BIGUANIDE-INDUCED CHANGES IN ACANTHAMOEBA-CASTELLANII - AN ELECTRON-MICROSCOPIC STUDY

Trophozoites and cysts of Aranthamoeba castellanii were exposed to chl orhexidine diacetate (CHA) and polyhexamethylene biguanide (PHMB); cha nges in cell ultrastructure and surface structure were examined by bot h transmission and scanning electron microscopy. PHMB caused a greater degree of structural and membrane damage; the cytoplasmic contents we re severely depleted and there were clusters of densely stained precip itates on the cell surface. Concentrations of CHA greater than 100 mu g ml(-1) produced shrinkage from the cyst wall. At high concentrations , PHMB induced a slight withdrawal of the cytoplasm from the wall and, unlike CHA, induced swelling of the cysts. These findings do not defi ne the mechanisms of action of CHA and PHMB, but provide evidence that a major target site for both agents is the plasma membrane. However, additional intracellular damage undoubtedly contributes to the lethal effects. The greater resistance of cysts may be associated with reduce d biguanide uptake.