Genetic and Functional Characterisation of pTet-like Plasmids of Campylobacter jejuni
Date: 21 November 2016
Thesis or dissertation
University of Exeter
PhD in Biological Sciences
With more than 400 million cases a year, Campylobacter jejuni is a leading cause of gastroenteritis worldwide. Farm animals, such as cattle poultry and pigs, act as an environmental reservoir for the bacterium. Although many studies have been performed in order to gain a better understanding of the disease and of the environmental ...
With more than 400 million cases a year, Campylobacter jejuni is a leading cause of gastroenteritis worldwide. Farm animals, such as cattle poultry and pigs, act as an environmental reservoir for the bacterium. Although many studies have been performed in order to gain a better understanding of the disease and of the environmental lifestyle of the bacterium, much remains to be uncovered. This work aim is to understand the role of the plasmid pTet in the bacterium lifestyle: firstly, analysing which genes are most represented in the plasmids pTet-like and if a difference in genetic set-up between pTet-like plasmids exists and what is its significance. Secondly, assessing whether the plasmids pTet-like are associated with bacterial characteristics, such as virulence, survival or adaptation to different ecological niches. Several plasmids have previously been described in C. jejuni: pTet is the most widely distributed plasmid amongst the known C. jejuni strains. This plasmid is about 45 kilobase pairs (kbp) long and 52 open reading frames (ORFs) can be identified in its sequence. Before this work, it was known to be found in about 20 % of C. jejuni strains, to encode for tetracycline resistance and a conjugative type four secretion system and to possess several genes involved in horizontal gene transfer. However, the function of this plasmid and the reason why it is so broadly distributed remain largely unknown. In the first part of this work I screened 4005 genome sequenced strains of C. jejuni for the presence of the plasmid. I investigated the genetic make-up of the pTet plasmids, sequencing 19 new plasmids and described a core and accessory sets of genes in this plasmid family using an innovative approach. I found that the core set of genes is predicted to be primarily involved with the maintenance and transfer of the plasmid pTet, while the accessory set of genes are predicted to be associated with metabolism, resistance and regulative functions. In the second part of this work I produced 11 C. jejuni deletion mutants. I tested these mutants for phenotypes associated with environmental survival and host invasion such as biofilm production, motility and infection of the model organism Galleria mellonella larvae. Two of the mutant strains showed an impaired phenotype, when compared to the wild type strain, in motility, production of biofilm and growth in MH broth. These findings indicate that the pTet plasmids have functions other than tetracycline resistance and horizontal transfer, and may help to explain the broad distribution of the plasmids in C. jejuni.
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