Enterohemorrhagic Escherichia coli (EHEC) are bacteria that inhabit the intestinal tract of humans and animals. While most E. coli strains are harmless, EHEC strains cause severe disease due to acquired genetic factors. Infection frequently results in an acute intestinal illness known as hemorrhagic colitis. EHEC is a common cause of foodborne outbreaks and can lead to life-threatening complications.
The Role of Shiga Toxin
EHEC causes illness by producing Shiga toxins (Stx), which are related to toxins made by Shigella dysenteriae. Because of this, EHEC strains, such as the common serotype O157:H7, are also called Shiga toxin-producing E. coli (STEC).
The toxin is an AB5 protein complex, consisting of five B subunits surrounding a single A subunit. The B subunits bind to the globotriaosylceramide (Gb3) receptor on human cells. Once bound, the toxin is internalized, and the A subunit becomes active.
The active A subunit acts as an enzyme that irreversibly inactivates the cell’s ribosomes, halting protein synthesis and leading to cell death. Initially, the toxin damages the large intestine, causing hemorrhagic colitis. If absorbed into the bloodstream, the toxin travels throughout the body, causing systemic harm.
How EHEC Spreads
EHEC naturally resides in the intestinal tract of healthy ruminants, such as cattle, goats, and sheep, which shed the bacteria in their feces. Humans primarily acquire the infection by ingesting contaminated food or water. The infectious dose is extremely low, sometimes requiring fewer than 100 bacterial cells to cause illness.
A common route is consuming undercooked ground beef, which can be contaminated during slaughter. Grinding meat mixes surface bacteria throughout the product, necessitating thorough cooking. Other high-risk foods include unpasteurized milk and raw fruit juices, such as cider, if processed without adequate heat treatment.
Produce can become contaminated if grown in fields fertilized with manure or irrigated with contaminated water. The bacteria can survive in bovine manure for over seven weeks, increasing environmental spread. Person-to-person transmission is a secondary route, occurring via the fecal-oral pathway, especially in institutional settings like daycare centers.
Symptoms and Severe Complications
Symptoms usually begin two to five days after exposure, starting with severe abdominal cramping and diarrhea. Within a day or two, the diarrhea typically becomes bloody, a condition known as hemorrhagic colitis. Patients generally experience little to no fever, which helps distinguish this infection from other diarrheal illnesses.
The acute illness usually resolves within about a week for most people. However, approximately 5% to 10% of those infected, primarily young children under five and the elderly, develop Hemolytic Uremic Syndrome (HUS). HUS is the systemic phase of the illness, typically appearing in the second week after the initial onset of diarrhea.
HUS is characterized by a triad of symptoms: acute kidney failure, hemolytic anemia, and thrombocytopenia. The Shiga toxin damages the endothelial cells lining blood vessels, especially in the kidneys. This damage causes microscopic blood clots to form, which destroy red blood cells (hemolytic anemia) and consume platelets (thrombocytopenia).
Damage to the small blood vessels in the kidneys leads to acute renal failure. Patients who develop HUS often require intensive medical care, including blood transfusions and dialysis. Recognizing early signs of HUS, such as reduced urination or extreme fatigue, requires immediate medical attention.
Reducing the Risk of Infection
Food safety and hygiene practices are effective in preventing EHEC infection. Ground meat, particularly beef, should be cooked to an internal temperature of at least 160°F (71°C) to eliminate the bacteria. A food thermometer must be used, as meat color is not a reliable indicator of safety.
Unpasteurized products, including raw milk, juice, and cider, should be avoided, especially by vulnerable populations. Raw produce, such as leafy greens, must be washed thoroughly under running water before consumption to remove surface contamination.
Preventing cross-contamination in the kitchen is essential. Raw meat must be kept separate from ready-to-eat foods. Cutting boards, utensils, and countertops used for raw meat must be washed with soap and hot water immediately after use. Hand hygiene, especially after using the restroom, changing diapers, or touching animals, is the best defense against transmission.