Spontaneous tandem amplification and deletion of the Shiga toxin operon in Shigella dysenteriae 1

Abstract

SummaryOnly one species of Shigella, Shigella dysenteriae 1, has been demonstrated to produce Shiga toxin (Stx). Stx is closely related to the toxins produced by Shiga toxin-producing Escherichia coli (STEC). In STEC, these toxins are often encoded on lambdoid bacteriophages and are major virulence factors for these organisms. Although the bacteriophageencoded stx genes of STEC are highly mobile, the stx genes in S. dysenteriae 1 have been believed to be chromosomally encoded and not transmissible. We have located the toxin genes of S. dysenteriae 1 to a region homologous to minute 30 of the E. coli chromosome, within a 22.4 kbp putative composite transposon bracketed by IS600 insertion sequences. This region is present in all the S. dysenteriae 1 strains examined. Tandem amplification occurs via the flanking insertion sequences, leading to increased toxin production. The global regulatory gene, fnr, is located within the stx region, allowing deletions of the toxin genes to be created by anaerobic growth on chlorate-containing medium. Deletions occur by recombination between the flanking IS600 elements. Lambdoid bacteriophage genes are found both upstream and within the region, and we demonstrate the lysogeny of Shigella species with STEC bacteriophages. These observations suggest that S. dysenteriae 1 originally carried a Stx-encoding lambdoid prophage, which became defective due to loss of bacteriophage sequences after IS element insertions and rearrangements. These insertion sequences have subsequently allowed the amplification and deletion of the stx region.

Mosaic structure of Shiga-toxin-2-encoding phages isolated from Escherichia coli O157:H7 indicates frequent gene exchange between lambdoid phage genomes

“…Only the stx # phage 933W appeared to be a fully functional phage, while the other λ-like sequences showed a mosaic structure, indicating extensive exchange of genes. Another STEC O157 : H7 isolate that is currently being sequenced also has many phage-like sequences in its genome (Yokoyama et al, 2000), and a similar phenomenon has also been observed in Shigella dysenteriae (McDonough & Butterton, 1999). The general position of the stx genes in Stx-encoding phages appears to be conserved, as they are located between the genes encoding the transcription antiterminator Q protein and the holin S gene of the lysis cassette (Plunkett et al, 1999 ;Unkmeir & Schmidt, 2000).…”

Genetic structure and chromosomal integration site of the cryptic prophage CP-1639 encoding Shiga toxin 1

“…The stx gene of S. dysenteriae type 1 is also encoded in a phage remnant. Here, the stx operon is flanked by a number of insertion elements and only a small number of phage genes remained (McDonough et al, 1999) so that the prophage character of this element is archetypal. On the basis of sequence analysis, CP-1639 is considered to be a victim of a single or multiple recombination events, that left behind a handicapped mosaic phage genome.…”

“…Thus, integration of Stx phage DNA into E. coli chromosomes may play a key role in the emergence of new STEC pathotypes (Schmidt, 2001). The stx genes may also be present in defective prophage genomes as is the case for stx of Shigella dysenteriae and for stx 1 in E. coli O157 : H7 strain EDL933 (McDonough et al, 1999;Perna et al, 2001). Although the number of published phage genomes and chromosomal integration sites is growing, only a few such sites are known for Stx phages.…”

A novel murine infection model for Shiga toxin–producing Escherichia coli

“…The lambda phage receptor was put into DBS100, creating DBS100/pTroy9, and induced by growing overnight at 37°C in lambda broth with 0.2% maltose and 10 mM MgSO4. This strain was then lysogenized with the antibioticmarked bacteriophages as described previously (118), resulting in DBS770 [C. rodentium (λstx2dact)] or DBS771 [C. rodentium (λstx2dact::kan R )].…”