Now, bacteria have much simpler chromosomes than eukaryotes—heck, they don’t even have a nucleus. Bacterial chromosomes are simple circular arrangements. More to the point, bacteria—even bacteria of different species—can swap and collect small circular chromosomes called plasmids, like they were dirty magazines or pirated software or something. One really popular form of plasmid among hip, modern bacteria, who detest authority and repression, is the kind of plasmid that carries a gene for resistance to antibiotics.  Such plasmids are available not only from seedy low-life bacteria who hang out around bacterial playgrounds and street corners, pushing their wares on the next generation of thrill-seeking microorganisms, but also from scientific supply houses.