Escherichia coli infection, commonly known as E. coli infection, is an illness caused by certain strains of bacteria that normally live peacefully in our intestines but can sometimes turn harmful and cause anything from stomach upset to serious health complications requiring urgent medical attention.

What Is Escherichia coli Infection?

Escherichia coli, or E. coli for short, is a type of bacteria that lives naturally in the intestines of people and animals. Most strains of E. coli are completely harmless and even helpful to our bodies. They assist with digestion, help produce certain vitamins, and protect us from other harmful germs that might try to settle in our gut. In fact, they make up a normal and necessary part of the collection of microorganisms that live inside us.^[1][2]

However, not all E. coli bacteria are friendly. While most strains live peacefully in our digestive system without causing any trouble, some types can make people very sick. These harmful strains are different because they can stick to the cells in our body and release toxins, which are poisonous substances that damage our tissues and organs. When these dangerous types of E. coli enter the body, they can cause infections that range from mild diarrhea to life-threatening conditions.^[3]

E. coli infections can affect different parts of the body, though most people associate them with digestive problems. The bacteria can cause infections in the gastrointestinal tract, urinary tract, bloodstream, and in rare cases, even the lungs or the protective membranes around the brain. The type of illness you develop depends on which strain of E. coli you encounter and where in your body the infection takes hold.^[2][4]

Epidemiology

E. coli infections happen all over the world and affect people of all ages. In the United States alone, there are approximately 265,000 cases each year of infections caused by Shiga toxin-producing E. coli, also known as STEC. This particular type is the most common cause of E. coli outbreaks and serious illness in the country. STEC infections have become a significant public health concern because they can lead to severe complications, especially in vulnerable groups.^[2]

The bacteria are found everywhere in our environment. E. coli naturally lives in the intestines of cattle, poultry, and other warm-blooded animals, where it causes no harm to the animals themselves. The bacteria can end up in soil, on vegetables, in water supplies, and on the floors of hospitals and long-term care facilities. Because of how widespread the bacteria are in nature, opportunities for human infection occur regularly through various routes of contact with contaminated materials.^[4][6]

While anyone can get an E. coli infection, certain groups of people face higher risks of getting sick and experiencing severe complications. Children younger than five years old are particularly vulnerable because their immune systems are still developing. Adults aged 65 and older also face increased risk due to weakened immunity that comes with aging. People with compromised immune systems from conditions like HIV/AIDS, cancer treatment, or organ transplants are at greater risk as well. International travelers may encounter E. coli strains their bodies have never been exposed to before, making them more susceptible to infection.^[1][14]

Causes

People get E. coli infections when harmful strains of the bacteria enter their body, usually through the mouth. The most common way this happens is by swallowing the bacteria after they have contaminated food or water. Understanding how the bacteria spread helps explain why certain foods and situations carry higher risks than others.^[1]

Food contamination is responsible for about 85 percent of current E. coli infections. Meat can become contaminated during the slaughtering process when bacteria from an animal’s intestines get onto the outer surface of the meat. During processing, especially when meat is ground up, the bacteria can spread throughout the entire product. This is why ground beef is one of the most common sources of E. coli infection. If the meat is not cooked thoroughly enough to kill the bacteria, eating it can make you sick.^[3][20]

Raw or unpasteurized milk and dairy products can also carry E. coli. Pasteurization is a heating process that kills harmful germs, including E. coli. When milk has not been pasteurized, bacteria from the cow’s intestines or from the milking environment can remain in the milk. Similarly, unpasteurized juices, especially apple cider, have been linked to E. coli outbreaks.^[3][18]

Fresh fruits and vegetables can become contaminated while they are still growing in fields. This can happen through several pathways: if the soil has been fertilized with improperly composted manure that contains E. coli, if contaminated water is used for irrigation, if wild animals carrying the bacteria leave droppings in the fields, or if farm workers or harvesting equipment transfer the bacteria to the produce. Leafy greens like spinach and lettuce, as well as sprouts, have been identified as sources of E. coli in multiple outbreaks.^[3][24]

Water contaminated with human or animal waste can transmit E. coli. This includes drinking untreated water from wells, lakes, or streams, as well as swallowing water while swimming in contaminated recreational water. The bacteria can also spread from person to person, especially when people do not wash their hands properly after using the bathroom or changing diapers. In homes, daycare centers, nursing homes, and hospitals, poor hygiene practices can lead to transmission of the bacteria from an infected person to others.^[1][13]

Direct contact with animals can also lead to infection. Farm animals, including cattle, sheep, and goats, as well as some pets, can carry E. coli without showing any signs of illness. Touching these animals or their living environment and then touching your mouth or eating without washing your hands first can transfer the bacteria into your body. Petting zoos and agricultural fairs have been linked to E. coli outbreaks, particularly among young children.^[24]

Risk Factors

Several factors increase the likelihood of getting an E. coli infection or experiencing serious complications from it. Age plays a significant role in vulnerability. Young children, especially those under five years old, have immune systems that are still maturing, making it harder for their bodies to fight off infections. Their smaller body size also means that fluid loss from diarrhea can lead to dehydration, which is when the body does not have enough water to function properly, more quickly than in adults.^[1]

