli O157 is obtained from it in a laboratory. This may be 1 to 3 days from the time the sample is received in the laboratory.
- Sample shipping time: The time required to ship the E. coli O157 bacteria from the laboratory to the state public health authorities that will perform “DNA fingerprinting.” This may take 0 to 7 days depending on transportation arrangements within a state and the distance between the clinical laboratory and public health department.
- Time to “DNA fingerprinting”: The time required for the state public health authorities to perform “DNA fingerprinting” on the E. coli O157 and compare it with the outbreak pattern. Ideally this can be accomplished in 1 day. However, many public health laboratories have limited staff and space, and experience multiple emergencies at the same time. Thus, the process may take 1 to 4 days.
SOURCE: Centers for Disease Control and Prevention (CDC)
Drexler, Madeline. Secret Agents: The Menace of Emerging Infections. New York: Penguin, 2003.
Nestle, Marion. What to Eat. New York: North Point Press, 2006.
Davies. M., et al. “Outbreaks of Escherichia coli O157: H7 associated with petting zoos—North Carolina, Florida, and Arizona, 2004 and 2005.” Morbidity and Mortality Weekly. 54: 1277–1281 (2005).