Battling Phages: How Bacteria Defend against Viral Attack

Bacteriophages (phages) are accomplished, bacteria-specific, viral predators with far-reaching impact: from the food and biotechnology industries 1 to global nutrient cycling 2 to human health and disease 3; wherever bacteria thrive, it seems, so do predatory phages. In order to survive the constant onslaught of phage, bacteria have evolved mechanistically diverse defense strategies that act at every stage of the phage life cycle (Fig 1) 4,5. Phages rapidly co-evolve to overcome these barriers, resulting in a constant, and often surprising, molecular arms race 6. In this review, I highlight the spectrum of “innate” strategies used by bacteria to evade phage predation, with particular attention paid to more recent findings in the field. For a discussion of the CRISPR-Cas adaptive immune system, readers are directed to several recent reviews 4–6. Open in a separate window Fig 1 An overview of bacterial defense systems against phage. Each step of the phage lytic replication cycle is shown in italics. For simplicity, the cell wall and outer membrane (for gram-negative bacteria) is not shown. Bacteria can use a range of antiphage systems that can target all stages of the phage life cycle.