Modelling the evolution of growth strategies in bacteria

Abstract

The concept of Darwinian fitness is central to biology. It is often assumed that fitness in bacteria directly translates to their growth rates, but this assumption does not explain how many bacterial strains can be experimentally induced to grow significantly faster than what is observed in the wild. It also fails to explain why closely related bacterial strains that possess nearly identical enzymes have such variable growth rates. In this thesis, I use a model of bacterial growth to study the evolution of bacterial growth rates. The model incorporates allocation of resources in the proteome and a growth- lag tradeoff that all bacteria experience upon nutrient shifts. Using simulations and analytical results, I demonstrate that an optimal allocation of resources in fluctuating environments determines the growth rates, and this optimal allocation is determined by the characteristics of the environment. In particular, I show that slow growing strains are actually optimal in conditions with a higher supply of poor nutrients and a low mortality. Further, stable coexistence emerges in some environments as a result of niche separation onto different nutrients. In summary, I show that the variable growth rates of bacterial strains can emerge from adaptation to different environmental conditions, rather than as a result of physiological limitations. These results have implications for how fitness in bacteria is viewed, and how species’ growth rates can provide a window into their environmental histories.