Bacterial toxins induce changes in membrane transport which underlie the loss of
electrolyte homeostasis associated with
diarrhea. Bacterial- and their secreted toxin-types which have been linked with
diarrhea include: (a) Vibrio cholerae (
cholera toxin, E1 Tor
hemolysin and
accessory cholera enterotoxin); (b) Escherichia coli (heat stable
enterotoxin, heat-labile
enterotoxin and
colicins); (c) Shigella dysenteriae (
shiga-toxin); (d) Clostridium perfringens (C. perfringens
enterotoxin, alpha-toxin, beta-toxin and theta-toxin); (e) Clostridium difficile (toxins A and B); (f) Staphylococcus aureus (alpha-haemolysin); (g) Bacillus cereus (
cytotoxin K and haemolysin BL); and (h) Aeromonas hydrophila (
aerolysin, heat labile
cytotoxins and heat stable
cytotoxins). The mechanisms of toxin-induced
diarrhea include: (a) direct effects on ion transport in intestinal epithelial cells, i.e. direct toxin interaction with intrinsic
ion channels in the membrane and (b) indirect interaction with ion transport in intestinal epithelial cells mediated by toxin binding to a membrane receptor. These effects consequently cause the release of second messengers, e.g. the release of
adenosine 3',5'-cyclic
monophosphate/guanosine 3',5'-monophosphate, IP(3), Ca2+ and/or changes in second messengers that are the result of toxin-formed Ca2+ and K+ permeable channels, which increase Ca2+ flux and augment changes in Ca2+ homeostasis and cause depolarisation of the membrane potential. Consequently, many voltage-dependent ion transport systems, e.g. voltage-dependent Ca2+ influx, are affected. The toxin-formed
ion channels may act as a pathway for loss of fluid and
electrolytes. Although most of the
diarrhea-causing toxins have been reported to act via
cation and
anion channel formation, the properties of these channels have not been well studied, and the available biophysical properties that are needed for the characterization of these channels are inadequate.