Effect of Hamster Liver Passage on the Virulence of Axenically Cultivated Entamoeba Histolytica *

Abstract

Abstract Virulence of axenically cultured Entamoeba histolytica as a function of lesion size in adult hamster liver was examined in a statistically analyzed model. Several potential sources of variation inherent in animal hepatic inoculation experiments were considered. The effects of growth stage in culture was examined. Log or exponential growth phase cells produce significantly larger lesions than cells from either lag or decline phase of axenic culture. A comparison of the size of lesions produced by unmanipulated HM-1 or substrains of HM-1 previously passed through the liver of hamsters (control strains) was made with the size of lesions produced by λ substrains (obtained by the passage of control strains through hamster liver one additional time). Control strains were paired with corresponding λ substrains and inoculated into the right or left hepatic lobe of the same animal. Statistical analysis of the data demonstrated an overall significant increase in mean lesion size due to a single additional liver passage. A similar comparison was made of the size of lesions produced by unmanipulated HM-1 and by substrains which had been passed through the liver two or four times. This revealed that a significantly larger increase in lesion size resulted from multiple hamster liver passages alternating with axenic culture. Hamsters killed 1 mo after inoculation with a substrain passed four times through hamster liver had large lesions characteristic of human amebic liver abscess. Hamsters inoculated with unmanipulated HM-1 had no grossly demonstrable lesions at 1 mo. Increased virulence conferred by hamster liver passage could still be demonstrated 45 wk after final isolation from liver of a substrain passed four times. These observations support the concept that multiple hepatic passages significantly enhance virulence of trophozoites of E. histolytica which have become minimally pathogenic during long-term axenic cultivation. Periodic hamster liver passage offers a method of restoring and maintaining virulence in E. histolytica kept in axenic culture. Comparative studies of attenuated axenic strains and those with enhanced virulence could reveal modifications necessary for pathogenicity of this protozoan.