Epsilon Open Archive

Abstract

Botulism is caused by botulinum neurotoxins (BoNTs) produced by the spore forming strictly anaerobic bacterium Clostridium botulinum. Seven different types of BoNTs (type A-F) have so far been established on the basis of neutralization with different antibodies. Botulism affects both humans and animals, and there are occasionally large-scale outbreaks of high mortality in animals. Especially large outbreaks of avian botulism have been reported from various countries, including Sweden. Other animals relatively commonly affected are cattle, horses, sheep and farmed fur animals. C. botulinum is a diverged species and can be organized into four groups, which reflect their genetic and physiological differences. C. botulinum group III strains producing BoNT types C, D and chimers C/D and D/C, are mainly connected to animal botulism. The gene encoding BoNT in C. botulinum group III strains is located on an unstable plasmid-like phage.

In this thesis, strains of the previously relatively uncharacterised C. botulinum group III were isolated and genotyped with pulsed-field gel electrophoresis. Several pulsotypes were formed, but the majority clustered closely together and represented most of the chimeric strains. Strains representing different pulsotypes and different animal and geographical origin, were selected for whole genome sequencing and the resulting genomes could be divided into four genomic lineages. Comparisons against genomes of Clostridium novyi and Clostridium haemolyticum revealed that they could be organized into the same genomic lineages (lineages II-IV), which resulted in the suggested collective term C. novyi sensu lato. The organization of all sequenced genomes was analysed. It revealed a relatively conserved chromosome and an abundance of highly dynamic plasmids. The plasmids, lineages and species were entwined because plasmids and toxin genes had moved across the lineage boundaries. Of the four lineages, only lineage I was C. botulinum specific, and this lineage includes strains of the most common pulsotype. One genome of lineage I was assembled into completion. It was smaller than C. botulinum group I and II genomes, but contained as much as five plasmids. Most of the identified putative toxin genes were found on these plasmids. Strains of lineage I may be more virulent than other C. botulinum group III strains, which is reflected by their domination in animal botulism cases today.