Older adults face similar challenges. As people age, their immune system naturally becomes less effective at fighting infections. The elderly are also more likely to have other health conditions that make them more vulnerable to complications. Both the very young and the very old are at increased risk of developing hemolytic uremic syndrome, or HUS, a life-threatening complication that can cause kidney failure.^[3][5]

People with weakened immune systems from any cause are at higher risk. This includes individuals with HIV/AIDS, those undergoing chemotherapy for cancer, people who take immunosuppressive medications after organ transplants, and those with conditions like diabetes that affect immune function. When the immune system cannot work at full strength, the body has a harder time controlling bacterial infections.^[1]

Interestingly, people who take medications that reduce stomach acid, such as antacids, have an increased risk of E. coli infection. Stomach acid normally acts as a protective barrier, killing many bacteria that are swallowed. When this acid is neutralized or reduced, more bacteria can survive the journey through the stomach and reach the intestines, where they can cause infection.^[14]

International travelers are another group at increased risk, particularly when visiting countries with less developed water treatment and food safety systems. The body’s immune system may not recognize E. coli strains that are common in other parts of the world, making travelers more susceptible to infection. This is one cause of what is commonly called traveler’s diarrhea.^[1][4]

Certain behaviors and circumstances also increase risk. Not washing hands properly, especially after using the bathroom, changing diapers, or handling raw meat, creates opportunities for the bacteria to spread. Eating undercooked ground beef or other meat, consuming unpasteurized dairy products or juices, and eating raw fruits and vegetables that have not been properly washed all raise the risk of infection. Living in or visiting places with many people in close contact, such as daycare centers, nursing homes, or dormitories, increases the chance of person-to-person transmission.^[7][18]

Symptoms

The symptoms of E. coli infection depend on where in the body the bacteria have taken hold, but most infections affect the digestive system. When harmful E. coli bacteria infect the gastrointestinal tract, they cause gastroenteritis, which is inflammation of the stomach and intestines. The experience can be uncomfortable and sometimes frightening, especially when symptoms are severe.^[2]

The most common symptom of an E. coli intestinal infection is diarrhea. In many cases, the diarrhea starts out watery and frequent. People may need to use the bathroom many times throughout the day. As the infection progresses, the diarrhea can become bloody, which happens when the toxins produced by the bacteria damage the lining of the intestines. Seeing blood in the stool can be alarming, but it is an important warning sign that medical attention may be needed.^[3][5]

Along with diarrhea, people typically experience stomach cramps and abdominal pain. These cramps can range from mild discomfort to severe pain that makes it difficult to go about normal activities. The pain often comes in waves and may be more intense before a bowel movement. Many people also feel nausea, which is the queasy, unsettled feeling in the stomach, and some experience vomiting. Loss of appetite is common because eating may seem to make the discomfort worse.^[2][8]

A low-grade fever may develop, though not everyone with an E. coli infection will have a fever. When fever does occur, it usually is not very high. Feeling extremely tired and weak is also typical, partly because of the infection itself and partly because of fluid and nutrient loss from diarrhea and vomiting.^[2][23]

When E. coli infects the urinary tract instead of the intestines, the symptoms are quite different. A urinary tract infection caused by E. coli can cause pain or a burning sensation when urinating. People often feel an urgent need to urinate frequently, but only small amounts of urine come out each time. The urine may appear cloudy and have a strong, unpleasant smell. Some people experience pain in the lower abdomen, pelvis, or lower back.^[2][8]

In rare cases, E. coli can cause infections in other parts of the body, leading to different symptoms. Bloodstream infections can cause high fever and chills. Pneumonia from E. coli causes coughing, difficulty breathing, and chest pain. Meningitis, an infection of the membranes surrounding the brain and spinal cord, though extremely rare, can cause severe headache, stiff neck, and sensitivity to light.^[2]

Prevention

The good news is that most E. coli infections can be prevented with simple, practical steps that focus on cleanliness and safe food handling. These measures work because they break the chain of transmission, stopping the bacteria from getting from contaminated sources into your body.^[18]

Hand washing is one of the most effective ways to prevent E. coli infection. Washing your hands thoroughly with soap and warm water is crucial at several key times throughout the day. Always wash your hands after using the bathroom, after changing diapers, after touching animals or being in farm environments, after handling raw meat, and before preparing or eating food. Proper hand washing means scrubbing all surfaces of your hands for at least 20 seconds and then rinsing well. This simple habit can prevent the bacteria from being transferred from contaminated surfaces or materials to your mouth.^[18]

Safe food preparation is essential in preventing E. coli infection. When handling food, follow four basic principles: clean, separate, cook, and chill. Clean means washing your hands, utensils, cutting boards, and countertops with hot, soapy water before and after handling food, especially raw meat. Separate means keeping raw meat, poultry, and seafood away from other foods that will not be cooked, such as salads or bread. Use separate cutting boards for raw meat and for other foods to avoid cross-contamination, which is when bacteria transfer from one food to another.^[18]

