Researchers captured E. coli bacteria in the act of becoming resistant to the antibiotic tetracycline. Some bacteria already contained a circular piece of DNA, called a plasmid, which carries antibiotic-resistance genes. Those resistant cells (green) pass the plasmid to sensitive cells (red). Once the plasmid has transferred (yellow dots), sensitive bacteria begin making proteins that make the microbes resistant to the antibiotic. The bacteria turn increasingly green as they become resistant to