Abortive infection: a killer bacteriophage resistance strategy of last resort

Bacteriophages are the most abundant biological entities on earth and outnumber their bacterial hosts by a factor of 10. As a result, bacteria have evolved numerous defense systems to resist viral infection. These systems operate at all stages of the phage life cycle and include cell surface modification, restriction-modification and CRISPRmediated resistance. Another notable defense strategy is phage abortive infection (Abi), where a bacterial host prevents the production of viable phage progeny and in the process protects the bacterial population as a whole, but at the expense of its life. For this reason, phage Abi can be viewed as "altruistic suicide". Recently, a link has been demonstrated between abortive infection activity and the near-ubiquitous prokaryotic toxin-antitoxin (TA) systems. Here, we review abortive infection and the emerging role for TA systems in mediating this powerful anti-viral activity.