Studies on a Morphologically Distinct Colchicine‐Resistant Variant of Entamoeba sp.*

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Abstract
Colchicine has a temperature-dependent cytotoxic effect on Entamoeba sp. (Laredo isolate) that is most apparent when the drug is applied during the initiation of cultures at a concentration of 7.5 mM or higher. Continued transfer of cultures in medium containing progressively increasing concentrations of colchicine resulted in a variant that grows prolifically in the presence of colchicine (7.5 mM) with a generation time comparable to that of the parent stock. Comparison of a number of parameters of the 2 variants revealed that colchicine resistance was accompanied by a change in cell shape, a reduced membrane permeability [which could partially be overcome by the addition of dimethyl sulfoxide (DMSO)] and a reduced tolerance to osmotic stress. The parent strain and resistant variant were equally susceptible to cycloheximide and puromycin suggesting that the acquired colchicine resistance may not be explained on the basis of an entirely unspecific generalized reduced ability for drug uptake. Colchicine resistance and altered structure were stable over a long period of time. The possible interdependence of these 2 parameters and their relation to cell motility in Entamoeba sp. are discussed.