The ingested organism resists expulsion from the small intestine with
the rapidly flowing chyme by adhering to the epithelium through attach-
ment or colonization factors in the form of fimbriae on the bacterial cell
surface. These can have different morphology and be either rigid (6–7 nm
diameter) or flexible (2–3 nm diameter) structures composed of 14–22 kDa
protein subunits. They are mannose resistant,i.e.they mediate haemag-
glutination in the presence of mannose, and particular colonization
fimbriae are restricted to certain O:H serotypes. They are encoded on
plasmids which frequently also encode for the diarrhoeagenic toxins.
Two toxin types are produced: the heat-stable toxins (ST), which can
withstand heating at 100 1 C for 15 min and are acid resistant, and the
heat-labile toxins (LT) which are inactivated at 60 1 C after 30 min and at
low pH. LTI bears a strong similarity to cholera toxin; it consists of five
B subunits (Mr11.5 kDa) which are responsible for binding of the toxin
to the epithelial cells and an A subunit (Mr25 kDa) which is translocated
into the epithelial cell where it activates adenylate cyclase. The subse-
quent increase in cAMP levels then inhibits Na^1 ,Cland water absorp-
tion by the villus cells and stimulates their loss from intestinal crypt cells
thus leading to profuse watery diarrhoea. LTII toxin produced by certain
ETEC strains has similar biological activity to LTI but does not cross
react with antiserum to LTI or cholera toxin.
Two types of ST have been recognized; the most common, STA,isa
low molecular weight, poorly antigenic polypeptide of less than 20 amino
acids produced from a 72 amino acid precursor. Its resistance to heat,
low pH and proteolytic digestion probably derive from its compact
three-dimensional structure which contains at least 3 disulfide linkages.
It acts by stimulating the production of cGMP by guanylate cyclase in
epithelial cells. The mechanism of action of STB, which can be distin-
guished from STA by its inability to produce fluid secretion in the
intestines of suckling mice, is not known but does not appear to operate
through the stimulation of cyclic nucleotide production.
7.8.3.2 EnteroinvasiveE. coli(EIEC). Infection by EIEC results in
the classical symptoms of an invasive bacillary dysentery normally
associated withShigella. LikeShigella, EIEC invades and multiplies
within the epithelial cells of the colon causing ulceration and inflamma-
tion, though EIEC strains do not produce Shiga toxin. Clinical features
are fever, severe abdominal pains, malaise and often a watery diarrhoea
which precedes the passage of stools containing blood, mucus, and faecal
leukocytes. Invasiveness is determined by a number of outer membrane
proteins which are encoded for on a large plasmid (E140 MDa). The
infective dose of EIEC appears to be substantially higher than for
Shigellaand this is thought to be a reflection of the organism’s greater
sensitivity to gastric acidity.
Chapter 7 219