Restriction-Modification Systems: Bacteria employ restriction enzymes that recognize and cleave DNA sequences specific to invading phages. Simultaneously, the host's DNA is protected by methylating specific nucleotides, making it immune to cleavage.
CRISPR-Cas Systems: Bacteria can acquire immunity against specific phages through the CRISPR-Cas adaptive immune system. CRISPR arrays contain short sequences derived from previous phage infections, which guide Cas proteins to recognize and cleave invading phage DNA.
Abortive Infection: Some bacteria employ abortive infection mechanisms that halt viral replication at an early stage, leading to the death of the infected cell and preventing the release of progeny phages.
Genome Rearrangements: Bacterial genomes can undergo extensive rearrangements, such as inversions or deletions, altering critical phage attachment sites or essential genes required for viral replication.
Anti-CRISPR Proteins: Bacteriophages themselves can produce anti-CRISPR proteins that inhibit the function of CRISPR-Cas systems, allowing them to successfully infect the host.
Host-Induced Mutations: Bacteria can rapidly acquire mutations that alter the phage receptor sites on their cell surfaces, making them resistant to infection by specific phages.
Biofilm Formation: Certain bacteria can form protective biofilms that encase the bacterial cells in a matrix of extracellular material, shielding them from phage infection.
Prophage Integration: Bacteriophages can integrate their genomes into the host's chromosome as prophages. In this state, they become dormant and no longer replicate, evading destruction by other phages.
The effectiveness of these defense mechanisms varies among bacterial species and depends on the specific phage-host interactions. Moreover, bacteriophages are constantly evolving and can overcome host defenses, leading to an ongoing arms race between bacteria and their viral predators.
