Bacteriophages of Shiga Toxin-Producing Escherichia coli and Their Contribution to Pathogenicity

Abstract

Shiga toxins (Stx) of Shiga toxin-producing Escherichia coli (STEC) are generally encoded in the genome of lambdoid bacteriophages, which spend the most time of their life cycle integrated as prophages in specific sites of the bacterial chromosome. Upon spontaneous induction or induction by chemical or physical stimuli, the stx genes are co-transcribed together with the late phase genes of the prophages. After being assembled in the cytoplasm, and after host cell lysis, mature bacteriophage particles are released into the environment, together with Stx. As members of the group of lambdoid phages, Stx phages share many genetic features with the archetypical temperate phage Lambda, but are heterogeneous in their DNA sequences due to frequent recombination events. In addition to Stx phages, the genome of pathogenic STEC bacteria may contain numerous prophages, which are either cryptic or functional. These prophages may carry foreign genes, some of them related to virulence, besides those necessary for the phage life cycle. Since the production of one or more Stx is considered the major pathogenicity factor of STEC, we aim to highlight the new insights on the contribution of Stx phages and other STEC phages to pathogenicity.

Keywords: STEC; Shiga toxins; Stx phages; antibiotic resistance genes; lambdoid prophages; pathogenicity; virulence factors.

Figure 1

Regulation scheme of stx expression in bacteriophage 933W of E. coli O157:H7 strain EDL 933 comprising relevant genes (colored in orange) and regulatory elements (not to scale), modified from Wagner and Waldor 2002 [30]. During the lysogenic state (indicated in grey arrows), transcription is inhibited through binding of the cI-encoded repressor protein to operator sites of the early promoters p_L and p_R (colored in grey); transcription is also terminated by downstream terminators (dark blue). Upon phage induction, autocleavage of the cI-encoded repressor protein allows transcription at p_L, resulting in the production of phage-encoded antiterminator protein N (red), which enables polymerase read-through at downstream terminators including t_L1 and t_R1. This, in turn, leads to the expression of the late-phase antiterminator Q (red), which facilitates transcription initiating at the late-phase promoter p_R’, transcending terminator t_R’, and resulting in the expression of downstream genes including stx (indicated in light blue arrows). Additionally, the expression of O- and P-encoded phage replication products leads to increased Stx production by amplifying stx copy numbers [30,39].

Figure 2

Scheme of putative functions of the phage-encoded O-acetyl esterase in the large intestine. Pathogenic STEC cells have to traverse the loose and the tight mucus layer to reach the epithelium for adherence and colonization. Mucinases and other proteases play a role in that process. Cleavage of O-acteyl residues from terminal O-glycans (e.g., Neu5,9Ac₂) by chromosomal and phage-encoded O-acetyl esterases results in deacetylated free sialic acids such as N-acetyl neuraminic acid, which can be metabolized by the bacteria [113]. The chemical structure of Neu5,9Ac₂ is shown. Honeycomb structure = mucin network. Paneth cells and goblet cells are indicated.

Figure 3

Influence of Stx prophages on the bacterial host transcriptome; blue arrows indicate upregulation of genes for acid resistance, anaerobic respiration, and motility and chemotaxis; orange arrows point to transcriptomic changes via sRNAs; red arrows symbolize downregulation of transport, C-, N-, energy, and fatty acid metabolism, and LEE genes. Details are described in the text.