Escherichia coli (E.coli) is a group of bacteria that have a close relationship with humans. Some strains exist in the human gut and do not cause any disease if the population is kept within certain limits. In fact it helps to fight off any pathogenic bacteria that may enter the gut and can also help with nutrition. Other strains of E.coli can cause severe infections in humans, particularly of the bowel and abdominal cavity, urinary tract, lungs and brain lining (meninges). E.coli infection can cause inflammation of the small intestine (enteritis) or large intestine (colitis) which is associated with severe diarrhea.
Diarrhea is the passing of loose or watery stool that exceeds 300g or 300 mL in a 24 hour period. This is usually passed out in more than three bowel movements in a day. Infectious diarrhea may be due to inflammation of the small intestine (enteritis) or large intestine (colitis). It is known as enterocolitis if both the small and large intestine are affected. When the stomach and small intestine are both affected, then it is known as gastroenteritis, and is associated with severe nausea and vomiting that is accompanied or followed by diarrhea.
The diarrhea arises as a result of bacterial toxins (enterotoxins) that inflame the stomach lining and cause water to be “dumped” into the lumen of the bowel from the bloodstream and tissue spaces. This is known as secretory diarrhea. Other types of bacteria invade the cells, destroy it and the lining of the intestine cannot absorb water and nutrients in the gut. This also leads to diarrhea, which may be bloody (hemorrhagic diarrhea) since the intestinal layer is damaged.
Spread of E.coli
In bacterial causes of infectious diarrhea, the pathogens are contracted through ingesting contaminated food or water (foodborne or waterborne). Sometimes the diarrhea is due to toxins that are present in food and is known as food poisoning. Read more on food infection and food poisoning.
With E.coli infection, food and water contaminated with small amounts of human or animal fecal matter may cause diarrhea when ingested. It can also be contracted when a person eats food prepared in unhygienic settings or where the hygiene standards are low – like if a person has not washed their hands after using the toilet and then handles food. Even a few bacteria can cause disease and with E.coli this can be less than 100 and even as low as 10 (low infecting dose).
Although may types of food may be contaminated with E.coli, in recent years there has been a focus on fresh uncooked vegetables as used in salads especially within eating establishments. Runoff water from animal grazing pastures may spread to neighboring fields where vegetables are grown. E.coli from the animal feces may then contaminate the vegetables.
Different Types of E.coli in Diarrhea
The strains of E.coli that can cause bowel infections and lead to diarrhea is known as diarrheagenic E.coli. Different strains of E.coli have different mechanisms of action. This means that it acts in different ways to cause disease, in this case diarrhea. Some E.coli invade or attach and destroy the cells lining the intestine (mucosal cells). Others produce an enterotoxin while a few strains may have the capability of both attaching to cells and producing toxins.
Enterotoxigenic E. coli (ETEC)
This strain of E.coli is a well known cause of traveler’s diarrhea and is frequently seen in developing countries. It causes a mild disease characterized by vomiting and diarrhea (gastroenteritis) which starts between 1 to 2 days after exposure (incubation period). ETEC causes secretory diarrhea as a result of the heat-stable enterotoxin rthat it releases. This causes large amounts of water to be released into the small intestine thereby resulting in diarrhea.
Enteropathogenic E.coli (EPEC)
This strain is an important cause of infant diarrhea. It is not associated with an enterotoxin but instead the bacteria attach to the mucosal lining of the small intestine. Here it destroys the mucosa and microvilli which ultimately affects water and nutrient absorption from the gut. The malabsorption leads to mild diarrhea that has the potential to become quite severe. Adults who are exposed to this type of E.coli are less likely to contract an infection unlike children.
Entero-invasive E. coli (EIEC)
This strain causes a Shigella-like dysentry. The bacteria do not produce an enterotoxin but invade and destroy the mucosal cells lining the bowels. Copious watery diarrhea and abdominal pain are the most prominent symptoms. There may be bloody stools but it is usually mild.
Entero-aggregative E. coli (EAggEC) or Enteroadherent E.coli (EAEC)
The entero-aggregative strain of E.coli, also known as aggregative non-EPEC, produces an enterotoxin when it adheres to the mucosal lining of the small intestine thereby causing secretory diarrhea. It is so named because of its pattern of aggregation when it attaches to mucosal cells, similar to a stacked brick pattern. EAggEC is known to cause persistent diarrhea in children, particularly in South America, South-east Asia and India. Enteroadherent E.coli (EAEC), also known as diffuse non-EPEC E.coli, also adhere to the lining of the small intestine and induce a toxic effect on the mucosal cells. EAEC causes childhood diarrhea and is also implicated in traveler’s diarrhea, particularly in Mexico and North Africa.
Enterohemorrhagic E. coli (EHEC)
These strains of E.coli or O serotypes have genes that allow it to adhere to the mucosal lining of the intestines and produce enterotoxins. The type of toxin is similar to that produced by Shigella dysenteriae (causes shigellosis or bacillary dysentery in humans) and is therefore known as shiga-like toxins or verotoxins. These E.coli are also known as Shiga-toxin E.coli (STEC) or verocytotoxic E.coli (VTEC). One of the better known of these E.coli serotypes is E.coli O157:H7, also referred to as E.coli O157, which is often responsible for outbreaks of E.coli infection. Other serotypes include O126, O111 and O103 which are also known as non-O157 STEC.
EHEC has a tendency to affect the large intestine (colitis) and causes bloody diarrhea (hemorrhagic colitis).
Signs and Symptoms of E.coli Diarrhea
The incubation period is between 1 to 7 days meaning that this time can elapse after exposure without any symptoms being present. E.coli infection causes diarrhea which can be watery and/or bloody. A person may not even feel unwell prior to the onset of the diarrhea. Abdominal cramps and constant abdominal pain are also prominent symptoms in E.coli diarrhea.
Nausea and vomiting is not always present and may be mild. Similarly, a fever may be mild or absent altogether. While most cases are self-limiting and will resolve within a few days even without treatment, it can persist and require medical attention. Complications associated with severe diarrhea includes dehydration and even malnutrition in persistent cases. Oral rehydration therapy is therefore essential during the symptomatic phase and even a short while after the diarrhea resolves.