Bacterial Defense: How Bacteria Combat Viral Attacks (Phages)

Bacteria employ sophisticated multi-layered defenses against bacteriophages (phages):

1. Restriction-Modification (RM) Systems

Innate immunity. Restriction endonucleases recognize and cleave specific unmethylated phage DNA sequences. Host DNA is protected by methylation at the same sites. >3,000 RM systems known.

2. CRISPR-Cas Systems

Adaptive immunity. Bacteria integrate short phage DNA fragments (spacers) into CRISPR arrays. Upon reinfection, CRISPR RNAs guide Cas nucleases to target and destroy matching phage DNA. Provides sequence-specific memory.

3. Abortive Infection (Abi) Systems

Altruistic suicide. Infected cell detects phage components and triggers premature death (metabolic arrest, membrane depolarization, toxin-antitoxin activation). Prevents phage replication and spread to clonal siblings.

Additional Mechanisms

• Surface receptor modification (prevent adsorption)
• Superinfection exclusion (prophage-encoded)
• Bacteriophage exclusion (BREX), DISARM, Thoeris, etc. — newly discovered systems.

This evolutionary arms race drives immense diversity in both phage offense and bacterial defense.