Exploitation of the cooperative behaviours of anti-CRISPR phages
Many bacteria encode CRISPR-Cas (Clustered Regularly Interspaced Short Palindromic Repeats; CRISPR-associated) adaptive immune systems to protect themselves against their viruses (phages)1. To overcome resistance, phages have evolved anti-CRISPR proteins (Acr), which naturally vary in their potency to suppress the host immune system and avoid phage extinction2,3,4,5. However, these Acr-phages need to cooperate in order to overcome CRISPR-based resistance4,5: while many initial infections by Acr-phages are unsuccessful, they nonetheless lead to the production of Acr proteins, which generate immunosuppressed cells that can be successfully exploited by other Acr-phages in the population4,5. Here we test the prediction that phages lacking acr genes (Acr-negative phages) may exploit this cooperative behaviour6. We demonstrate that Acr-negative phages can indeed benefit from the presence of Acr-positive phages during pairwise competitions, but the extent of this exploitation depends on the potency of the Acr protein. Specifically, \"strong\" Acr proteins are more exploitable and benefit both phage types, whereas \"weak\" Acr proteins predominantly benefit Acr-positive phages only and therefore provide a greater fitness advantage during competition with Acr-negative phages. This work further helps to explain what defines the strength of an Acr protein, how selection acts on different Acr types in a phage community context, and how this can shape the dynamics of phage populations in natural communities.