CRISPR immunity drives rapid phage genome evolution in Streptococcus thermophilus

Area of Science:

• Microbial Ecology
• Molecular Biology
• Evolutionary Biology

Background:

• Bacteria utilize CRISPR-Cas systems for adaptive immunity against bacteriophages (phages), which are critical in shaping microbial communities.
• Phage predation and bacterial CRISPR-Cas immunity engage in an evolutionary "arms race," driving reciprocal adaptations.
• CRISPR-Cas systems provide adaptive immunity by integrating phage genetic material to recognize and neutralize future infections.

Purpose of the Study:

• To investigate the impact of CRISPR-Cas adaptive immunity on the evolutionary trajectory of phage genomes.
• To characterize the mechanisms of phage adaptation and genome evolution in response to bacterial CRISPR-Cas immunity.
• To assess the role of multiple phage infections in bacterial-phage coevolutionary dynamics.

Main Methods:

• Conducted long-term coevolution experiments involving the bacterium Streptococcus thermophilus and its infecting phage (phage 2972).
• Employed massive deep-sequencing to track population dynamics and genomic changes in both bacteria and phages over extended periods.
• Analyzed mutation rates and genome rearrangements in phage populations under selective pressure from CRISPR-Cas immunity.

Main Results:

• CRISPR-Cas immunity in S. thermophilus drove the fixation of single nucleotide polymorphisms exclusively in CRISPR-targeted regions of the phage genome.
• Phage genomes exhibited mutation rates significantly higher than their bacterial hosts, facilitating rapid adaptation.
• The presence of multiple phages enhanced phage persistence by enabling recombination, leading to chimeric genomes that replaced CRISPR-targeted sequences.

Conclusions:

• CRISPR-Cas adaptive immunity is a fundamental driver of phage genome evolution, inducing targeted mutations and rearrangements.
• Phage recombination and the formation of chimeric genomes are key mechanisms for overcoming CRISPR-Cas defenses.
• Coexistence of multiple phages is crucial for phage persistence in natural microbial systems by facilitating adaptation and immune evasion.