Strains of E. histolytica from axenic cultures were studied with regard to their virulence and pathogenicity. Inoculation of amebae from axenic cultures into the liver of weanling hamsters did not result in hepatic abscesses. Reassociation of amebae with various bacterial strains resulted in hepatic abscess regularly when amebae and bacteria were reassociated for an interval of about 6 to 12 hours before inoculation into the liver. Loss of virulence upon axenization, however, was found to decline slowly over a period of from 3 to 5 months. In order to determine if there was some factor elaborated by, or associated with, bacteria that would cause increased amebic virulence, we added filtered bacterial cultures, bacteria killed by heat or X-radiation, or ground or sonicated bacteria to amebae from axenic cultures, which were then inoculated into livers of hamsters or guinea pigs. Hepatic abscess could not be produced by these methods. Similarly, growth of axenic amebae and bacteria in parabiotic chambers separated by a semipermeable membrane did not result in enhanced amebic virulence. These experiments suggest that increased virulence is associated only with contact of amebae with live bacteria. There may be the transfer of a factor from the bacteria to the amebae that alters virulence.
Career Scientist, Health Research Council, City of New York (I-450).