Shiga toxins (Stx1 and Stx2) are potent substances produced by certain strains of Escherichia coli (E. coli). These toxins can cause severe illness in humans, making them a significant public health concern.
Sources of Shiga Toxins
Shiga toxins primarily originate from Shiga toxin-producing Escherichia coli (STEC), with E. coli O157:H7 being a well-known strain. These bacteria commonly reside in the intestines of healthy ruminant animals, including cattle, sheep, goats, and deer, which act as reservoirs without showing illness. Humans become infected by ingesting food or water contaminated with animal feces. Common transmission routes include consuming undercooked ground beef, unpasteurized milk or juices, and contaminated produce. Exposure also occurs through contact with infected animals, their environments, and person-to-person spread due to inadequate hygiene.
Cellular Mechanism of Shiga Toxins
Shiga toxins are AB5 toxins, consisting of an A subunit and five B subunits. The B subunits bind to a specific receptor called globotriaosylceramide (Gb3) on the surface of human cells, especially those lining blood vessels in the kidneys and intestines. This binding facilitates the toxin’s entry into the cell. Once inside, the toxin undergoes retrograde transport within the cell.
Within the cell, the A subunit is cleaved, and its active component (A1 fragment) is released into the cytoplasm. This A1 fragment targets the 60S ribosomal subunit, inactivating the ribosome, which is the cellular machinery responsible for protein synthesis. By halting protein production, the toxin causes the affected cell to die. Stx2 is generally considered more potent than Stx1, exhibiting higher affinity for ribosomes and greater catalytic activity.
Impact on Human Health
Infection with Shiga toxin-producing bacteria can lead to various health issues, with symptoms typically appearing three to four days after exposure. Initial symptoms often include severe abdominal cramps and diarrhea, which can progress to bloody diarrhea (hemorrhagic colitis). Vomiting and a mild fever may also occur. Most individuals recover within about a week with supportive care.
A serious complication of STEC infection, particularly in young children and older adults, is Hemolytic Uremic Syndrome (HUS). HUS develops when Shiga toxins enter the bloodstream and damage the lining of small blood vessels, especially in the kidneys. This damage leads to a triad of conditions: hemolytic anemia (destruction of red blood cells), thrombocytopenia (low platelet count), and acute kidney failure. HUS can result in significant kidney damage, sometimes requiring dialysis, and can be life-threatening.
Reducing Exposure Risk
Minimizing Shiga toxin exposure risk involves careful food handling and hygiene. Thoroughly cook all meats, especially ground beef, to a safe internal temperature. Avoid consuming unpasteurized dairy products, such as raw milk, and unpasteurized juices or ciders. Always wash raw fruits and vegetables thoroughly under running water before consumption, even if they will be peeled.
Prevent cross-contamination in the kitchen by keeping raw meats separate from ready-to-eat foods and using different cutting boards and utensils. Wash hands thoroughly with soap and water for at least 20 seconds, especially after using the bathroom, changing diapers, handling raw meat, or having contact with animals. Additionally, avoid swallowing water from lakes, ponds, or untreated swimming pools.