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Can a bacterial infection cause blood in stool?

Blood in stool, also known as hematochezia, can be caused by a variety of conditions, including bacterial infections of the gastrointestinal tract. Bacterial infections that can lead to blood in the stool include salmonella, campylobacter, shigella, E. coli, C. difficile, H. pylori, and more. While blood in stool due to a bacterial infection is usually an acute problem that resolves with treatment of the infection, it can sometimes become a chronic issue if the underlying infection is not properly addressed.

What causes blood in stool?

There are many potential causes of blood in stool, including:

Bacterial infections

As mentioned, bacterial infections are a common cause of blood in stool. Specific types include:

  • Salmonella – Causes salmonellosis food poisoning
  • Campylobacter – Leading cause of bacterial diarrhea
  • Shigella – Highly infectious causes shigellosis or bacillary dysentery
  • E. coli – Certain strains cause bloody diarrhea
  • Clostridium difficile – Overgrowth causes pseudomembranous colitis
  • H. pylori – Chronic infection can damage the stomach lining

These bacteria can invade the intestinal wall and cause inflammation, ulcers, and bleeding. Stool may appear bloody or tarry.

Viral infections

Viral gastroenteritis and other viral infections affecting the intestines may cause transient blood in stool. This includes viruses like norovirus, rotavirus, adenovirus, and cytomegalovirus.

Parasitic infections

Protozoa like Entamoeba histolytica and Giardia can disrupt the intestinal wall and lead to dysentery with bloody diarrhea. Worms like hookworm, whipworm, roundworm, and tapeworm can also cause blood in stool.

Inflammatory bowel diseases

Chronic conditions like Crohn’s disease and ulcerative colitis often result in gastrointestinal bleeding due to chronic intestinal inflammation. Blood in stool may be mild or severe.

Intestinal ischemia

Restricted blood flow to the intestines can cause ischemic colitis. This may occur due to low blood pressure, blood clots, or narrowing of blood vessels. Stool with blood typically occurs after eating.

Colon cancer and polyps

Bleeding from colorectal cancer or precancerous polyps is a common cause of blood in stool in adults. Stool may be dark or tarry with a foul odor.

Anal fissures and hemorrhoids

Tears in the anal lining or swollen, enlarged hemorrhoids can bleed and cause bright red blood in stool or on toilet paper. Pain and itching may occur.


Blood thinners, NSAIDs, antibiotics, and anti-platelet drugs can sometimes cause GI bleeding. Stool may appear tarry or maroon.

Other causes

Bleeding disorders, trauma, ulcers, inflammatory bowel disease, vascular malformations, and more.

How do bacterial infections cause blood in stool?

There are a few ways that bacterial infections of the intestines can lead to the passage of bloody stools:

Disruption of the intestinal barrier

Many pathogenic bacteria like salmonella and shigella invade the intestinal wall and get taken up by cells in the lining. This leads to inflammation, damage to the barrier, and increased intestinal permeability. Cells may slough off, along with blood.

Inflammation and ulceration

Bacteria trigger an inflammatory reaction in the intestines. This causes redness, swelling, ulceration, and erosion of the tissue. Small ulcers and wounds in the intestinal lining can bleed.

Toxin production

Some bacteria release toxins that break down colonic mucosa and lead to inflammation orabloody diarrhea. For example, shiga toxin from E. coli O157:H7 and C. difficile toxin can destroy cells and lead to hemorrhage.

Malabsorption and intestinal hurry

Diarrhea and intestinal hurry caused by infections reduce time for absorption. This can irritate the GI lining. Bacteria may also impair absorption of vitamin K needed for blood clotting.

Common bacterial infections that cause bloody stool


Salmonella infections often cause diarrhea that may be bloody. Sources include undercooked poultry, eggs, produce, and contaminated water. Salmonella invades the small intestine and causes inflammation, mucosal sloughing, and bloody diarrhea 4-7 days after ingestion.


Shigella bacteria are a common cause of bloody diarrhea worldwide. Sources include contaminated food and water. Shigella invades colonic tissue and causes ulcers, pain, and stools mixed with blood and mucus.


Campylobacter infections typically cause abdominal pain, fever, and diarrhea with blood. Undercooked poultry is often the source. Campylobacter disrupts the intestinal barrier and causes inflammation and hemorrhage.

E. coli

Certain strains of Escherichia coli, like O157:H7, produce shiga toxin leading to hemorrhagic colitis. Symptoms of abdominal cramping and bloody diarrhea start 2-4 days after ingesting contaminated food or water.

