The causative agent of anthrax is a low-GC, Gram-positive, spore-forming bacterium. The spore is a definitive feature of its ecology, evolution, and use as a bioweapon. Particular strains of B. cereus and B. thuringensis isolates have been shown to be the closest relatives to B. anthracis. Because these related species are common in the environment and as industrial microbes, differentiation between them and B. anthracis is crucial. Based upon 16S rRNA sequences, there are few, or no, differences between B. anthracis and its closest relatives. However, B. anthracis contains two large plasmids that carry essential virulence genes for protein toxins (pXO1) and the biosynthesis of a poly-D glu-tamic acid capsule (pXO2). Lack of either plasmid dramatically attenuates the bacterium.
B. anthracis is a zoonotic disease and probably pathogenic to many mammals. However, its virulence differs greatly among the species—herbivores are most commonly affected in nature. The frequency of the disease in herbivores could be the result of ingestion of spores along with their food, followed by invasion from the gastrointestinal tract. Humans most commonly exhibit cutaneous anthrax, acquired by handling spore-contaminated wool or animal hides. Gastrointestinal anthrax from eating spore-contaminated meat is the next most common form, with inhalational anthrax being relatively rare. There is no evidence of strain differences between infection routes.
Because of the historical and recent importance of B. anthracis as a biological weapon, genomic and genetic analyses are highly developed for this pathogen. Multiple strain genomes have been completely sequenced, including the Ames isolate from the first anthrax-letter victim of the 2001 U.S. attacks.4,5 High-resolution DNA genotyping based upon MLVA can uniquely identify most laboratory strains, with some differentiation possible among different isolates of the same strain.1
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