Drug sensitivities of several strains of Entamoeba histolytica grown at 37°C and strain Laredo and E. invadens grown at room temperature in the CLG medium were compared. Strain Laredo and E. invadens were similar in response in that they were more resistant to 3 or 4 drugs tested than were strains which were propagated at 37°C. In this respect and the fact that it no longer propagates at 37°C in our laboratory the Laredo strain is more like E. invadens than like E. histolytica. We have previously shown that the same is true for its free amino acid content. One recently isolated strain of E. histolytica (D1) grown at 37°C was, in general, more resistant than were several strains which have been grown in synthetic medium for years in this laboratory. This supports the idea that the testing of recent isolates is important in studies of this kind, since such isolates have not been subjected to any selective pressures that culture factors may exert.
One drug, paromomycin was equally effective at low concentrations against all strains of Entamoeba tested. It is suggested that testing of drugs at both 37°C and room temperature may be useful as a preliminary screening device for drugs to be used in the treatment of amebiasis, since effectiveness at both temperature extremes might indicate that a drug has a “broad spectrum” of activity against Entamoeba.