Cooking food thoroughly is critical because E. coli bacteria are destroyed by heat. Ground beef and other ground meats should be cooked to an internal temperature of at least 71 degrees Celsius or 160 degrees Fahrenheit. Using a meat thermometer is the only reliable way to know that meat has reached a safe temperature. You cannot tell if meat is safely cooked just by looking at its color. Ground beef should be cooked until no pink remains and the juices run clear. When eating at restaurants, do not hesitate to send back a hamburger that appears undercooked.^[7][18]

After cooking or using the cutting board for raw meat, never place cooked meat back on the same unwashed plate or cutting board. The bacteria from the raw meat can contaminate the cooked food. Always use a clean plate for cooked meat or wash the original plate thoroughly with hot, soapy water before reusing it.^[7]

Choosing pasteurized products is another important preventive measure. Only drink and eat pasteurized milk, dairy products, and juices. Check the label to confirm that products are pasteurized. If there is any doubt about whether a product has been pasteurized, it is safer not to buy or consume it. Juice made from concentrate is considered as safe as pasteurized juice.^[18][21]

Washing raw fruits and vegetables under running water before eating them helps remove bacteria that may be on the surface. This is important even for produce that will be peeled, because bacteria on the outside can be transferred to the inside when you cut through the skin.^[21]

Water safety is another key factor in prevention. At home, most tap water in developed countries is safe to drink because it has been treated. However, when camping, hiking, or traveling to areas where water safety is uncertain, take precautions. Use bottled water or treat water before drinking, cooking, or brushing your teeth. Avoid swallowing water when swimming in lakes, ponds, streams, or swimming pools, as these water sources can sometimes harbor E. coli bacteria.^[18]

If you or a family member has diarrhea, take extra steps to prevent spreading the infection to others. The person with diarrhea should not swim in pools or recreational water until they have been symptom-free for at least a week. Careful hand washing after bathroom use and proper disposal of soiled diapers become even more important when someone in the household is ill.^[18]

Pathophysiology

Understanding what happens inside the body during an E. coli infection helps explain why the symptoms occur and why some infections become serious. The disease process begins when harmful strains of E. coli bacteria are swallowed and make their way through the digestive system to the intestines.^[3]

Once in the intestines, the harmful E. coli bacteria have special abilities that allow them to cause disease. Many disease-causing strains can adhere, or stick tightly, to the cells that line the intestinal wall. This adhesion is the first step in establishing an infection. The bacteria use special structures on their surface to latch onto the intestinal cells, preventing the normal movement of the bowels from washing them away.^[2]

After attaching to intestinal cells, the bacteria begin releasing toxins. Different strains of E. coli produce different types of toxins, which is why the severity of symptoms varies. Shiga toxin-producing E. coli, or STEC, produces particularly dangerous toxins called Shiga toxins. These toxins are named after their similarity to toxins produced by another harmful bacteria called Shigella. Shiga toxins damage the lining of the small intestine and can also affect blood vessels throughout the body.^[3]

The toxins cause inflammation and damage to the cells lining the intestines. This damage leads to several problems. The damaged intestinal lining cannot absorb water and nutrients properly, which results in watery diarrhea as fluid accumulates in the intestines. As the toxins continue to damage tissue, they can cause bleeding from the intestinal wall, leading to bloody diarrhea. The inflammation and damage also trigger pain signals, causing the abdominal cramps and discomfort that people experience.^[3]

In some cases, particularly with STEC infections, the toxins enter the bloodstream and travel to other parts of the body. The kidneys are especially vulnerable to these toxins. When Shiga toxins reach the kidneys, they damage the tiny blood vessels inside them. This damage can lead to hemolytic uremic syndrome, a serious condition characterized by three main problems: destruction of red blood cells, which causes anemia or low red blood cell count; damage to the kidneys that can lead to kidney failure; and low platelet count, called thrombocytopenia, which affects the blood’s ability to clot properly.^[3][16]

It is estimated that up to 10 percent of patients with STEC infection may develop HUS. This complication is more common in young children and the elderly. HUS can cause permanent kidney damage and requires intensive medical treatment, including dialysis to filter the blood when the kidneys cannot work properly. In severe cases, HUS can be fatal.^[3]

When E. coli normally found in the intestines enters the urinary tract, a different disease process occurs. The bacteria travel up through the urethra, the tube that carries urine out of the body, and can reach the bladder, causing cystitis or bladder infection. If the bacteria continue upward, they can reach the kidneys, causing a kidney infection called pyelonephritis. In the urinary tract, E. coli causes inflammation and irritation, leading to the burning pain during urination and the frequent urge to urinate that characterize urinary tract infections.^[2][4]

The body’s immune system responds to the E. coli infection by triggering inflammation and sending white blood cells to fight the bacteria. This immune response contributes to symptoms like fever and fatigue. In most cases, the immune system successfully controls and eliminates the infection within a week or so. However, in vulnerable individuals or with particularly virulent strains, the bacteria can overwhelm the body’s defenses, leading to more severe illness.^[4]