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Bacillus cereus and other non-anthracis Bacillus species

Lisa Noonan, MSc, MBChB, DTMH, FRACP
Joshua Freeman, MBChB, FRCPA
Section Editor
Daniel J Sexton, MD
Deputy Editor
Elinor L Baron, MD, DTMH


The Bacillus cereus group is comprised of seven closely related species: B. cereus, B. mycoides, B. pseudomycoides, B. thuringiensis, B. weihenstephanensis, B. toyonensis, and B. anthracis [1-5]. Most human non-anthracis Bacillus spp infections are caused by B. cereus, although infections with other species within the B. cereus group have also been described [1,2]. Not all isolates are associated with invasive disease. B. cereus is associated with nosocomial and opportunistic infections, particularly in immunocompromised patients, intravenous drug users, and patients with indwelling or implanted devices. B cereus is also associated with syndromes such as food poisoning due to toxin production.

Issues related to B. cereus and other non-anthracis Bacillus species will be reviewed here. Issues related to B. anthracis are discussed separately. (See "Microbiology, pathogenesis, and epidemiology of anthrax" and "Clinical manifestations and diagnosis of anthrax" and "Treatment of anthrax".)


Members of the B. cereus group are catalase-positive, aerobic (or facultatively anaerobic), spore-forming gram-positive bacilli [6]. Occasionally, B. cereus may appear gram variable or even gram negative with age. In Gram stains of body fluids, B. cereus appears straight or slightly curved with square ends arranged either alone or in short chains. Junctions between members of the chain are clearly visible. Gram staining of B. cereus taken from agar colonies tends to have a more uniform bacillary appearance ranging in size from 3 by 0.4 microns up to 9 by 2 microns. B. cereus present in tissue sections may appear long and filamentous. Spores are not always visible on Gram stain but, when apparent, they are located centrally, do not distort the bacillary shape, and are clear in appearance. Spores can be stained using specific dyes (eg, malachite green) that are absorbed by spores in the presence of heat.

Bacillus species are easily recovered on blood and chocolate agars and grow optimally at environmental temperatures (25 to 37°C). All species except B. anthracis are motile and beta-hemolytic on blood agar. Colonies have an irregular perimeter and appear dull gray and opaque on sheep blood agar. On egg yolk agar, members of the B. cereus group produce a zone of opacification due to lecithinase production.

Individual species within the B. cereus group cannot usually be differentiated by diagnostic laboratories (with the exception of B. anthracis, which is nonmotile and nonhemolytic). Newer methods for identifying microorganisms such as matrix-assisted light desorption/ionization–time of flight mass spectrometry (MALDI-TOF MS) can reportedly differentiate between some members of the B. cereus group (eg, between B. cereus sensu stricto and B. thuringiensis), but this may not be achievable using standard databases and identification algorithms [7]. Therefore, distinguishing individual species within the B. cereus group usually requires specialized molecular testing by a reference laboratory [8].

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Literature review current through: Nov 2017. | This topic last updated: Feb 09, 2017.
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