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Home Current Topics Phages Appear To Provide B. anthracis a “Dynamic Alternative Lifestyle”
Phages Appear To Provide B. anthracis a “Dynamic Alternative Lifestyle” Print E-mail
"Infection with bacteriophages dramatically alters the survival capabilities of Bacillus anthracis; freeing them from the bleak prospect of dormancy," say Raymond Schuch and Vincent Fischetti of Rockefeller University in New York, N.Y.  They portray this "deadly bacterium" as leading a "dynamic, alternate environmental lifestyle," one that includes a pivotal role for its collaborative viruses. They investigated several distinct phage families, including some that are highly B. anthracis-specific, revealing that "lysogeny profoundly alters the capacity of this microbe to sporulate, produce exopolysaccharide, form biofilms, change its morphology, and ultimately achieve longterm survival in the soil." Details of their research appear in the August issue of PLoS ONE (www.plosone .org).

 These findings also speak to Louis Pasteur's 19th-century hypothesis that B. anthracis finds a safe harbor in earthworm intestines. "The earthworm gut is a particularly rich source of both anthrax bacilli and their phage," says Schuch. Additionally, as with B. cereus, phage-induced filamentous growth may help B. anthracis cells attach to worm intestinal walls from which smaller rods and spores are shed into the gut and then from the worm. "Furthermore, bacteriophages accompanying the rods and spores seed the soil with infecting particles, making it possible for additional bacilli to expand into new niches," he adds.

 Electron microscopy reveals at least three distinct viral families of B. anthracis- infecting phage in earthworm guts, namely Siphoviridae, Myoviridae, and Tectiviridae, according to Schuch. Moreover, each worm gut contains multiple phage types, some of which affect sporulation, he says. For example, under certain conditions, phages silence the genes that increase spore production and under other conditions they promote sporulation- sometimes providing spores with an extra exosporium layer to better withstand harsh conditions. Bacteriophages appear to exert their control over B. anthracis via sigma factors, which activate host-encoded gene loci that, in turn, induce novel phenotypes, says Schuch, noting that the viral sigma subunit confers promoter specificity, determining which genes are expressed. "And while it's interesting that B. anthracis phages are using transcriptional regulatory proteins to effect host-cell change, it isn't surprising considering the pleiotropic effects of such regulators," he says.

 Several years ago Schuch and Fischetti discovered that B. anthracis acquired resistance to the antibiotic fosfomycin after being infected by phages, thereby demonstrating a gene flux between virus and bacterium and suggesting that this microbe has a major growth phase outside the mammalian host. "It isn't unreasonable to expect that B. anthracis, which are released in staggering numbers after host death, are a target for a variety of B. anthracis-active exogenous phages in enzootic locations," they say.

 However, anthrax expert Paul Keim from Northern Arizona University in Flagstaff disagrees. "B. anthracis was a soil bacterium that did a niche shift to become the catastrophic disease-causing pathogen and, in the process, lost its extensive environmental component," he says.

 "The phage work reported in this paper is really nice, and I enjoyed reading it," Keim continues. "But I'll bet Schuch and Fischetti a pitcher of beer that, on further testing, all their ‘B. anthracis-specific' phages routinely infect one or more of B. anthracis' near relatives, B. cereus and B. thuringiensis. There's a real mixture of B. cereus and B. thuringiensis among the relatives, and many cause disease, although not generally as severe as anthrax." Moreover, the "genetic backbones" for both the pXO1 and pXO2 virulence plasmids are not restricted to B. anthracis but can also be found in related B. cereus and B. thuringiensis. "That's how close these strains are to one another," he says.

 "I totally agree with Keim," Schuch says. "The phages we work with are probably also infecting some B. cereus [and] the lines between these strains and B. anthracis sometimes appear transient and blurry. But we have shown that bona fide B. anthracis- clonal, with no soil activity-can regain its soil functions through phage infection, retaining all of the genetic markers that originally made it into a B. anthracis strain; and when the phage is lost, the bacterium also loses its soil activity."

 Marcia Stone
 Marcia Stone is a science writer based in New York City. More of her work can be seen at http://www.mstoneworks .net.