Lysogeny with Shiga Toxin 2-Encoding Bacteriophages Represses Type III Secretion in Enterohemorrhagic Escherichia coli

Abstract

Lytic or lysogenic infections by bacteriophages drive the evolution of enteric bacteria. Enterohemorrhagic Escherichia coli (EHEC) have recently emerged as a significant zoonotic infection of humans with the main serotypes carried by ruminants. Typical EHEC strains are defined by the expression of a type III secretion (T3S) system, the production of Shiga toxins (Stx) and association with specific clinical symptoms. The genes for Stx are present on lambdoid bacteriophages integrated into the E. coli genome. Phage type (PT) 21/28 is the most prevalent strain type linked with human EHEC infections in the United Kingdom and is more likely to be associated with cattle shedding high levels of the organism than PT32 strains. In this study we have demonstrated that the majority (90%) of PT 21/28 strains contain both Stx2 and Stx2c phages, irrespective of source. This is in contrast to PT 32 strains for which only a minority of strains contain both Stx2 and 2c phages (28%). PT21/28 strains had a lower median level of T3S compared to PT32 strains and so the relationship between Stx phage lysogeny and T3S was investigated. Deletion of Stx2 phages from EHEC strains increased the level of T3S whereas lysogeny decreased T3S. This regulation was confirmed in an E. coli K12 background transduced with a marked Stx2 phage followed by measurement of a T3S reporter controlled by induced levels of the LEE-encoded regulator (Ler). The presence of an integrated Stx2 phage was shown to repress Ler induction of LEE1 and this regulation involved the CII phage regulator. This repression could be relieved by ectopic expression of a cognate CI regulator. A model is proposed in which Stx2-encoding bacteriophages regulate T3S to co-ordinate epithelial cell colonisation that is promoted by Stx and secreted effector proteins.

Shiga Toxin 2-Encoding Bacteriophages in Human Fecal Samples from Healthy Individuals

“…They are involved in the pathogenic profile of their bacterial hosts, in the development of the genome plasticity of host bacteria, in the regulation of pathogenicity factors, and in the survival and dissemination of stx in the environment (2,5,6). One or more Stx phages may be present in the genome of an STEC strain, and their incorporation can lead to the emergence of new pathogenic strains, as observed in recent outbreaks (7,8).…”

Strain-Dependent Cellular Immune Responses in Cattle following Escherichia coli O157:H7 Colonization

“…While research to determine the complete genome sequences of the two challenge strains is ongoing, it is already known that the PT21/28 strain contains both Stx2a and Stx2c lysogens, whereas the PT32 strain contains Stx2c only. The toxin type and level of production may, in part, account for differences, but we also know that there are complex regulatory interactions between prophages and bacterial host gene expression, including colonization factors (86) and core genome regulatory elements (87). Additional in vitro and in vivo work with isogenic Stx and Stx-encoding bateriophage knockout strains is planned to explore these concepts in more detail.…”

Impact of Spontaneous Prophage Induction on the Fitness of Bacterial Populations and Host-Microbe Interactions

“…2). The colonization of the gastrointestinal tract of ruminants was assumed to rely on the spontaneous induction of a subset of STEC serovar O157:H7 cells (75). It was proposed that the presence of a small fraction of induced STEC cells would lead to a sufficient release of Shiga toxin and thus to diarrhea, enabling the spread of the remaining uninduced lysogenic cells (23).…”

“…In Shiga toxin-producing E. coli (STEC) bacteria, the spontaneous induction of prophages increases the fitness of the entire bacterial population within the host environment by priming the host's epithelial cells for infection (23,75,76). In the proposed model, a small fraction of cells induces the Stx-encoding prophages, leading to Shiga toxin release and, in consequence, priming the gut epithelial cells by the expression and relocation of nucleolin and further receptors to the epithelial cell surface.…”