Abstracts (first author)
Environmental variation and the evolution of virulence
Environmental opportunist pathogens are common in nature. Two well known examples are Vibrio cholera in humans and Flavobacteria in fish. Yet studies on epidemiology and evolution of pathogenicity are centred on obligate pathogens. Environmental opportunist pathogens do not require host-to-host contacts for transmission, and spend most of their time in the outside-host environment where their abundance depends on biotic and abiotic conditions. Such environmental bacteria can spontaneously gain and loose virulence factors that are likely to be associated with increased growth potential and energetic costs. Non-obligate pathogens have not tightly coevolved to evade host immune system, and pathogen transmission is likely to be strongly dose-dependent. This leads to a paradox: in the absence of hosts pathogenicity is selected against due to reduced competitive ability and this in turn prevents transmission to hosts. How can environmental opportunistic pathogens exist? We propose that environmental variation is a plausible mechanism explaining transition to more virulent forms.
We present a model which combines a SIR system to environmental virulent and non-virulent bacterial strains. We assume the pathogenic strain is a fast growing whereas the competing strain is a better competitor. This system is associated with bi-stability between two regimes: virulent strain with infections and non-virulent strain with no infections. Starting from a lower density the virulent strain is unable to grow or cause infections in a stable environment. If both bacterial strains are subjected to environmental variation, the virulent strain can overcome between-strain competition and increase in density close to infective dose and cause sporadic outbreaks or persistent infections, and thus gain significant fitness increases. Infective cycles may in turn promote further evolution from environmental opportunism to obligate pathogenicity, especially in the case of persistent infections.