Tiny Viruses Provide B. anthracis with Environmental Options

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Bacillus anthracis Image credit: Ray Schuch

The long-held belief that B. anthracis is safely sequestered in spores outside its  mammalian host has been challenged by two scientists from New York  City.

To the great dismay of bioterrorism hunters who want their microbial definitions cut and dry, Raymond Schuch and Vincent A. Fischetti of Rockefeller University report that Bacillus anthracis has an active life in the soil, much like that of its close relatives B. cereus and B. thuringiensis. Furthermore, they say, this activity is mediated by tiny viruses called bacteriophages, or ‘phages’ for short.

In 2006, Drs. Schuch and Fischetti noticed that phage-infected anthrax bacteria became resistant to the common natural antibiotic fosfomycin, which made them suspect that B. anthracis is nowhere near as quiescent in soil as scientists thought. The more they investigated, the more it appeared that this bacterium has a far more dynamic interaction with its earthy neighbors than generally assumed; in fact B. anthracis appears to behave pretty much like other soil bacteria which are well known for their gene-exchanging promiscuity .

Bacteriophages Free B. anthracis from the Bleak Prospect of Dormancy

Phages shuttle genes around and are known shapers of a bacterium’s genomic architecture, and B. anthracis as well as B. cereus and B. thuringiensis have more than their fair share of phages. Thus, Schuch and Fischetti began to investigate a number of bacilli-infecting phage families, some of which were very specific to B. anthracis, and discovered that infection profoundly alters the capacity of the bacteria to live within biofilms, form spores, change shape and, ultimately achieve long-term survival in the soil.

“It isn’t unreasonable to expect B. anthracis, which are released in staggering numbers after host death and are a target for a variety of phages, to have an active growth phase in soil,” say Schuch and Fischetti.

Earthworms Provide a Safe Harbor for Anthrax Bacteria

The investigators also recently confirmed Louis Pasteur’s century old hunch that B. anthracis finds safe harbor in earthworm intestines. “The earthworm gut is a particularly rich source of anthrax bacilli and their phages,” says Schuch, who has squeezed gut muck out of countless worms and thinks that by giving the bacteria the genetic clout they need to form the slimy, protective communities known as biofilms, the phages enable them to colonize the grazing annelids. “Additionally,” he notes, “as with B. cereus, the growth of phage-induced filaments help B. anthracis attach to a worm’s intestinal walls.” Once they are successfully ensconced inside, the bacteria replicate, shedding B. anthracis and their phages out of the worm and into the surrounding soil making it possible for additional bacilli to expand into new niches.

Toggling the Bacterial Genome Between Community Building and Spore Production

Furthermore it appears that phages toggle the bacterium’s genome between the biofilm formation and sporulation. When conditions permit, some phages silence the genes that ramp up spore production and these bacteria survive; but when the going gets tough, others promote sporulation: even providing the spores with an extra tough layer to better withstand harsh environmental conditions.

The phages manipulate the bacterial genome with molecules called sigma factors, which determine which host genes are expressed. And since different phages encode different sigma factors, the traits expressed by a bacterium depend on which phages are infecting it. “And while it’s interesting that B. anthracis phages use sigma proteins to effect host cell change,” says Schuch, “it isn’t surprising considering the wide-ranging activities of these regulators.”

“What this means,” says Fischetti, “is that sequencing the genome may not be enough. There are more than 1,000 known isolates of anthrax and little genetic variation between one isolate and the next…it is a really boring genome. But what we see here is that the phage DNA, which works together with the B. anthracis DNA is important and shouldn’t be overlooked.”

References:

Schuch, Raymond and Fischetti, Vincent A. “The Secret Life of the Anthrax Agent Bacillus anthracis: Bacteriophage-Mediated Ecological Adaptations.” PLoS One 2009;4(8): e6532. Doi:10.1371/journal.pone.0006532.

Schuch, Raymond and Fischetti, Vincent A. “Detailed Genomic Analysis of the Wβ and γ Phages Infecting Bacillus anthracis: Implications for Evolution of Environmental Fitness and Antibiotic Resistance.” J Bacteriology 2006;188(8): 3037-3051.

This article was originally published on Suite 101.

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