Genotypic heterogeneity within Giardia lamblia isolates demonstrated by M13 DNA fingerprinting

There has been considerable speculation regarding the possible relationship between the phenotypic and genotypic heterogeneity seen among human isolates of Giardia lamblia and the wide clinical spectrum of human giardiasis. Several workers have suggested that human giardiasis may be a mixed infection consisting of variant strains or subgroups which are present in the same infection and which are selectable, but it is not clear whether these apparent variant strains represent a truly heterogeneous infection or whether the genotypic heterogeneity observed is due to the susceptibility of the Giardia genome to a high rate of structural genetic rearrangement. We have therefore studied variation in Giardia intestinalis genotypes in 19 isolates in vitro and in vivo by using the technique of M13 DNA fingerprinting. Genotypes of isolates changed with time when cultured under standard conditions and when pressured with bile. Sequential isolates and their clones taken from a patient with chronic giardiasis both before and after several treatments with metronidazole had different genotypes. Finally, clones of isolate WB had different initial genotypes, which changed after 4 months in culture. These findings suggest that the apparent genotypic heterogeneity at least in these G. intestinalis isolates is more likely to be due to the plasticity of the Giardia genome than to the presence of a truly mixed population of strains within the same infection.