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Deaths Blamed on Mysterious Microbe with Anthrax Genes

By Kate Ruder

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Last year two hospital patients from different cities in Texas died of severe pneumonia that appeared to be caused by inhalation anthrax. Yet neither patient was infected with the bacterium that causes anthrax, Bacillus anthracis.

Instead, DNA tests indicated that both patients became infected by another species of bacteria that carried the lethal anthrax genes. The bacterium, called Bacillus cereus, typically causes mild food poisoning.

B. cereus from Louisiana patient (top) and B. anthracis. Both microbes have capsules that may protect them from a person’s immune system.
When the Texas cases came to light, researchers at the U.S. Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, were sequencing the genome of a strain of B. cereus isolated from a man in Louisiana who, in 1994, developed severe anthrax-like symptoms.

The researchers now report that B. cereus can indeed carry lethal genes of the anthrax bacterium. The Texas and Louisiana patients were all metal workers and appeared to have inhaled the dangerous bacteria, although scientists do not know how.

These are the first cases in which anthrax genes have been discovered in an organism other than B. anthracis, and the findings raise concerns about the prevalence of the previously unknown pathogen—and our preparedness to detect and respond to it in the event of a biological attack.

For example, tests used to detect anthrax during a suspected bioterrorism incident might not pick up the equally pathogenic bacteria B. cereus.

Alex Hoffmaster of the CDC found the Louisiana strain of B. cereus that tested positive for anthrax genes while he was looking back at samples from patients with fatal or near-fatal disease.

“Either I had made a mistake or this was a really interesting isolate,” says Hoffmaster.

He sent the CDC sample to The Institute for Genomic Research (TIGR) in Rockville, Maryland, and scientists there sequenced its genome in just a few short weeks. They then compared it to the genomes of related bacteria such as the B. anthracis Ames strain used in the 2001 U.S. mail attacks and other strains of B. cereus.

The CDC bacterium has genes nearly identical to anthrax toxin genes that cause disease in humans and other animals. And the bacterium caused anthrax-like illness in mice, the researchers report.

“It was completely unexpected to sequence the genome and see toxin genes in B. cereus identical to ones found in B. anthracis,” says Jacques Ravel of TIGR, who led the sequencing effort.

The findings appear in Proceedings of the National Academy of Sciences this week.

How did the strains of B. cereus acquire anthrax genes? “That is still a mystery,” says Ravel.

He and others suspect that the two species of Bacillus bacteria, which both live in the soil, might have swapped genes at some point during evolution. There is no evidence that someone engineered the bacteria to carry these genes.

Hoffmaster, A. R. et al. Identification of anthrax toxin genes in a Bacillus cereus associated with an illness resembling inhalation anthrax. Proceedings of the National Academy of Sciences 101, 8449-8454 (June 1, 2004).

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