Selection of specific genotypes of Giardia intestinalis by growth in vitro and in vivo.

This study examined whether allelic changes observed when clinical isolates of Giardia intestinalis made in suckling mice were adapted to in vitro growth occurred as a result of gene switching (alternate isoenzymes) or through selection of organisms with different genotypes from mixed infections. Samples were compared electrophoretically at 20 enzyme loci. Marked allelic differences were detected between the uncloned clinical isolates grown in mice and the axenic cultures established from them. Furthermore, the allelic profiles of the uncloned isolates changed during the course of in vivo or in vitro growth. In contrast, all clones produced from each isolate retained identical allelic profiles, regardless of whether they were grown in vivo or in vitro. These findings argue against gene switching as an explanation for the observed allozyme changes and support preferential selection of organisms with specific genotypes by growth conditions. The data indicate the presence of at least 2 and possibly up to 4 distinct genotypes within each clinical isolate. The genetic differences detected between clinical isolates in suckling mice were of similar magnitude to those that separate different axenic isolates of G. intestinalis into cryptic species. Conversely, the genetic differences between the isolates were limited when sampled after establishment in vitro. These findings have significant implications for research on Giardia and other medically important parasites and raise the possibility that culture may exert a similar selective bias on the genotypes isolated from infections with other parasitic protozoa.