The Ecology and Evolution of Bacterial Virulence

Abstract

Infections by bacterial pathogens are one of the biggest causes of human mortality each year globally. Increasingly they are being caused by bacteria that can survive in, and adapt to, non-host associated environments. It is therefore crucial we understand what may cause bacteria in these settings to both become more pathogenic and to successfully colonise new environments. The aim of this thesis is to improve our understanding of the effect of metals on bacterial virulence, and the effect of pathogen invasion into a community. Anthropogenic pollution is increasing concentrations of metal ions to toxic levels in many environments around the world, and whilst their effect on antimicrobial resistance is known, their effect on virulence is not. Copper selected for increased virulence in populations of the opportunistic pathogen Pseudomonas aeruginosa at environmentally relevant concentrations. This was largely due to copper selecting for increased siderophore production, which is both a metal detoxification mechanism, and a virulence factor. Conversely, copper decreased the virulence of multiple wastewater influent bacterial communities. It is most likely that this was due to changes in community composition, and decreases in community productivity. As these results demonstrated that copper can affect bacterial virulence, it was then tested whether liming, which is a common metal remediation method, can reduce this selective effect in metal polluted river water and sediments. Liming was found to have little effect on either virulence or antibiotic resistance, demonstrating that even remediated environments can remain as reservoirs for pathogens. Finally, pulse mortality events facilitated the invasion of P. aeruginosa into a bacterial community. Furthermore, it was shown that the combined effect of pulse mortality and invasion greatly reduced resident diversity. This demonstrates that disturbance events may increase the risk posed by bacterial pathogens. These results demonstrate that ever-increasing metal pollution is likely to affect bacterial virulence, and that pathogen colonisation can detrimentally affect resident communities.