Mucosal adherence

Most bacteria causing diarrhoea must first adhere to specific receptors on the mucosa. A number of different molecular adhesion mechanisms have been elaborated; for example, adhesions at the tip of the pili or fimbriae which protrude from the bacterial surface aid adhesion. For some pathogens this is merely the prelude to invasion or toxin production but others such as enteropathogenic Escherichia coli (EPEC) cause attachment-effacement mucosal lesions on EM and produce a secretory diarrhoea directly as a result of adherence. Enteroaggregative E. coli (EAggEC) adhere in an aggregative pattern with the bacteria clumping on the cell surface and its toxin causes persistent diarrhoea in developing countries. Diffusely adhering E. coli (DAEC) adheres in a uniform manner and may also cause diarrhoea in children and in developing countries.

Mucosal invasion

Invasive pathogens such as Shigella spp., enteroinvasive E. coli (EIEC) and Campylobacter spp. penetrate into the intestinal mucosa. They destroy the epithelial cells and produce the symptoms of dysentery: low-volume bloody diarrhoea, with abdominal pain.

Toxin production

Gastroenteritis can be caused by different types of bacterial toxins:
Enterotoxins, produced by the bacteria adhering to the intestinal epithelium, induce excessive fluid secretion into the bowel lumen, leading to watery diarrhoea, without physically damaging the mucosa, e.g. cholera, enterotoxigenic E. coli (ETEC). Some enterotoxins preformed in the food primarily cause vomiting, e.g. Staph. aureus and Bacillus cereus. A typical example of this is 'fried rice poisoning', in which B. cereus toxin is present in cooked rice left standing overnight at room temperature.
Cytotoxins damage the intestinal mucosa and, in some cases, vascular endothelium as well.

Clinical syndromes

Bacterial gastroenteritis can be divided on clinical grounds into two broad syndromes: watery diarrhoea (usually due to enterotoxins, or adherence), and dysentery (usually due to mucosal invasion and damage). With some pathogens such as Campylobacter jejuni there may be overlap between the two syndromes.