Diarrhea: Is It Food Poisoning or Infection?

Food poisoning is a general term that refers to gastrointestinal illnesses (usually diarrhea and/or vomiting) caused by food that is contaminated with bacteria, parasites, viruses, or toxic substances. The actual cause of most individual episodes of food poisoning is usually not pursued (for example, a culture is not done), because most episodes are mild or moderate in severity and are over in a few hours to a few days. In fact, the diagnosis of food poisoning usually is made only presumptively, based on the patient's symptoms and the circumstances. Even in outbreaks of suspected food poisoning that involve many people, when careful studies are done, a specific cause is found no more than half of the time.

Bacteria are the cause of most outbreaks of food poisoning for which a specific cause is determined. Bacteria cause food poisoning in three ways. After reaching the intestines, they may multiply and release toxic substances into the intestine that cause diarrhea and/or vomiting without damaging the intestine itself. The bacteria also may multiply within the intestines and produce toxic substances that damage the lining of the intestine or they may invade and damage the intestine directly. Finally, some bacteria produce toxic substances before the food is eaten that cause diarrhea and/or vomiting. These bacteria do not need to multiply within the intestines, and the toxic substances they produce do not damage the intestine.

To know with certainty that a bacterium is causing food poisoning, the bacterium must be cultured, usually from stool and rarely from vomitus. If the food suspected of causing the poisoning still is available, the food can be cultured. Identification of the causative bacterium may require a determination of the bacterial subtype since not all bacteria of one type, for example, E. coli, cause illness. In some cases, the bacteria isolated from the culture may be tested to see if they produce toxic substances. In food poisoning caused by toxic substances formed by bacteria in food before the food is ingested, for example, staphylococcal food poisoning, the toxic substance can be sought in the food, stool, or vomitus. The identification of toxic substances, however, is complex and is not done by most bacteriology laboratories.