Phages preying on Bacillus anthracis, Bacillus cereus, and Bacillus thuringiensis: past, present and future

Abstract

Many bacteriophages (phages) have been widely studied due to their major role in virulence evolution of bacterial pathogens. However, less attention has been paid to phages preying on bacteria from the Bacillus cereus group and their contribution to the bacterial genetic pool has been disregarded. Therefore, this review brings together the main information for the B. cereus group phages, from their discovery to their modern biotechnological applications. A special focus is given to phages infecting Bacillus anthracis, B. cereus and Bacillus thuringiensis. These phages belong to the Myoviridae, Siphoviridae, Podoviridae and Tectiviridae families. For the sake of clarity, several phage categories have been made according to significant characteristics such as lifestyles and lysogenic states. The main categories comprise the transducing phages, phages with a chromosomal or plasmidial prophage state, γ-like phages and jumbo-phages. The current genomic characterization of some of these phages is also addressed throughout this work and some promising applications are discussed here.

Figure 1

Timeline displaying the main milestones for B. thuringiensis research (grey flags) alongside the discovery of its phages (blue flags).

Figure 2

Transmission electron micrograph of the Twort-like phage vB_BceM_Bc431v3. Phage particles display isometric heads 85.4 ± 3 nm in diameter with individual capsomers. The phage possesses a long contractile tail 180 ± 3 nm in length by 12 ± 4 nm in width. Reproduced from El-Arabi et al. (2013), Virol. J. (reference [68]).

Figure 3

Transmission electron micrographs of phage of phage vB_BanS-Tsamsa particles negatively stained with 2% uranyl acetate on carbon-coated copper grids. (A) Preparation overview; (B) Close-up of single phage particle; (C) Details of the phage tail distal end; (D) Details of the phage head structure. Individual capsomers are visible. Scale bars represent 100 nm. Reproduced from Ganz et al. (2014), PLoS One (reference [125]).

Figure 4

Genome comparisons of γ-like isolates. Predicted genes and direction of transcription are represented as block arrows. For phage Wβ, ORFs are colored according to gene function, as indicated by legend at the bottom. wp indicates numerical gene designations for phage Wβ. Conserved regions are grey-shaded, the color intensity indicating the nucleotide identity levels (from 64% to 100%). The comparisons were done by BLASTn, and similarities with E values lower than 0.001 were plotted. Blue brace above wp28 and wp29 indicates the genes affected by the deletion in the lysogeny control module that characterize the lytic γ-like isolates. When present, Fosfomycin resistant gene is indicated by magenta ORFs (arrows) in the lytic isolates. For further description, see the main text. The figure was produced using Easyfig 2.1 program [171] using data extracted from GenBank annotations and from [118,120]. GenBank accession numbers are listed in Table 4.

Figure 5

Genome comparisons of phages vB_BceS-IEBH and 250. ORFs are plotted as arrows and some predicted gene functions are indicated. The conserved regions are grey-shaded, the color intensity indicating the identity levels (from 64% to 100%). The 7.5- and 9.5-kb regions with the lowest nucleotide identity are indicated by double-head arrows. The plasmid-like regions for vB_BceS-IEBH and 250 are highlighted in rose and yellow boxes, respectively. Comparisons were done by BLASTn, and similarities with E values lower than 0.001 were plotted. The figure was produced using Easyfig 2.1 program [171]. GenBank accession numbers are indicated in Table 4.

Figure 6

Transmission electron micrograph of phage Wip1 particles negatively stained with 2% uranyl acetate. Phage Wip1 has the typical tail-less isometric heads of Tectiviridae with an internal lipid membrane (highlighted by the arrow). Scale bar represents 25 nm. Reproduced from Schuch et al. (2009), PLoS One (reference [180]).

Figure 7

Transmission electron micrograph of phage 0305ϕ8-36. Phage particles are in contact with each other displaying aggregation. The length bar is 0.1 μm; magnification calibration was checked with diffraction grating. The tails of all phages particles have partially contracted. Reproduced from Serwer et al. (2007), Virol. J. (reference [191]).