Isolation of Different-Sized Substrains from Three Stock Cultures of Entamoeba histolytica, with Observations on Spontaneous Size Changes Affecting Whole Populations

SYNOPSIS. Three stock cultures of E. histolytica , strains 200, K9, and 22, have each been shown by microisolation methods to consist of at least two substrains or variants differing in size. Cultures were grown with Trypanosoma cruzi in Phillips medium, and on egg slants with mixed bacteria and a single species of bacteria. For each strain, the variant that was bigger in one medium was also bigger in the other two media, in spite of the fact that absolute sizes changed greatly depending upon the medium. This shows that size patterns were primarily determined by endogenous factors, although absolute size was strongly influenced by the culture medium. In all cases the small variants remained larger than E. hartmanni.
In the initial weeks after microisolation, the variants showed consistent size patterns. After a few months, the big substrains of E.h. K9 and 22 showed a spontaneous reduction in size, eventually becoming smaller than the corresponding small variants. The change in E.h. K9 was shown to be due to a shift in the entire population, rather than to selection of individual mutants. In the E.h. 200 cultures, big and small variants retained their distinctive sizes throughout the 17 months during which they were observed.