B. anthracis Its Phage: Surprising Dynamics In Soil

It appears that Bacillus anthracis in soil is nowhere near as quiescent as scientists think, say Raymond Schuch and Vincent A. Fischetti of Rockefeller University in New York City.⁽¹⁾ Their work on phage-mediated gene transfer reveals that this bacterium has a far more dynamic interaction with its environment than previously assumed.

This startling discovery emerged from Schuch and Fischetti’s recent genomic and functional analysis of two historically important phages, Wβ and γ. Their intent was to access the contribution of lysogeny to the phenotype of B. anthracis — a bacterium previously distinguished only by its virulence plasmids pXO1 and pXO2. Of particular interest was the role of Wβ and γ as vectors driving its ecological adaption since B. anthracis is a single, genetically homogeneous group in an otherwise heterogeneous B. cereus lineage including B. cereus and B. thuringiensis. The emergence of an fierce animal pathogen from this fairly benign bacterial family had to be dependent on more than just the acquisition of a couple of plasmids encoding anti-host toxins and capsular structure.

Phages are known shapers of bacterial genome architecture; and B. anthracis as well as B. cereus have more than their fair share of phages. Among the morphologically and genetically diverse phages harbored by these bacilli are a family of tailed, double-stranded members of the Siphoviridae which are very much like the γ lytic phage.

Schuch and Fischetti had reason to suspect that the γ lytic phage they were investigating evolved from a parental Wβ, its genetics reflecting a different lifestyle, lytic versus lysogenic respectively. Conveniently, Wβ is encoded in B. cereus ATCC 11950. But inducing the W phage from its lysogenic state in B.cereus by standard methods proved impossible. Then, fortuitously, a separate study of fosfomycin resistance (Fos^r) in B. cereus enabled the isolation and purification of Wβ and it was morphologically identical to γ. Additional investigation showed that the γ lytic phage evolved from the temperate Wβ through mutations at key loci controlling host recognition, lysogenic growth and, perhaps, phenotypic host modification. Furthermore, the γ phage’s shift from a temperate to a lytic lifestyle was traced to a large 2,003bp deletion in the Wβ lysogeny module.

The team also discovered that Wβ is very similar in gene order and sequence to B. anthracis prophages and can recombine to create hybrid phage such as γ. Preliminary evidence revealed at least three phage proteins that could impact on spore surface structure. And while genes encoding surface proteins and antibiotic resistance may not be virulence factors in the classic sense, they do make B. anthracis better able to survive within the highly competitive soil environment.

The demonstration of phage-encoded fosfomycin resistance has several important implications. For one thing, soil-dwelling bacteria are threatened with a myriad of antibiotics and counter with a corresponding number of sensing and evading strategies. Soil is a huge reservoir of antibiotic resistance. Phage-mediated transfer of Fos^r clearly demonstrates that the horizontal movement of antibiotic resistance is not restricted to plasmid, transposon, gene cassette, and integron vehicles or transduction and transformation —at least not in the soil environment. Furthermore, the induction of Wβ from its lysogenic state by fosfomycin appears ecologically important because its lytic pathway genes sit near a consensus antibiotic-inducible σ^W promoter.

But the pivotal findings of this investigation, say Schuch and Fischetti, is the gene flux between bacterial chromosome and infecting phage in B. anthracis and that spore antigens and resistance mechanisms are moved around in the process. Indeed, the very fact that phage can acquire and transmit growth-associated characteristics offers proof that B. anthracis has a major growth phase outside the mammalian host. This is a remarkable and unexpected finding.

Detailed studies of phages that impact on B. anthracis in the wild are in progress.

1. Schuch, R. and V. A. Fischetti. 2006. Detailed Genomic Analysis of the Wβ and γ Phages Infecting Bacillus anthracis: Implications for Evolution of Environmental Fitness and Antibiotic Resistance. Journal of Bacteriology 188:3037-3051.

[Suggested Visual: Fig. 1C. p3040 legend: Fosfomycin, a broad-spectrum antibiotic produced in the soil by Streptomyces spp., causes B. cereus ATCC 1150 to form donut-shaped bacterial colonies on agar plates. The central clearing zones provide a pool of infectious W particles for harvesting.]

[Suggested Visual: Fig. 1 A, B and E. p3040 legend: A and B. anthracis γ phage virons showing its isometric head and long nonconractile tail. E. A single intact Wβ phage viron.]