Bacteria have a range of immune mechanisms, but it is unclear why this diverse armamentarium evolved. The most important immune mechanisms are (1) Surface Modification (SM) (2) Abortive infection (Abi) (3) Restriction Modification (R-M) (4) CRISPR-Cas and (5) prokaryotic Argonaute (pAgo), all of which can occur as stand-alone mechanisms or in combination. The individual mechanisms differ in key as ...