Figure 1. CRISPR–mediated arms race.

Ellipses - bacteria; Pentagons - phage; red and blue rectangles - acquired spacers; red and blue circles, regions of the phage genome corresponding to the acquired spacers (protospacers); stars, mutated protospacers generating CEMs. WT, Wild type bacteria and phage; BIMX and CEMX, bacteria with spacers for acquired resistance to WT phage and the first-order CRISPR Escape mutants (CEMX), respectively; BIMX² and CEMX², bacteria with spacers for acquired resistance to CEMX and the second-order CRISPR–escape mutants, respectively. Panel A: Infection and phage replication relationships. Solid black lines, phage adsorption and replication; broken red lines - phage adsorption and loss. Panel B: Changes in state. BIMX are produced by WT phage infecting WT cells and BIMX² are produced by CEMX infecting BIMX. CEMX are produced by P₀ infecting and replicating on B₀ and CEMX² are produced by CEMX infecting and replicating on BIMX.