Figure 1. Electron micrograph of a plasmid (pBR322). The diameter of the upper ring is approximately 0.25 µm. (Photograph: Michael Hoppert, Goettingen, Germany.)

Figure 2. A plasmid of enterotoxinogenic E. coli strain, an ETEC strain carrying various resistance genes. The double ring depicted consists of 130 440 base pairs. The tra region (in green) contains the genes for plasmid transfer by conjugation. Genes in red confer resistance against eight different antibiotics. Also present are genes encoding a toxin (colicin) and a virulence factor (hemolysin). (Diagram: Elzbieta Brzuszkiewicz, Goettingen, Germany.)