Clostridium difficile

C. difficile is an opportunistic infection that often occurs after antibiotic treatment. It releases toxins that damage the colonic mucosa causing pseudomembranous colitis with bloody diarrhea.


Yersinia infections are contracted from pork products. Yersinia invades intestinal tissue leading to bloody stools, fever, and right lower quadrant abdominal pain mimicking appendicitis.

Risk factors

Factors that increase risk of bloody stool from a bacterial infection include:

  • Food or water contaminated with bacteria
  • Recent use of antibiotics – increases C. diff risk
  • Weak immune system – HIV, chemotherapy, steroids
  • Travel to developing countries with poor sanitation
  • Young children or elderly adults
  • Inflammatory bowel diseases
  • Abdominal adhesions or history of bowel surgery
  • Decreased stomach acid – allows more bacterial survival

When to see a doctor

It is important to consult a doctor for evaluation if you notice blood in your stool. This is especially urgent if you have:

  • Large volume bloody stool
  • Persistent bloody diarrhea
  • Severe abdominal pain
  • High fever over 101°F (38°C)
  • Signs of dehydration – dizziness, dry mouth, rapid heart rate, little urination

Call 911 or go to the ER if you are experiencing dizziness, weakness, or rectal pain and cannot pass stool or gas. This may indicate a medical emergency like a bowel obstruction or perforation.

Diagnosing the cause

To determine the cause of blood in stool, your doctor may:

  • Ask about your symptoms, bowel habits, diet, and medical history
  • Perform a physical exam of your abdomen
  • Order stool tests to look for blood and bacterial cultures
  • Do a complete blood count to check for anemia
  • Check inflammatory markers like sedimentation rate and C-reactive protein
  • Conduct endoscopic procedures like colonoscopy or sigmoidoscopy
  • Obtain CT scans or capsule endoscopy to view the small intestine

These tests help identify any inflammation, ulcers, polyps, cancers, or other sources of bleeding in the gastrointestinal tract.

Treating bacterial infections


Bacterial infections require prescription antibiotics to eradicate the bacteria and stop further intestinal damage. Supportive therapy includes:

  • IV fluids and electrolytes for dehydration
  • Antidiarrheals like loperamide to slow bowel movements
  • Iron supplements for anemia
  • Pain medications

Severe C. difficile infections may require fecal transplant of healthy gut bacteria.

Dietary changes

Eat a bland, low fiber, low residue diet during active infection. Avoid dairy, caffeine, alcohol, high fat foods, raw fruits and vegetables. Stick to bananas, rice, applesauce, toast, and lean proteins.


Take probiotic supplements with healthy gut bacteria to restore intestinal microbial balance after antibiotics. Yogurt with active cultures also helps.


Surgery may be required if there is intestinal perforation, abscess, peritonitis, sepsis, or toxic megacolon resulting from infection. This removes infected or damaged portions of the bowel.


You can take steps to avoid bacterial infections that cause bloody stools:

  • Wash hands thoroughly with soap and warm water before preparing food
  • Cook meats thoroughly to kill bacteria
  • Avoid unpasteurized dairy products
  • Wash fruits and vegetables thoroughly
  • Only drink filtered or bottled water when traveling
  • Avoid food from street vendors with questionable food handling practices

Getting vaccinated can also help prevent infections from salmonella, shigella, and other pathogens.

When to see a gastroenterologist

You should see a gastroenterologist if:

  • There is prolonged bloody diarrhea lasting over 2 weeks
  • You have frequent recurrences of bloody stools
  • There is no infection found but blood in stool continues
  • There are signs of moderate to severe colitis
  • You have risk factors for colon cancer like family history or inflammatory bowel disease
  • Further evaluation with colonoscopy is needed

A gastroenterologist specializes in treating conditions of the GI tract and can diagnose and treat any underlying cause of persistent blood in stool.

The bottom line

Blood in stool is often caused by bacterial infections that invade and damage the intestines. Salmonella, shigella, campylobacter, E. coli, C. difficile, and other bacterial pathogens disrupt the intestinal barrier leading to inflammation, ulcers, and bloody stools. Most infections resolve with antibiotics and supportive treatment. However, some cases can become chronic without proper diagnosis and care. See a doctor if bloody diarrhea is severe or persists more than 2 days. Preventive steps like handwashing, food safety, and vaccination can help reduce risk of bacterial infections that cause bloody stool.