Anthrax Toxin Genes Found in Another Microbial Species
For the first time, researchers have found anthrax toxin genes in a naturally occurring microbe other than Bacillus anthracis, the bacterium that causes anthrax. The microbe that contains those anthrax genes is a strain of Bacillus cereus isolated from a patient with a medical condition similar to inhalation anthrax.
May 17, 2004
Rockville, MD — For the first time, researchers have found anthrax toxin genes in a naturally occurring microbe other than Bacillus anthracis, the bacterium that causes anthrax.
The microorganism that contains those anthrax genes is a newly identified strain of the soil microbe Bacillus cereus that was isolated from a patient with severe pneumonia similar to inhalation anthrax.
The study, which will appear in the June 1, 2004 issue of the Proceedings of the National Academy of Sciences, was to be posted online during the week of May 17.
The research collaboration involved The Institute for Genomic Research (TIGR) and the Centers for Disease Control and Prevention (CDC). The National Institute of Allergy and Infectious Diseases (NIAID) funded TIGR's role in the research through the Pathogen Functional Genomics Resource Center at TIGR.
The significance of this strain was first noted during a CDC retrospective research study on Bacillus isolates associated with severe disease. TIGR then sequenced the complete genome of the B. cereus isolate that caused the anthrax-like illness and compared its DNA sequence to those of previously-sequenced strains of B. anthracis and B. cereus, which are closely related bacteria.
The genome analysis found that the virulent B. cereus isolate included a plasmid (a small circular DNA structure outside the cell's single chromosome) that was nearly identical to the anthrax plasmid that contains anthrax toxin genes.
TIGR President Claire M. Fraser, Ph.D., a senior author of the PNAS paper, says the discovery of a novel pathogen such as this virulent B. cereus strain illustrates the importance of being able to quickly recognize and identify new and emerging pathogens.
"These findings show that genomics can rapidly assist public health experts in responding to novel pathogens," says Fraser.
B. cereus, most commonly associated with food poisoning, has also been suspected to be the cause of some cases of fatal respiratory disease. Last year, two cases of such a fatal pulmonary illness in the United States were attributed to B. cereus isolates that also contained the anthrax toxin genes.
Bacteriologists consider B. anthracis to be a member of the B. cereus family of rod-shaped bacteria, and recent genome studies have confirmed that the two species share many genes. The main difference between most strains of B. anthracis and B. cereus is that anthrax bacteria have plasmids that include genes that create the toxin.
That plasmid found in the virulent B. cereus isolate has 99.6 percent of the genetic sequence of the anthrax bacillus' toxin-coding plasmid, which is called pX01. It is not known exactly how the B. cereus strain acquired the anthrax toxin genes.
While a second plasmid was also found in the virulent B. cereus isolate, scientists say it differs significantly from the second anthrax plasmid (called pX02). However, the second B. cereus plasmid does include a novel sugar capsule cluster which is not found in other strains of that species, and researchers say it could potentially play a major role in the virulence of this B. cereus isolate.
The lead authors of the study were Alex R. Hoffmaster of the CDC and Jacques Ravel and David A. Rasko of TIGR. As part of the research project, scientists led by Tim Read at the U.S. Naval Medical Research Center's Biological Defense Directorate used laboratory mice to confirm the B. cereus isolate's virulence.
The full study, "Identification of anthrax toxin genes in a Bacillus cereus associated with an illness resembling inhalation anthrax," will be available on the PNAS website.