Evolution of Virulence in Emerging Epidemics: From Theory to Experimental Evolution and Back

The experimental validation of theoretical predictions is a significant step in the demonstration of the predictive power of a model. In a previous study, we monitored the evolution of the temperate phage λ spreading in continuous cultures of Escherichia coli . This experimental work confirmed the influence of the epidemiological dynamics on the evolution of transmission and virulence of the virus. A variant with larger propensity to lyse bacterial cells was favoured in emerging epidemics (when the density of susceptible cells was large), but counter-selected when most cells were infected. Although this approach qualitatively validated an important theoretical prediction, no attempt was made to fit the model to the data nor to further develop the model to improve the goodness of fit. Here, we show how theoretical analysis and model fitting can be used to estimate key parameters of the phage life cycle and yield new insights on the evolutionary epidemiology of the phage λ . First, we show that adding an additional stage to model the phage life cycle improves the fit of the model to the data. Second, we carry out a theoretical analysis that yields useful approximations that capture the effects of epidemiological dynamics on selection and differentiation across distinct compartments. Finally, we infer the parameters of our new model and, in particular, we estimate key phenotypic traits characterizing the two strains of the virus used in our experiment. This work illustrates how the interplay between experimental and theoretical approaches can help enhance our understanding of evolutionary epidemiology of infectious diseases.