Restriction modification systems are bacterial defence systems that pair restriction endonucleases, which cut foreign DNA at specific recognition sequences, with methyltransferase enzymes that methylate the bacterium’s own DNA and thereby protect it from being cut. This self-recognition mechanism guards bacteria against invading bacteriophages and plasmids. In food microbiology these systems affect how starter cultures, probiotics, and pathogens resist phage attack, with consequences for fermentation reliability, and they also matter in strain typing and in the genetic improvement of food microbes.