Biosafety in Microbiological and Biomedical Laboratories—6Th Edition
Section VIII-A: Bacterial Agents Bacillus anthracis Bacillus anthracis, a Gram-positive, non-hemolytic, and non-motile bacillus, is the etiologic agent of anthrax, an acute bacterial disease among wild and domestic mammals, including humans. Like all members of the genus Bacillus, under adverse conditions, B. anthracis has the ability to produce spores that allow the organism to persist for long periods (i.e., years), withstanding heat and drying, until the return of more favorable conditions for vegetative growth.1 It is because of this ability to produce spores coupled with significant pathogenic potential in humans that this organism is considered one of the most serious and threatening biowarfare or bioterrorism agents.2 Most mammals are susceptible to anthrax; it mostly affects herbivores that ingest spores from contaminated soil and, to a lesser extent, carnivores that scavenge on the carcasses of diseased animals. In the United States, it occurs sporadically in animals in parts of the West, Midwest, and Southwest. Human case rates for anthrax are highest in Africa and central and southern Asia.3 The infectious dose varies greatly from species to species and is route-dependent. The inhalation anthrax infectious dose (ID) for humans has been primarily extrapolated from inhalation challenges of non-human primates (NHPs) or studies done in contaminated wool mills. Estimates vary greatly but the median lethal dose (LD50) is likely within the range of 2,500–55,000 spores.4 It is believed that very few spores (ten or fewer) are required for cutaneous anthrax infection.5 Anthrax cases have been rare in the United States since the first half of the 20th century.
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