The coevolutionary dynamics of the Ralstonia solanacearum and its bacteriophages – implications for biocontrol of bacterial wilt

The Ralstonia solanacearum species complex (RSSC) is a destructive closely related bacterial plant pathogen that cause immense economic and agricultural damage worldwide. RSSC consists of three species Ralstonia pseudosolanacearum, Ralstonia solanacearum and Ralstonia syzygii. Due to their remarkable similarities, both genetically and phenotypically, the boundaries to define each species of RSSC as completely separate from each other becomes unclear and instead are grouped together in a species complex. Control of RSSC is extremely difficult as a result of geographical distribution, broad host range, virulence, persistence in natural reservoirs and now climate change. Several strategies have been proposed to control RSSC including, interplanting, resistant cultivars and biocontrols. However, many remain insufficient. Lytic bacteriophage (phages) are viruses which can infect and kill bacteria; they have often been purposed as a potential biocontrol for RSSC. Numerous studies have found that phages are effective at suppressing RSSC and even providing plant protection in greenhouse and field experiments. However, infectivity level, infectivity range and phage resistance evolution are major obstacles in developing phages as effective biocontrols.
RSSC and its phage engage in antagonistic coevolutionary dynamics, the reciprocal adaptation and counter-adaptation between interacting host and parasite populations. This thesis aims to exploit the coevolutionary trajectories of phages and RSSC to develop highly effective biocontrols through experimental evolution. We first pre-adapted (“trained”) phages against three genotypes of Ralstonia solanacearum in mono- or combination cultures (Chapter 2) with one-sided experimental evolution, with the aim of improving infectivity and infectivity range of phages. We found that multiple genotype trained phages were much more effective at suppressing two genotypes of R. solanacearum compared to their single genotype trained phages. Unfortunately, phage resistance evolved rapidly against trained phages. Next, coevolution experiments on four phages against one genotype of R. solanacearum was tested to see if evolving interacting populations would result in increased infectivity levels (Chapter 3). However, rapid evolution of resistance in R. solanacearum drove phages to extinction, suggesting coevolutionary dynamics were highly asymmetrical. Finally, combination experiments were conducted to see if having multiple phages in combination would suppress R. solanacearum resistance evolution and characterise highly effective phages in our collection (Chapter 4).
Overall, this thesis provides insights and predictions into asymmetrical evolution and resistance in R. solanacearum but also highlights the potential of bacteriophages to control R. solanacearum.