Data from: Costs of CRISPR-Cas mediated resistance in Streptococcus thermophilus

CRISPR-Cas is a form of adaptive sequence-specific immunity in microbes. This system offers unique opportunities for the study of co-evolution between bacteria and their viral pathogens, bacteriophages. A full understanding of the co-evolutionary dynamics of CRISPR-Cas requires knowing the magnitude of the cost of resisting infection. Here, using the gram-positive bacterium Streptococcus thermophilus and its associated virulent phage 2972, a well-established model system harbouring at least two type II functional CRISPR-Cas systems, we obtained different fitness measures based on growth assays in isolation or in pairwise competition. We measured the fitness cost associated with different components of this adaptive immune system: the cost of Cas protein expression, the constitutive cost of increasing immune memory through additional spacers, and the conditional costs of immunity during phage exposure. We found that Cas protein expression is particularly costly, as Cas deficient mutants achieved higher competitive abilities than the wild type strain with functional Cas proteins. Increasing immune memory by acquiring up to 4 phage-derived spacers showed small or non-detectable fitness costs. In addition, the activation of CRISPR-Cas during phage exposure does not induce significant fitness costs. Together these results suggest that the costs of CRISPR-Cas system arise mainly due to the maintenance of the defence system, while increasing the CRISPR array itself (up to 4 new spacers) or the deployment of immunity do not entail a significant fitness cost. We discuss the implications of these results for the evolution of CRISPR-Cas mediated immunity.