The sequences of these two new B. cereus group members will contribute significantly to studies of the evolution of host range and virulence, said Jean Challacombe, among the leads of the 46 listed authors on the paper, "Pathogenomic Sequence Analysis of Bacillus cereus and Bacillus thuringiensis Isolates Closely Related to Bacillus anthracis" in the May issue of the journal.
The paper points to the relatively small genetic diversity among B. anthracis isolates, because unlike the two new samples, "B. anthracis rapidly kills its host and therefore has little opportunity for genetic exchange," Challacombe said.
Other members of the B. cereus family show far greater genetic diversity and while they may cause illness or infection, they and their hosts tend to survive. One of the organisms, B. thuringiensis 97-27, was isolated from the wound of a soldier in Bosnia, and appears to be an opportunistic pathogen. The other, Bacillus cereus E33L was obtained from the soil environment in Namibia and is probably not disease causing, despite having been collected from the carcass of a zebra.
The B. cereus group of bacteria comprises a diverse set of organisms that are widely distributed in the environment. This group includes 1) B. cereus, which can cause a variety of infections, most notably food poisoning, 2) B. thuringiensis, which produces toxins that kill insects and is currently used as a biopesticide on such crops as fruit trees and vegetables, and 3) the deadly B. anthracis, an animal and human pathogen that gained public attention in 2001 due to its presence in letters that resulted in the deaths of U.S. Postal Service workers.
In addition to food poisoning, B. cereus bacteria have been associated with a variety of infections, including abscesses, bacteriaemia and septicemia, cellulitis, ear and eye infections, endocarditis, meningitis, kidney infections, osteomyelitis, and pulmonary and wound infections.
It is unlikely that the B. cereus was the fatal infection for the zebra, however. "We don't know what killed the zebra because the carcass was dragged around by other animals before the swab was taken, so it is strongly likely that the E33L isolate got onto the carcass from the soil," said Challacombe.
There is considerable debate in regard to the systematic classification of members of the B. cereus group. Historically, these organisms were classified into three species (B. cereus, B. thuringiensis, and B. anthracis) on the basis of distinct physical or functional differences that defined them.
While the relationship between these organisms is still not clearly understood, recent molecular approaches have revealed extensive similarities between genomes and relatively few consistent differences warranting the segregation of isolates into discrete species classified as B. anthracis, B. cereus, and B. thuringiensis. In a classification scheme examining much of the B. cereus group as members of an asexually derived clonally population, B. thuringiensis 97-27 and B. cereus E33L are both members of the B. anthracis lineage.
Importantly, the B. anthracis lineage provides a molecular-based distinction that separates commercially important B. thuringiensis strains from pathogenic B. anthracis, say the paper's authors.
Electronic reprints of the journal paper can be downloaded from the Journal of Bacteriology Web site at http://jb.asm.org/cgi/reprint/188/9/3382 online.
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