Is human giardiasis caused by two different Giardia species?

Abstract

We have recently sequenced the genome of the human Giardia intestinalis assemblage B isolate GS.1 comparisons to the earlier sequenced genome of the human assemblage A isolate WB showed that the average amino acid identity in 4,300 orthologous proteins was only 78%. Here we discuss these results in the light of new genome sequencing data from the hoofed-animal assemblage E (isolate P15, isolated from a pig) and further characterization of assemblage A and B isolates from humans. There is a highly conserved set of core genes (4,557 genes, 91% of genome) common to all isolates. The largest genomic differences are found in variable, Giardia-specific gene families and a large number of chromosomal rearrangements were detected, even between different chromosomes. Surprisingly, the assemblage E and A isolates are more similar at the amino-acid level than the two human isolates are to each other. This strengthens our earlier data suggesting that humans are infected by two different species of Giardia.

Keywords: comparative genomics; diarrhea; genotype; intestinal parasite; protozoa.

Figure 1

The intestinal protozoan parasite Giardia intestinalis is flagellated and binucleated. It has special cytoskeletal structures like the adhesive disc and the median body, giving it the characteristic “happy-face” look. Currently we have the genome sequence of three different G. intestinalis isolates; WB-assemblage A, GS-assemblage B and P15-assemblage E. assemblage A and B parasites can be found in humans and certain sub-genotypes have also been found in other mammals like dogs, cattle, rabbits, monkeys, sheep and moose. Assemblage E parasites are mainly found in hoofed animals. We compared orthologs defined by reciprocal BLAST searches and synteny information. Ortholog alignments were created using the transAlign program, which increases the accuracy of a nucleotide alignment by using the corresponding amino acid alignment as a template. Members of large gene families are not included in this comparison. The average amino acid identity between the different isolates is 78% (A–B), 81% (E–B) and 90% (A–E). Many big questions remain to be answered but further genome and transcriptome sequencing can answer many of these questions.