C. dif cile is a Gram-positive rod that readily forms spores. Its early reputation for fastid-ious growth is responsible for its species epithet. Like the other clostridia described in this section, C. dif cile has a most important medical feature: its ability to produce toxins. In this species, two distinct large polypeptide toxins, A and B, with similar structure (45% homology) are released during late growth phases of the vegetative organism, perhaps at the time of cell lysis. Both toxins act in the cytoplasm by disrupting proteins involved in signal transduction, particularly those involving the actin cytoskeleton. The A toxin causes cell rounding and the disruption of intercellular tight junctions followed by altered membrane permeability and fluid secretion. The net effect is that of an enterotoxin, al-though inflammation and cytoxic activity are also present. The B toxin lacks the entero-toxic properties of the A toxin but has cytotoxic potency at least 10 times higher. The two toxins appear to act synergistically by a mechanism yet to be determined.