Microbial Forensic Investigation of the Anthrax-Letter Attacks

This chapter focuses on the microbial forensic investigations of the anthrax-letter attack in the United States in 2001. The anthrax-letter attack represented a defining moment that has dramatically shaped the biodefense infrastructure and research efforts. Spores of Bacillus anthracis contained in or on letters mailed from a New Jersey location infected 22 persons, killing 5. The responsible strain was identified as a laboratory strain that was used commonly for research and development of vaccines and therapeutics. The Federal Bureau of Investigation developed a strain repository of all known sources of the Ames strain. Distinctive morphological variants were observed among colonies grown from the spore-containing letters. DNA was isolated from each variant and then whole genome sequenced to identify the genetic basis for these phenotypes. Four polymerase chain reaction-based assays for detecting the four different variants were developed and then used to screen the Ames strain repository. Only cultures derived from the U.S. Army Medical Research Institute for Infectious Diseases spore stock RMR-1029 contained all four mutants associated with the morphological variants. The FBI concluded that the letter spores were derived from this source. The microbial forensics of the Amerithrax investigation relied heavily on genetics and comparative genomics to provide invaluable investigative leads, which suggested that (i) the strain of B. anthracis used in the attack was more likely obtained from a laboratory source than from the environment and (ii) that a B. anthracis spore preparation known as RMR1029 at the U.S. Army Medical Research Institute for Infectious Diseases, Fort Detrick, Maryland, was a potential source or at least a direct lineage source from which the spores in the letters originated.