After a seven-year investigation of the anthrax letter attacks of 2001, the FBI was preparing in July to charge a single scientist, Bruce Ivins of the U.S. Army Medical Research Institute of Infectious Diseases, with the crimes. To trace the source of the anthrax, the FBI used “microbial forensics,” a new approach that combines established microbiology techniques with DNA sequencing. By 2002, sequencing had narrowed possible sources to about 20 laboratories worldwide. Ivins’s strange protocols allowed inspectors to narrow the search further.
Anthrax is a stable bacterium: Growing it once seldom produces detectable mutations. Contrary to common practice, however, Ivins dumped material from 35 different anthrax cultures into a single flask, thus mixing a number of mutant strains. Examining agar plates cultured from anthrax in the letters, investigators spotted mutant colonies and then sequenced them. This yielded a characteristic genetic signature of four mutations that was traceable to Ivins’s flask. ...
