Figure 3.

Transition from in vivo to in vitro equilibrium. The tradeoff between fitness and virulence is represented by the downward sloping efficient frontier (blue line), and the green dotted lines represent the indifference curves. Each indifference curve represents the combination of fitness and virulence that provide the same payoff to the bacteria and we assume that the payoff landscape is independent on the virulence and increasing on fitness, which determines the form of the indifference curves (P1 < P2 < P3). (A) When facing the in vivo conditions (intra- and extracellular defense systems are adopted by the host) the payoff of the bacterium in the red zones is equal to zero (if the bacteria are too virulent, they will kill the host along with the bacteria themselves; if the bacteria are not virulent enough relative to their fitness, they will be easily detected by the host's immune system and eliminated), and the optimal strategy for the bacterium is characterized by the tangency point between the highest indifference curve and the efficient frontier (point A). (B) When moving to the in vitro conditions (only intracellular defense system is adopted by the host), the zone for which the bacterium's payoff is reduced, so the equilibrium changes (point B) and the bacterium achieves a higher payoff (P3 > P2). However, when the bacterium faces back the in vivo conditions, the equilibrium point B gets into the red zone. This means that the bacteria are not virulent enough (relative to their fitness) to evade the host's immune system and they are eliminated.