Figure 2.

Evolution of host-triggered aggressiveness in an obligate intracellular pathogenic bacterium. In this scheme, the ultimate amplitude of aggressiveness is proportional to virulence (ability to induce symptoms and evading the host's immune system) and fitness (ability to replicate inside the cell in vitro). First, a virulent strain that sickens the animal delivers effector proteins (i) to counteract host immune response and (ii) to scavenge the host cell. Effector synthesis is an energy-consuming process that leads to a moderate fitness in vitro(development cycle of 5 days for virulent Gardel strain). Second, by the mean of iterative passages in vitro, the resulting attenuated strain gained skills to grow in cell culture but lost the ability to hijack host immunity and is eliminated when injected in the animal. Thus, this strain exhibits a greater fitness in vitro (development cycle of 4 days for attenuated Gardel strain) but a decreased virulence once inside the animal (host's success in eliminating the bacterium). This behavior highlights the co-evolutionary arms race between a bacterial pathogen and its host.