Short, mobile segments of DNA that can move within and between genomes, carrying the machinery for their own relocation. In food microbiology they matter because they shuffle genes — including antibiotic-resistance and virulence genes — among bacteria, and because their distinctive patterns help type pathogens and detect genetically modified organisms in testing. Their movement drives bacterial evolution in food environments, which is why surveillance of resistance genes in the food chain pays close attention to mobile genetic elements. Whole-genome sequencing now tracks their role in the spread of resistance traits.