Restoration of virulence to rat of axenically grown Entamoeba histolytica by cholesterol and hamster liver passage.

A virulent strain of Entamoeba histolytica (NIH-200) from a symptomatic human case, cultured in vitro for more than three years, was unable to infect rat caecum or hamster liver. When grown with a mixed bacterial flora, it infected rat caecum but did not produce ulceration. After passaging amoebae growing with bacteria in hamster liver, amoebic hepatic lesions were regularly produced. The amoebae from liver lesions produced ulceration in the caeca of rats. Expoxure to cholesterol of amoebae growing with bacteria or feeding rats orally with cholesterol and injecting these amoebae along with bacteria also led to the production of ulcers in rats. Thus virulent amoebae which have been cultured in vitro for a long time become attenuated but their virulence can be revived.