A stool culture is used, often along with other tests such as an O&P test that detects parasites in the stool and/or a Clostridium difficile toxin test, to help determine the cause of your prolonged diarrhea.

Stool cultures may be ordered when you have had diarrhea for several days and when you have blood and/or mucus in your loose stools. This is especially true when you have eaten food or drunk fluids that you or your doctor suspect may have been contaminated with a pathogenic bacteria, such as undercooked meat or raw eggs, or the same food that has made others ill. Recent travel outside the United States may also suggest possible food contamination. When your doctor suspects that you may have a parasitic infection, he may also order an O&P test. When your diarrhea begins during or after antibiotic treatment, then your doctor may order a Clostridium difficile toxin test along with a stool culture.

If you have had a previous pathogenic bacterial infection of your gastrointestinal tract and have been treated for it or gotten better on your own, your doctor may order one or more stool cultures to verify that the pathogenic bacteria are no longer detectable. This can be important because in some cases people can become carriers of the bacteria. Carriers are not ill themselves, but they can infect other people.

Results are frequently reported out with the name of the pathogenic bacteria and whether it was “isolated” (found in your stool sample) or “not isolated” (that bacteria was not found). Negative results usually reflect the fact that the stool culture was checked for pathogens at several intervals and none were found (not isolated). A report may say something like: “no Campylobacter isolated”, “no Salmonella or Shigella isolated”, etc. If the culture is negative for the major pathogens, then it is likely that your diarrhea is due to another cause. It is also possible that the pathogenic bacteria are present in the gastrointestinal tract but were not found in this particular stool sample. If your doctor suspects this is the case and your symptoms continue, he may order another stool culture.

If your stool culture is positive for pathogenic bacteria, then that is the most likely cause of your prolonged diarrhea. The stool culture report may state “Salmonella enteritidis isolated” (which means that you have an infection caused by this particular pathogenic bacterium). Most diarrheal disease is caused by a single pathogen, but it is possible to have an infection with more than one.

Severe pathogenic bacterial infections of the gastrointestinal tract and those causing complications may be treated with antibiotics, but many uncomplicated cases are best untreated with antibiotics. Patients with competent immune systems will usually get better on their own within a week or so. Patients are instructed in how to prevent the spread of the infection and are treated and monitored for symptoms such as dehydration.

Pathogenic bacterial infections are monitored on a community level. Other than travel-related cases, health officials want to try to determine where your infection came from so that they can address any potential public health concerns. For instance if your salmonella or shigella infection is due to eating food from a particular restaurant, the health department will want to investigate whether or not other people have also become ill from their food. They will visit the restaurant to determine the source of the infection and take steps to ensure that the spread of the infection is stopped.