Unexpected evolutionary benefit to phages imparted by bacterial CRISPR-Cas9

Area of Science:

• Microbiology
• Molecular Biology
• Genetics

Background:

• Bacteria and phages engage in constant evolutionary battles using diverse defense and counter-defense strategies.
• The CRISPR-Cas system is a key bacterial adaptive immunity mechanism targeting invading phage DNA.

Purpose of the Study:

• To investigate the impact of the CRISPR-Cas system on phage evolution dynamics.
• To explore the potential for CRISPR-Cas pressure to accelerate phage mutation rates.

Main Methods:

• Infection of *Escherichia coli* with CRISPR-Cas9 by phage T4.
• Analysis of phage genome mutations under CRISPR-Cas9 selective pressure.
• Comparison of mutation frequencies with and without CRISPR pressure.

Main Results:

• CRISPR-Cas9 poorly cleaved the modified phage T4 genome.
• Phage mutants rapidly evolved under CRISPR-Cas9 pressure, with mutations accumulating within a single plaque.
• Mutation frequencies were approximately six orders of magnitude higher than spontaneous rates.

Conclusions:

• The CRISPR-Cas system acts as a double-edged sword, potentially accelerating phage evolution.
• This interaction may drive the coevolution of bacteria and phages, contributing to their ecological abundance.
• CRISPR-Cas pressure can be a significant evolutionary driver for bacteriophages.