When Anthrax Isn't Anthrax

Bacillus cereus is a Gram-positive, rod-shaped bacterium that rarely causes problems for humans. A strain of the bacterium picked up genes that encode the anthrax toxin. Image by Scott Rose.

A patient who had symptoms of inhalation anthrax infection and who eventually succumbed to the disease was actually infected by a different bacterial species that had acquired anthrax toxin genes. Within a matter of days, pathologists at The Methodist Hospital and scientists from two other institutions sequenced and analyzed the pathogen's genome, and determined it was not likely to be a bioterrorism weapon.

In an upcoming issue of the Archives of Pathology & Laboratory Medicine, lead investigator and Chair of the Department of Pathology and Genomic Medicine James M. Musser, MD, PhD, and colleagues report that the germ in this case is a strain of Bacillus cereus possessing a key plasmid and other genes the give Bacillus anthracis its virulent qualities. A plasmid is a small chromosome independent of the main bacterial chromosome, which -- under the right conditions can be replicated and transmitted to other bacterial species.

"This patient had an unusually severe, rapidly progressing infection caused by Bacillus cereus, an organism not typically associated with fatal infections," says Musser. "We determined the whole genome sequence of this virulent bacterium within a few days after the patient was hospitalized. The investigative team was able to rule out a possible bioterrorism threat, determine the reasons for the strain's unusually high virulence, and correctly manage health care workers who may have been exposed. This work was important because it generated actionable intelligence using genome sequence information, moving use of this important technology into the clinic."

Musser also serves as the director of the Center for Molecular and Translational Human Infectious Disease Research at The Methodist Hospital Research Institute, and holds the Fondren Distinguished Endowed Chair.

Because the patients infection mimicked inhalation anthrax, genomic sequencing of this bacterial strain was necessary to determine whether it had been genetically modified for nefarious purposes, Musser says. "This strain lacked unusual genetic factors likely to be present in an engineered bioterrorism agent," he said. "It lacked an unusual spectrum of toxins, and diverse antibiotic resistance genes that would have made it harder to treat -- there were no signatures of man-made genetic alterations."

The patient, a welder from a town about 75 miles southwest of Houston, died less than three days after seeking medical help. The overwhelming infection suggested a weakened immune response to initial infection by the B. cereus strain. Welders have an increased risk of developing lung infections, due to exposure to particulate matter and metals. No secondary B. cereus infections occurred, indicating the infection had not spread to others.

How, when, and where this strain of B. cereus acquired toxin genes similar to those present in the anthrax bacterium is not yet known. Musser and other members of Methodist's investigative team plan to publish a second paper with a detailed analysis of the B. cereus strain's genome. The anthrax plasmid pX01 possesses a number of genes responsible for the bacterium's virulence, including protective antigen (pag), edema factor (cya), and lethal factor (lef). Proteins encoded by these three genes combine to form the anthrax toxin. All three genes were present in the organism isolated from the patients lung and other tissue samples.

B. cereus normally inhabits soil, and is rarely a source of devastating infections. Some strains of the species cause food sickness. B. cereus is closely related to B. anthracis and another species, B. thuringiensis, which causes disease in some insects. The three Bacillus species possesses unique genetic elements on their main chromosomes, but can be distinguished from each other by the plasmids and bacterial viruses they possess.

All of the genome sequencing and analysis was done at The Methodist Hospital and The Methodist Hospital Research Institute. Musser says the studies were made possible by Methodists Stephen Beres, PhD, and Randall J. Olsen, MD, PhD, with additional important contributions made by other Methodist physicians and researchers from the Great Lakes Regional Center for Biodefense and Emerging Infectious Diseases based in Chicago.

"Until recently, it would have taken months or years to do what we did in days," Musser says. "We can do the genomic sequencing and analysis even faster with the next generation of genomic sequencers. That is something that will benefit patients and the greater community as well. In our Department of Pathology and Genomic Medicine we are working rapidly to implement this technology for routine use in patient care in The Methodist Hospital System."