Time to Get Cereus!
Abstract & Commentary
Synopsis: A patient with a disease resembling anthrax led to the identification of anthrax-like virulence factors in an isolate of Bacillus cereus.
Source: Hoffmaster AR, et al. Identification of Anthrax Toxin Genes in a Bacillus cereus Associated with an Illness Resembling Inhalation Anthrax. Proc Natl Acad Sci USA. 2004;101:8449-8454.
A previously healthy patient presented with a 2-day history of nausea, vomiting, hemoptysis, shortness of breath, and fever. His chest X-ray was abnormal, and his WBC on admission was 12,000/mm3, subsequently rising to a peak of 22,400/mm3. Cultures of sputum and of blood yielded a Gram-positive bacillus identified using traditional phenotypic characteristics, including biochemical reactions, as Bacillus cereus. The patient required mechanical ventilation for 44 days, but eventually recovered.
Sequencing of the organism’s 16S rRNA confirmed its identity as B. cereus while multilocus sequence typing found that it was closely related to, but distinct from, Bacillus anthracis. The patient’s isolate, however, contained a circular plasmid, named by Hoffmaster and colleagues as pBCXO1, that had 99.6% similarity with the B. anthracis toxin-encoding plasmid, pXO1. In addition, a polysaccharide capsule cluster was encoded on a second plasmid, pBX218, thus providing an analog to the B. anthracis capsule genes encoded on its other plasmid, pXO2. The virulence of the patient isolate was confirmed by mouse inoculation experiments.
Comment by Stan Deresinski, MD
B. cereus, a cause of food poisoning, is an uncommon cause of invasive infection. These infections mostly occur in immunocompromised patients, and have included post-traumatic or post-cataract surgery endophthalmitis, prosthetic valve endocarditis, native valve endocarditis in injection drug users, and meningitis in neonates and hematopoietic stem cell recipients.1 Other reported infections include those of cerebrospinal fluid shunts and of vascular access.
B. anthracis, on the other hand, is a highly virulent organism that causes potentially fatal disease regardless of precipitating events or immunocompromise. This virulence is the consequence of the presence of the expression of genes carried by 2 plasmids, pXO1 and pXO2, that encode the lethal toxin complex and the poly-g-D-glutamic acid capsule, respectively. The virulent B. cereus, isolated from the patient discussed here, had acquired a plasmid encoding the anthrax toxins and a second plasmid capable of encoding polysaccharide capsular material.
As indicated by Hoffmaster et al, in a comment on the evolutionary plasticity of the microbial world, "depending on the number extent of lateral gene transfer, nature could produce an unlimited number of variations and combinations." Thus, when using standard clinical laboratory techniques, notions such as "anthrax bad, cereus not so bad" are potentially dangerous over simplifications that may have a number of important clinical and other implications, and that potentially apply to other organisms. Thus, in some instances, the identification of an isolate such as B. cereus may lead to its being inappropriately disregarded as a contaminant. The lack of association of severe virulence with such an isolate, may lead to unnecessary searches for other etiologies of a patient’s perilous clinical state. Finally, an engineered bioterrorism agent that clinical laboratories identify simply as B. cereus could lead to significant delay in the identification of a sinister attack.
1. Drobniewski FA. Bacillus cereus and Related Species. Clin Microbiol Rev. 1993;6:324-338.
Stan Deresinski, MD, FACP Clinical Professor of Medicine, Stanford; Associate Chief of Infectious Diseases, Santa Clara Valley Medical Center, is Editor for Infectious Disease Alert.