Genome-plasticity and adaption in Helicobacter pylori

Abstract

Helicobacter pylori is a gastric bacterial pathogen that infects an estimated half. of the human population. The infection is primarily acquired during early childhood and causes a persistent infection that is usually not eradicated by the host immune system. Although most infected individuals. remain asymptomatic, a significant proportion develops peptic ulcer disease and gastric cancer. This thesis aimed to study diversity of the cag pathogenicity island (PAI) and Lewis antigen expression in the lipopolysaccharide (LPS). These two traits in H. pylori may both play a role in persistence of infection and disease progression.

The cag PAI of H. pylori codes for a type IV secretion system and promotes cellular proliferation and conformational changes in host epithelial cells. We used microarray to study the complete composition of cag PAI genes and found that a significant part of clinical isolates carried incomplete islands. There was a correlation between harboring a complete cag PAIs and induction of IL-8 in host cells, as well as diagnosis of severed~ in the colonized host. Moreover, we observed clonal variants with different cag PAI genotypes that co-existed within one host. In order to further analyze the structure of the rag PAI, the complete region was sequenced in four of the isolates. The general layout of the cag PAI was similar in all four isolates, but some distinct features were observed, including the presence of an IS606 element in one isolate and a large rearrangement and/ or insertion at the 3'end of the cag PAI of another isolate. An interesting finding was the discovery of a previously undescribed gene at die HP0521 locus, designated HP0521B, in three of the isolates. This gene showed no significant matches in the database, but was present in about half of Swedish clinical isolates, as determined by PCR. None of the observed genetic variants did however affect the strains ability to express a functional type IV secretion system.

The LPS of H. pylori contains carbohydrate Lewis antigens, which may be involved in the interaction with host cells and immune evasion. We observed extensive diversity within H. plod populations that occurred in vivo over time and in different gastric regions. Moreover, we described a mechanism by which H. pylori varies Lewis glycosylation patterns and suggest a molecular ruler regulatory mechanism for the alpha1,3fucosyltransferases FutA and FutB. Variation in the sizes of O-antigen polymers being fucosylated depends on the number of heptad repeats in the C-terminal region of FutA and FutB, where one heptad repeat corresponds to one O-antigen repeat unit. Diversity of Lewis expression may arise in a random fashion or be induced during certain conditions. We could demonstrate that phase variation of Lewis expression occurs during in vitro passages on agar plates, and that the size of the bottleneck in each transfer may affect the frequency of diversification. Moreover, after transfer to a new animal host, a relatively higher degree of diversity is induced, as exemplified by extensive LPS diversity in clonal descendants after experimental infection in mice. Our data suggest that although microdiversity exist after in vitro passages, diversity is more pronounced after in vivo passages, possibly due to adaptation to the new host.

The presence of bacteria with different genetic backgrounds and hence different properties at a given time, in a certain niche, provides a great advantage for the bacterial population. When the environmental conditions change, clones with a survival advantage will expand, whereas less fit variants will be restrained or even disappear. Since an H. pylori infection can result in various outcomes, the diversity of the bacterium is likely significant for the disease development. Studies of microdiversity between subclones and their interplay can provide valuable information about the population fitness and why H. pylori causes persistent infections as well as gastro-duodenal disease.