Comparative genomics of Shiga toxin encoding bacteriophages

Abstract

BackgroundStx bacteriophages are responsible for driving the dissemination of Stx toxin genes (stx) across their bacterial host range. Lysogens carrying Stx phages can cause severe, life-threatening disease and Stx toxin is an integral virulence factor. The Stx-bacteriophage vB_EcoP-24B, commonly referred to as Ф24B, is capable of multiply infecting a single bacterial host cell at a high frequency, with secondary infection increasing the rate at which subsequent bacteriophage infections can occur. This is biologically unusual, therefore determining the genomic content and context of Ф24B compared to other lambdoid Stx phages is important to understanding the factors controlling this phenomenon and determining whether they occur in other Stx phages.ResultsThe genome of the Stx2 encoding phage, Ф24B was sequenced and annotated. The genomic organisation and general features are similar to other sequenced Stx bacteriophages induced from Enterohaemorrhagic Escherichia coli (EHEC), however Ф24B possesses significant regions of heterogeneity, with implications for phage biology and behaviour. The Ф24B genome was compared to other sequenced Stx phages and the archetypal lambdoid phage, lambda, using the Circos genome comparison tool and a PCR-based multi-loci comparison system.ConclusionsThe data support the hypothesis that Stx phages are mosaic, and recombination events between the host, phages and their remnants within the same infected bacterial cell will continue to drive the evolution of Stx phage variants and the subsequent dissemination of shigatoxigenic potential.

Coculture of Escherichia coli O157:H7 with a Nonpathogenic E. coli Strain Increases Toxin Production and Virulence in a Germfree Mouse Model

“…The model proposed by Gamage et al suggested that a lytic infection of these strains by phage that are spontaneously released by E. coli O157:H7 increases the number of phage particles, as well as Stx2 levels. This model was developed using a single strain of E. coli O157:H7, although stx 2 -bearing phage are known to be highly polymorphic (29)(30)(31). We hypothesized here that strain-specific differences between these phage would impact the level of toxin amplification observed.…”

Bacteriophage secondary infection

“…One distinction that can be made with little ambiguity is that between infections that are productive versus infections that instead display lysogenic cycles, with both infection types at least potentially serving as primary infections, that is, infections of otherwise not phage-infected bacteria. Logically, therefore, the primary infection of a bacterial lysogen can be viewed as a secondary infection of a lysogenic infection, see for example, (Davis et al, 1999;Espeland et al, 2004;Fogg et al, 2007;Mann , 2003;Slavcev and Hayes, 2002;Smith et al, 2012;Weinfeld and Paigen, 1964;Werner and Christensen, 1969;Yamada et al, 2007). One can also describe such secondary infections within this context as superinfections (Sturino and Klaenhammer, 2006) and indeed the term "superinfection" generally appears to be more common within the phage literature than "secondary infection", including within the context of the concept of superinfection immunity as well as that of superinfection exclusion.…”

Evolutionary aspects of toxin-producing bacteria

“…Examples can readily be found in some of the emerging or reemerging pathogens, including heterogeniety of the genes for diphtheria toxin, tox, and its iron-dependent regulator, dtxR, in clinical isolates of Corynebacterium diphtheriae from epidemic outbreaks [71]; production of superantigen variants among group A streptococci [63], exotoxindiversity among community-acquired versus hospital-acquired methicillin-resistant Staphylococcus aureus strain lineages [72]; cholera toxin production by Vibrio cholerae isolates [7,73]; heat-labile enterotoxin (a homologue of cholera toxin) production by enterotoxigenic Escherichia coli strains (ETEC) [74]; and the widespread production of Shiga toxin variants among the Shigella and E. coli strains [75][76][77]. Indeed, studies on Shiga toxin (stx) gene-containing phages indicate that they are transmitted not only by temperate bacteriophages between different bacteria in vivo (i.e., in the intestines of humans and animals [78][79][80]), but also extraintestinally in aquatic environments, such as oceans [81], sewage, and other fecally contaminated water sources [76,82,83], and irrigation water, soil, and crops [84,85].…”

“…Indeed, it appears that no two sequenced Stx-encoding phages are the same, and indeed, other than the stx genes remaining linked to the λ-like transcriptional activator Q gene, nearly all of the phages are mosaics showing appreciable evidence of genomic recombination and gene-swapping events [77]. The two classes of Shiga toxins found in STEC, Stx1 and Stx2, have similar structures and mechanisms of action.…”