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Figure 1: Bacillus anthracis: String of Pearls Reaction. Bacillus anthracis is an aerobic gram-positive endospore-forming rod-shaped bacterium that causes anthrax. The CDC has classified it as a category A bioterrorism agent. Three forms of anthrax affect humans; cutaneous, gastrointestinal, and pulmonary. There are almost 200 Bacillus species that have been characterized. B. anthracis is susceptible to penicillin and incubation on medium containing low levels of penicillin causes the bacterium to swell and form a chain of cells that resemble a string of pearls. Other Bacillus species are resistant to penicillin and do not exhibit this "String of Pearls" reaction.

Figure 2: Bacillus anthracis: String of Pearls Reaction (Labeled view)

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