In vitro activity of metronidazole on Entamoeba histolytica in axenic culture

Various strains of axenic Entamoeba histolytica were exposed to concentrations of metronidazole ranging from 0·5 μg./ml. to 50 μg./ml. Observations were made every 4 hours with the aid of light and phase microscopy, as well as electron microscopy. Neutral red staining was also employed in an effort to distinguish live from dead amoebae. All strains survived 120 hours in 0·5 and 1·0 μg./ml. without discernible effect. At 2 μg./ml. strains of F-22 and 301 died within 24 hours, but strain HK-9 survived. Amoebae of all strains studied died at 4 μg./ml. within 20 hours. Early non-specific changes were visible by electron microscopy. In vitro amoebacidal levels of metronidazole are noted to be well below the serum concentration attained during therapy for amoebiasis.