Bacillus anthracis: String of Pearls Reaction

  • Author: Neal Chamberlain 1
    Affiliations: 1: Department of Microbiology/Immunology, A.T. Still University of Health Sciences/KCOM, Kirksville, Missouri, 63501
  • Citation: Neal Chamberlain. 2015. Bacillus anthracis: string of pearls reaction.
  • Publication Date : August 2015
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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.


The Gram stained slide used to obtain these images was from a commercial source (Ward's Science) and was purchased in the 1960's. The  manufacturer grew B. anthracis on medium containing 0.05-0.5 units of penicillin/ml for 3-6 hours at 37oC. The organisms were examined at 1000x magnification.


1. Murray PR, Rosenthal KS, Pfaller MA. 2009. Medical Microbiology. 6th edition. Chapter 24, Bacillus. Mosby/Elsevier, Philadelphia, PA. Pg. 247-253.

2. The Centers for Disease Control and Prevention. Accessed 11/19/2014. Bioterrorism Agents/Diseases by Category. http://www.bt.cdc.gov/agent/agentlist-category.asp.

3. Jernigan JA, et al. 2001. Bioterrorism-Related Inhalational Anthrax: The First 10 Cases Reported in the United States. Emerg Infect Dis. 7:933-944.

    4. Jernigan DB, et al. 2002. Investigation of Bioterrorism-Related Anthrax, United States, 2001: Epidemiologic Findings. Emerg Infect Dis. 8:1019-1028.

    5. Leise JM, Carter CH, Friedlander H, and Freeid SW. 1958. Criteria for the Identification of Bacillus anthracis. J Bacteriol. 7:655-660.

    6. Knisely, R.F. 1965. Differential Media for the Identification of Bacillus anthracis. J Bacteriol. 90:1778-1783.

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