Higher phage virulence accelerates the evolution of host resistance

<p>Pathogens vary strikingly in their virulence and the selection they impose on their hosts. While the evolution of different virulence levels is well studied, the evolution of host resistance in response to different virulence levels is less understood and as of now mainly based on observations and theoretical predictions with few experimental tests. Increased virulence can increase selection for host resistance evolution if resistance costs are outweighed by the benefits of avoiding infection. To test this, we experimentally evolved the bacterium <em>Vibrio alginolyticus</em> in the presence of two variants of a filamentous phage that differ in their virulence. The bacterial host exhibited two alternative defence strategies: (1) super infection exclusion (SIE) whereby phage-infected cells were immune to subsequent infection at a cost of reduced growth, and (2) surface receptor mutations (SRM) providing resistance to infection by preventing phage attachment. While SIE emerged rapidly against both phages, SRM evolved faster against the high compared to the low virulence phage. Using a mathematical model of our system we show that increasing virulence strengthens selection for SRM due to the higher costs of infection suffered by SIE immune hosts. Thus, by accelerating the evolution of host resistance, more virulent phages caused shorter epidemics.</p>