Anthrax-like Disease Caused by Bacillus cereus
By Amesh A. Adalja, MD, FACP, September 2, 2011
A man in Texas recently died of an anthrax-like illness that was caused by a toxin-producing strain of Bacillus cereus. Of the Bacillus species known to cause human disease, only B. anthracis is generally recognized as capable of causing fulminant disease. B. cereus, the other major Bacillus species that causes human disease, is primarily associated with “food poisoning” and endopthalmitis. However, this recent report of anthrax-like disease caused by B. cereus illustrates that, under certain circumstances, B. cereus can be a deadly pathogen—a fact that has been known but often overlooked.1
Microbiology of Bacillus
The genus Bacillus comprises many species, all of which are spore-forming gram-positive rods that are naturally found in soil throughout much of the world. The B. cereus group includes 6 species, among them B. anthracis and B. cereus, the ancestor from which B. anthracis is descended. The virulence of B. anthracis is the product of 2 plasmids: pXO1, which contains the genetic material for 2 exotoxins, and pXO2, which contains the genetic material for the antiphagocytic capsule. In the clinical setting, distinguishing the species microbiologically to identify B.anthracis is relatively simple.
Characteristics that distinguish B. anthracis:
- nonhemolytic on blood agar
- nonmotile on agar
- catalase positive
- colonies have a “whipped egg” appearance and a “comet tail” or “Medusa’s head” morphology.
Importantly, antimicrobial susceptibility also differs among the species: B. cereus is often resistant to beta-lactam antibiotics, with the exception of carbapenems.1,2
Severe Disease with Naturally Occurring pXO1-containing B. cereus
Fulminant disease from B. cereus in immunocompromised hosts is well described, but its ability to cause severe disease in healthy hosts is not widely appreciated. However, reports dating back to at least 19653 have described cases clinically similar to anthrax that were caused by B. cereus. In 2004, to further characterize these strains, Hoffmaster and colleagues retrospectively identified the presence of the pXO1 plasmid in a B. cereus strain isolated from a patient who had a life-threatening pneumonia.4 In subsequent reports, 2 fatal B. cereus pneumonia cases (2003 and 2004) that also harbored the pXO1 plasmid were described. Both of these patients were metal workers in Texas whose exposure to B. cereus–laden dust and noxious metal fumes was thought to predispose them to this infection; the same was suspected of 2 other cases reported in 1997.5,6
A B. cereus strain harboring both virulence plasmids also has been isolated from fatal cases of anthrax-like disease in chimpanzees in Côte d'Ivoire.7 The most recent human case, reported this year in a welder from Texas who was infected with a B. cereus strain containing the pOx1 plasmid, was fatal.8
Not Just a Contaminant
When a Bacillus species is recovered from a patient sample, it often is regarded as an environmental contaminant that has made its way into a sterile culture. Typically, no treatment is initiated, and the sample is discarded once it is shown not to be B. anthracis. The Bacillus is not routinely identified to the species level. Because B. cereus may be life-threatening if it has acquired the pOx1 plasmid, its presence in a culture collected from a patient with bacteremia, sepsis, and/or pneumonia should merit further investigation. This is particularly important because B. cereus is resistant to the most commonly prescribed antimicrobials for community-acquired pneumonia (standard beta-lactam antibiotics), which makes a rapid and accurate bacteriological diagnosis essential to timely initiation of appropriate antibiotic therapy. Additionally, attention to a patient’s occupational history should be emphasized, as metal workers appear to be disproportionately represented among the reported cases (at least anecdotally). In recognition of this organism’s pathogenicity, laboratories should consider expanding testing of Bacillus isolates from patients to exclude not just B. anthracis but also B. cereus.
Martin GJ, Friedlander AM. Bacillus anthracis (anthrax). In: Mandell GL, Bennett JE, Dolin R, eds. Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, PA: Churchill Livingstone; 2010.
Fekete T. Bacillus species and related genera other than Bacillus anthracis. In: Mandell GL, Bennett JE, Dolin R, eds. Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, PA: Churchill Livingstone; 2010.
Stapler T, Caneuscu V, Voiculescu M. Bronchopneumonia with lethal evolution determined by a microorganism of the genus Bacillus (B.cereus). Rom Med Rev 1965; 2:7–9.
Hoffmaster AR, Ravel J, Rasko DA, et al. Identification of anthrax toxin genes in a Bacillus cereus associated with an illness resembling inhalation anthrax. Proc Natl Acad Sci U S A 2004; 101: 8449-8454.
Avashia SB, Riggins WS, Lindley C, et al. Fatal pneumonia among metalworkers due to inhalation exposure to Bacillus cereus containing Bacillus anthracis toxin genes. Clin Infect Dis 2007; 44:414-416.
Miller JM, Hair GD, Hebert M, et al. Fulminating bacteremia and pneumonia due to Bacillus cereus. J Clin Microbiol 1997;35: 504-507.
Klee SR, Brzuszkiewicz EB, Nattermann H, et al. The genome of a Bacillus isolate causing anthrax in chimpanzees combines chromosomal properties of B.cereus with B.anthracis virulence plasmids. PLoS One. 2010 July 9;5(7):e10986.
Wright AM, Beres SB, Consamus EN, et al. Rapidly progressive, fatal, inhalation anthrax-like infection in a human. Arch Pathol Lab Med 2011; http://www.archivesofpathology.org/doi/pdf/10.1043/2011-0362-SAIR.1. Accessed August 29, 2011.