Trypanosoma cruzi: Inoculation Schedules and Re-isolation Methods Select Individual Strains from Doubly Infected Mice, as Demonstrated by Schizodeme and Zymodeme Analyses1

Abstract

Groups of mice received double infections with the Y and F strains of T. cruzi, the 1st inoculum of either strain being followed by a 2nd inoculum of the other strain on day 5, 15, 30-40, or 60-65. Parasites were reisolated from blood into culture, either directly or with an intermediate passage in .gamma.-irradiated mice, at intervals between 7 and 35 days after the 2nd inoculation. Strain identification in the reisolated material was by electrophoresis of [kinetoplast] kDNA fragments generated by the EcoRI restriction endonuclease and by electrophoresis for glucosephosphate isomerase isozymes. Both strains were identified in 22% of reisolates originating from the experimental mice and only one of them was present in the remaining reisolates, strain F being the most frequent. In some instances either Y or F was reisolated from the same blood source, depending on whether culturing had been preceded or not by passage through a mouse. These results are certainly related to strain differences in the various aspects of host-parasite relationship and, possibly, growth rates in culture. More than 1 strain of T. cruzi can coexist in the same host; the timing and method of parasite isolation from the vertebrate host act as selective factors and further passages (in mice or cultures) may completely eliminate one (or more) strain from originally mixed trypanosome populations, and kDNA restriction fingerprints and isozyme profiles are simple, sensitive, and reliable techniques for strain identification both in single and mixed preparations.