Understanding the Evolution of Emerging Bacterial Pathogens in Response to Host Resistance

Abstract

Understanding the evolution of parasites and hosts following a host-shit event is increasingly recognised as being of great importance to public health and the clinical/veterinary sciences in predicting the behaviour and evolutionary consequences of emerging infectious diseases. Microbial pathogenesis and virulence are remarkably complex traits, and only by considering them in the context of their hosts can we begin to unravel key questions as to how and why disease emerges and persists. The period immediately following a host-shift event, where a pathogen circulating in one host species successfully jumps into another is critical – whether such outbreak events “burn out” or become endemic, and what the ramifications of this might be are difficult to model and predict. That microbial pathogens and their hosts are in a close coevolutionary relationship has been evident since the early days of our understanding of disease, but it is only relatively recently that the ecological, molecular, genomic and bioinformatic tools all required to understand the subject have become widely available and applicable. In the work presented within this thesis, we utilise an exceptionally well monitored and studied novel host-pathogen interaction – that of the avian bacterial pathogen Mycoplasma gallisepticum and its recently infected novel host the House Finch (Haemorhous mexicanus). Approximately 25 years ago this pathogen jumped from its established host in chickens into the wild passerine finch species, triggering an epidemic which has been well monitored from the outset We aim to address how host-pathogen coevolution drives particularly the evolution of microbial virulence. Our current understanding of such host-pathogen interactions within an evolutionary context centres around the mathematical and ecological framework of the “Trade-off hypothesis”, but many of the assumptions linking pathogen virulence, transmission and replication have been difficult to test or to integrate with our modern understanding of how microbial virulence is manifested on a molecular level. In exploring these issues throughout this text, we consider that this work has progressed our understanding in this field and goes some way towards this integration of evolutionary theory and more descriptive classical microbiology / molecular biology.