Table 1.

A summary of the major defense mechanisms described in the manuscript.

Name	Mechanism	Phylogenetic breadth
Restriction-modification	The restriction enzyme cleaves specific patterns in the incoming foreign DNA, while the modification enzyme protects host DNA from cleavage by unique biochemical modification.	Appear in ~90% of all sequenced prokaryotic genomes [13]
CRISPR/Cas	Fragments of phage DNA are integrated into CRISPR loci, which are then transcribed and processed into short non-coding RNAs. These RNAs, along with the associated Cas proteins, guide the way to interfere with the phage nucleic acids, in a yet unknown mechanism.	Appear in ~40% and ~90% of all sequenced bacterial and archaeal genomes, respectively [27]. The distribution of CRISPR/Cas-bearing species across the phylogenetic tree of life is highly patchy [38, 59].
Abortive infection	Premature cellular death occurs upon phage entry, blocking the expansion of the phage to neighboring cells. Notably, abortive infection systems include a large collection of mechanisms with little or no known evolutionary relationship, apart from a very similar phenotype.	Currently known abortive infection systems display a sporadic phylogenetic distribution in Gamma-proteobacteria and in Firmicutes [44], yet recent reports suggest that some systems may have an even broader phylogenetic range [54, 111].