Stool during an E. coli infection typically starts as watery diarrhea and, depending on the type of bacteria involved, may progress to visibly bloody diarrhea within a day or two. The blood is usually bright red and can make the stool look streaked or entirely reddish. Some infections also produce stool with noticeable mucus. Not every E. coli infection looks the same, though, because different strains affect the gut in different ways.
How Stool Changes by Strain
There are six recognized types of diarrhea-causing E. coli, and each produces somewhat different stool. The strain that gets the most attention is STEC (Shiga toxin-producing E. coli), which includes the well-known O157:H7. STEC infections are the ones most likely to cause bloody diarrhea, sometimes described as looking like the stool is mostly blood with little solid material. The blood tends to be bright red because the damage occurs in the large intestine, close to the exit.
Other strains produce watery diarrhea without blood. ETEC and EPEC, common causes of traveler’s diarrhea, result in loose, watery stools that may be yellow or light brown. EAEC can cause watery diarrhea with visible mucus mixed in, giving stool a slimy appearance. One strain, EIEC, sits in the middle: it usually causes watery diarrhea that occasionally turns bloody.
Why Some Infections Turn Bloody
STEC bacteria produce Shiga toxins that directly damage the lining of the intestine. The bacteria attach to the intestinal wall and destroy the tiny finger-like projections (villi) that normally absorb nutrients. Once attached, the toxins enter intestinal cells and shut down their ability to make proteins, which kills the cells. This creates raw, inflamed patches along the intestinal wall that bleed freely into the stool.
The toxins also trigger a strong inflammatory response. White blood cells flood the damaged areas, and the resulting inflammation further injures blood vessels in the gut wall. This is why bloody diarrhea from STEC can look so alarming: it’s not just surface irritation but actual tissue destruction. In lab analysis, stool from these infections contains high levels of inflammatory cells, which signals more severe disease.
The Typical Timeline
Symptoms usually begin 3 to 4 days after exposure, though they can appear as early as one day or as late as a week after contact with the bacteria. In STEC infections, the pattern often follows a predictable sequence: the first day or two brings watery diarrhea and stomach cramps, then the diarrhea becomes bloody around days 2 to 3 of illness. The bloody phase can last several days.
Most otherwise healthy adults recover within a week. The diarrhea gradually becomes less frequent and less bloody, returning to a watery consistency before resolving. During recovery, stools may remain loose and lighter in color for a few days even after the infection clears.
What It Looks Like in Children
In infants and toddlers, E. coli infection shows up in diapers as frequent watery or bloody stools, often accompanied by a strong, unusually foul odor. Young children are more vulnerable to the toxin-producing strains, and blood in their diapers can range from faint pink streaks to dark red. Children are also more likely to have stool mixed with mucus. Because small children can’t describe their symptoms, visible blood or mucus in the diaper is often the first clear sign that something beyond a normal stomach bug is happening.
Signs of a Serious Complication
About 5 to 10% of people diagnosed with STEC infections develop a serious complication called hemolytic uremic syndrome (HUS), which is most common in children under 5. HUS occurs when Shiga toxins enter the bloodstream and damage small blood vessels, particularly in the kidneys.
The warning signs show up not just in stool but in urine. If someone with bloody diarrhea starts urinating less frequently, stops urinating altogether, or notices blood in their urine, these are red flags for HUS. Other signs include unusual paleness, swelling in the face or hands, and extreme fatigue. HUS typically develops about a week after diarrhea begins, sometimes just as the diarrhea itself seems to be improving.
How to Tell It Apart From Other Causes
Bloody stool has many possible causes, so appearance alone can’t confirm E. coli. A few features help distinguish it. E. coli-related bloody diarrhea usually comes with intense abdominal cramping but relatively little fever, especially with STEC. This sets it apart from infections like Salmonella or Shigella, which tend to cause higher fevers. The blood in E. coli stool is typically bright red and mixed throughout rather than sitting on the surface, which would be more suggestive of hemorrhoids or an anal fissure.
Context matters too. If bloody diarrhea starts 3 to 4 days after eating undercooked ground beef, drinking unpasteurized milk, or contact with farm animals, E. coli is a strong possibility. A stool culture is the only way to confirm the diagnosis, and it’s worth requesting early because the results take a couple of days and can guide what happens next.