Bacterial infections in the colon are most often caused by swallowing harmful bacteria through contaminated food, untreated water, or contact with infected people or animals. The five most common culprits are Campylobacter, Salmonella, Shigella, E. coli, and C. difficile, each reaching the colon through slightly different routes and causing damage in different ways.
The Bacteria Most Likely to Infect the Colon
Campylobacter is the single most common cause of bacterial diarrhea in the United States, responsible for 4% to 11% of all diarrhea cases. It’s isolated from stool samples twice as often as Salmonella and seven times more often than Shigella. The usual source is raw or undercooked poultry, unpasteurized milk, or contaminated drinking water.
Salmonella reaches the colon primarily through raw or undercooked eggs, meats, unpasteurized milk, and contaminated produce like fruits and vegetables. You can also pick it up from handling reptiles like turtles or pet ducklings. Salmonella causes a range of illness from mild stomach upset to serious bloodstream infections.
Shigella spreads easily from person to person through the fecal-oral route, meaning even tiny amounts of the bacteria on hands, surfaces, or in food or water can cause infection. One subtype, S. sonnei, is behind nearly 80% of bacterial dysentery cases in the U.S.
Most E. coli strains living in your gut are harmless, but a handful of subtypes have evolved specific tools to make you sick. Some produce toxins, others physically invade the cells lining the colon, and some do both. The most well-known, E. coli O157:H7, lives in the intestines of cattle and reaches people through undercooked beef, contaminated water, raw milk, unpasteurized apple cider, and even petting zoos or day care centers. This strain causes bloody diarrhea and can lead to serious complications.
How C. difficile Differs From Other Causes
C. difficile infections work differently from the bacteria above. Rather than being introduced from contaminated food, C. difficile typically takes hold after antibiotics wipe out the normal protective bacteria in your gut. Broad-spectrum antibiotics are especially likely to cause this disruption because they kill a wide range of bacteria, including the beneficial ones that normally keep C. difficile in check.
Without that competition, C. difficile spores germinate and the bacteria multiply rapidly. The damage comes from two toxins the bacteria release, which destroy cells lining the colon, break down the barriers between cells, and trigger intense inflammation. This is why C. difficile infections cause watery diarrhea, fever, stomach pain, and nausea, usually while you’re still taking antibiotics or shortly after finishing a course. About 1 in 9 people who recover from a C. difficile infection will get it again within 2 to 8 weeks.
How Bacteria Reach and Damage the Colon
Nearly all bacterial colon infections follow the fecal-oral route. Bacteria from the feces of an infected person or animal contaminate food, water, or surfaces. You ingest the bacteria, they survive the journey through your stomach, and they colonize the colon. Once there, different bacteria use different strategies to cause disease.
Some bacteria, like certain Shigella species, physically invade the cells lining the colon wall, destroying tissue directly. Others, like some E. coli strains, attach to the colon lining and release toxins that damage cells without invading them. Still others combine both approaches, sticking to the wall, releasing toxins, and burrowing into tissue. The result is inflammation, which your body experiences as diarrhea (sometimes bloody), cramping, and fever.
Risk Factors That Make Infection More Likely
Certain conditions lower your colon’s natural defenses. Stomach acid is one of the body’s first barriers against swallowed bacteria. People who regularly take proton pump inhibitors (PPIs) for acid reflux reduce that barrier, and research has linked PPI use to a higher rate of gastrointestinal infections. The drugs appear to shift the balance of gut bacteria in ways that make colonization by harmful species easier.
A weakened immune system from conditions like HIV, chemotherapy, or organ transplant medications also raises risk significantly, since the body is less able to fight off bacteria before they establish an infection. Inflammatory bowel disease can compromise the colon lining, making it more vulnerable to bacterial invasion. Travel to regions with less reliable water treatment is another major risk factor, which is why traveler’s diarrhea is so common. The longstanding advice for travelers still holds: boil it, cook it, peel it, or forget it.
Recent antibiotic use is the dominant risk factor specifically for C. difficile. The broader the antibiotic and the longer the course, the greater the disruption to your gut’s protective bacteria.
Symptoms and What to Watch For
Most bacterial colon infections produce some combination of diarrhea (watery or bloody), abdominal cramping, fever, nausea, and loss of appetite. The severity ranges widely. A mild Campylobacter infection might feel like a bad stomach bug lasting a few days, while Shigella or E. coli O157:H7 can cause intense bloody diarrhea that requires medical attention.
Dehydration is the most immediate concern with any bacterial diarrhea. Frequent loose stools drain fluid and electrolytes fast, especially in young children and older adults. Oral rehydration solutions are the first-line treatment for mild to moderate fluid loss.
Some infections carry the risk of serious complications. E. coli O157:H7 can trigger hemolytic uremic syndrome, a condition where toxins destroy red blood cells and damage the kidneys. Severe C. difficile infection can, in rare cases, lead to toxic megacolon, a dangerous swelling of the colon that can result in perforation, sepsis, or shock if untreated. Bloody diarrhea, high fever, or signs of dehydration like dizziness, dry mouth, and reduced urination are signals that an infection needs prompt evaluation.
How Bacterial Colon Infections Are Diagnosed
Stool tests are the primary way to identify which bacteria are responsible. Standard stool cultures can detect Salmonella, Shigella, Campylobacter, and pathogenic E. coli strains, though cultures take time to grow. For C. difficile, labs use a combination of rapid antigen tests (results in under an hour) and molecular PCR tests that detect the genes responsible for toxin production. Toxin-specific tests can confirm that the C. difficile present is actually producing the toxins that cause disease, rather than simply being carried harmlessly.
One challenge with testing is that a positive result doesn’t always mean the bacteria is causing the current symptoms. People can carry C. difficile or certain E. coli strains without being sick, so doctors typically only test when symptoms are present.
Treatment: Antibiotics Versus Supportive Care
Not every bacterial colon infection needs antibiotics. Many cases of Campylobacter and Salmonella resolve on their own within a week with rest and fluids. In fact, using antibiotics for E. coli O157:H7 can actually increase the risk of hemolytic uremic syndrome, so they’re generally avoided for that strain. For Shigella and C. difficile, targeted antibiotics are usually necessary because these infections are less likely to clear without treatment.
Rehydration is the cornerstone of care regardless of the specific bacteria involved. For mild to moderate dehydration, oral rehydration solutions work well for both children and adults. If vomiting makes drinking difficult, fluids can sometimes be given through a tube in clinical settings.
Preventing Bacterial Colon Infections
The CDC organizes food safety around four steps: clean, separate, cook, and chill. Washing your hands with soap for at least 20 seconds before, during, and after food preparation is the single most effective habit. Use separate cutting boards for raw meat and produce, and wash utensils and countertops with hot soapy water between food items.
Cooking temperatures matter because different bacteria die at different heat thresholds. Whole cuts of beef, pork, veal, and lamb need to reach 145°F internally (followed by a 3-minute rest). Ground meats require 160°F. All poultry, including ground chicken and turkey, must hit 165°F. Leftovers and casseroles also need to be reheated to 165°F. A food thermometer is the only reliable way to confirm these temperatures.
Beyond the kitchen, rinse fresh fruits and vegetables under running water before eating, avoid unpasteurized dairy and juices, and wash your hands after contact with animals. For C. difficile specifically, the most important prevention strategy is avoiding unnecessary antibiotic use. Taking antibiotics only when truly needed, and choosing the narrowest effective option, helps preserve the gut bacteria that naturally keep harmful species from gaining a foothold.