Mechanisms of genetic adaptation in Helicobacter pylori

Abstract: Helicobacter pylori is one of the most common bacteria worldwide, estimated to infect half of the human population. The infection is usually established in childhood and persists for a lifetime, unless treated. Most individuals never experience symptoms from H. pylori infection and are not aware of that their stomachs are invaded by spiral-shaped bacteria. However, about 20% of the infected individuals develop severe diseases such as peptic ulcers and gastric cancers. Why some persons are asymptomatic while others develop disease is not fully understood, but it seems to be a combination of bacterial, host and environmental factors. Since H. pylori was discovered in 1982 research has been devoted to solving the mystery of H. pylori associated disease development, and much has been learned, but still there are many unanswered questions. H. pylori is described as a panmictic population where the clonal structure is lost due to high recombination frequencies, which makes genetic studies challenging. This thesis aimed to investigate potential virulence associated genes: Restriction-modification (R-M) systems and Lipopolysaccharide (LPS), and to describe how these genes are expressed, what function they may have and the underlying mechanisms of H. pylori adaptation. In paper I, the H. pylori type II MTase, M.HpyAIV, was characterized and functionally described. The frequency and distribution of GANTC sites in the fully sequenced H. pylori strains was mapped. In a M. HpyAIV knockout mutant strain, the H. pylori catalase was significantly down-regulated, which implies that M.HpyAIV affect gene expression. In paper II, the H. pylori type I R-M systems were in focus. The presence of the different subunits was investigated and we found that these R-M systems were rather conserved in our clinical isolates. However, genetic analyses revealed high allelic diversity of the specificity subunits involved in DNA recognition. In paper III intra-individual strains from biopsies obtained at different regions of the stomach were investigated. A molecular ruler model for how fucosylation of O-antigens may occur was suggested and correlated to genetic structures of the fucosyltransferase genes. In paper IV, isolates in different environmental settings were described and we showed that the phenotype of O-antigen chains altered when H. pylori strains were passaged in vivo in mice. Furthermore, we revealed that recombination events occurred between the two orthologous alpha1,3-FucTs in intra-individual isolates, which may alter their activity. The studies of this thesis contribute to the understanding of inter- and intra- individual diversity of H. pylori isolates and how these bacteria may evolve in order to adapt to new hosts and new environments. The way H. pylori alters its genotype has an impact on the expression and activity of the enzymes, which in turn may be of importance in H. pylori pathogenesis.