Innate and adaptive immunity in bacteria: mechanisms of programmed genetic variation to fight bacteriophages

► Bacteria and their viruses (bacteriophages) are locked in an evolutionary contest for survival. ► Random mutation creates resistance at low frequency; not fast enough to win this contest. ► Programmed genetic variation mechanisms allow fast evolution of the bacterial immune pathways. ► Changes in bacteriophage receptors and restriction-modification systems provide a generic (innate) immune response. ► Acquisition of bacteriophage sequences into CRISPR loci mediates a sequence-specific adaptive immune response in bacteria.