Pathogen environmental persistence, its evolutionary dynamics, and its interactions with abiotic factors and other traits.

Abstract

Environmental persistence is a life history trait of extreme importance to many pathogens, not only because it influences their ability to maintain onward transmission to new hosts, but also as it has the potential to drastically influence the evolutionary trajectories of other pathogen traits such as virulence. By understanding the evolution of environmental persistence and the associations between it and other traits and environmental factors, we can build knowledge on how to mitigate and potentially prevent outbreaks of pathogens dangerous to both humans and other species. In this thesis we aimed to explore the environmental persistence of bacterial pathogens both within the literature and in a practical system. For the former, in chapter one, we conducted a comparative analysis of 47 papers on 29 species of bacterial pathogens, and extracted data which allowed us to investigate potential correlations between environmental persistence and other traits (including generation time and transmission mode), as well as other abiotic factors (including humidity and surface of exposure). For the latter, in chapter two we conducted a persistence assay on 27 isolates of the emerging pathogen Mycoplasma gallisepticum, where every isolate was exposed to environmental conditions for differing time points and then checked for viability in order to quantify their environmental persistence. Previous data collected on each isolate’s virulence was also used to explore any associations between those two traits. The results of our comparative analysis showed significant associations between environmental persistence and humidity, bacterial cell wall type (gram-positive or gram-negative), surface of exposure (indirect or direct), temperature, and transmission mode. The results of our Mycoplasma gallisepticum experiments showed no significant difference between the environmental persistence values of different isolates against their year of sampling and no significant association between environmental persistence and virulence. Though in part contrary to expectations, our results on the whole indicate support for the theory that environmental persistence is a significant trait to consider when examining the evolution of bacterial pathogens, as well as when examining potential trade-offs with and the evolution of other bacterial pathogen traits.