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No description available.

No description available.

The protozoan parasite Giardia intestinalis (syn. G. lamblia, G. duodenalis) is one of the most common causes of diarrheal disease throughout the world, where an estimated 500 million people are infected annually. Despite efforts in trying to elucidate factors associated with virulence in G. intestinalis little is currently known. The disease outcome is highly variable in Giardia infected individuals, ranging from asymptomatic carriers to severe disease. The reasons behind the differences in disease outcome are vaguely understood and studies trying to link infectivity to different Giardia assemblages or sub-assemblages have rendered conflicting results. Prior to this study, little was known about the prevalence and genetic diversity of different G. intestinalis assemblages across the world. In this thesis, molecular characterization of clinical G. intestinalis samples from Eastern Africa and Central America, has been performed, enabling a better understanding of the prevalence of different Giardia genotypes in endemic areas (Papers I and II). A correlation between Giardia colonization and the presence of Helicobacter pylori in the human host was established. We found that the currently available genotyping tools provide low resolution when used to characterize assemblage A Giardia. Also, genotyping of assemblage B isolates at these loci is troublesome due to the polymorphic substitutions frequently found in the sequencing chromatograms. This ambiguity was investigated by using micromanipulation to isolate single assemblage B Giardia cells (Paper III). Both cultured trophozoites and cysts from giardiasis patients were analyzed. The data showed that allelic sequence heterozygosity (ASH) does occur at the single cell level, but also that multiple sub-assemblage infections appear to be common in human giardiasis patients. Furthermore, genome-wide sequencing followed by comparative genomics was performed in order to better characterize differences between and within different Giardia assemblages. The genome of a non-human infecting, assemblage E isolate (Paper IV) was sequenced.  The genomes of two freshly isolated human infecting assemblage AII isolates were also sequenced (Paper V). Subsequent, comparative analyses were performed and included the genomes of two human infecting isolates, WB (AI) and GS/M (B). Several important differences were found between assemblages A, B and E, but also within assemblage A; including unique gene repertoires for each isolate, observed differences in the variable gene families and an overall difference in ASH between the different isolates. Also, a new multi-locus genotyping (MLG) strategy for genotyping of assemblage A Giardia has been established and evaluated on clinical samples from human giardiasis patients.

Giardia lamblia is a eukaryotic parasite of the upper small intestine of humans and animals. The infecting trophozoite cells do not invade the epithelium lining of the intestine, but attach to the brush border surface in the intestinal lumen. The giardiasis disease in humans is highly variable. Prior to this study, the molecular mechanisms involved in establishment of infection or cause of disease were largely uncharacterized. In this thesis, the molecular relationship between Giardia and the human host is described. The interaction of the parasite with human epithelial cells was investigated in vitro. Changes in the transcriptome and proteome of the parasite and the host cells, and changes in the micro-environment of the infection have been identified using microarray technology, and 1- and 2-Dimensional SDS-PAGE protein mapping together with mass spectrometry identification. The first large-scale description of cellular activities within host epithelial cells during Giardia infection is included in this thesis (Paper I). We identified a unique activation of the host immune response and induction of apoptosis upon infection by Giardia. Four important virulence factors of the parasite, directly linked to the success of Giardia infection, were characterized and are presented in Papers II and III. The parasite was shown to have immune-modulating capacities, and to release proteins during host-interaction that facilitate the establishment of infection. Additional putative virulence factors were found among Giardia genes transcriptionally up-regulated during early infection (Paper IV). In summary, this thesis provides important insights into the molecular mechanisms of the host-parasite interaction.