Escherichia coli O157:H7 Shiga toxin-encoding bacteriophages: integrations, excisions, truncations, and evolutionary implications - PubMed (original) (raw)

Escherichia coli O157:H7 Shiga toxin-encoding bacteriophages: integrations, excisions, truncations, and evolutionary implications

Nurmohammad Shaikh et al. J Bacteriol. 2003 Jun.

Erratum in

Abstract

As it descended from Escherichia coli O55:H7, Shiga toxin (Stx)-producing E. coli (STEC) O157:H7 is believed to have acquired, in sequence, a bacteriophage encoding Stx2 and another encoding Stx1. Between these events, sorbitol-fermenting E. coli O157:H(-) presumably diverged from this clade. We employed PCR and sequence analyses to investigate sites of bacteriophage integration into the chromosome, using evolutionarily informative STEC to trace the sequence of acquisition of elements encoding Stx. Contrary to expectations from the two currently sequenced strains, truncated bacteriophages occupy yehV in almost all E. coli O157:H7 strains that lack stx(1) (stx(1)-negative strains). Two truncated variants were determined to contain either GTT or TGACTGTT sequence, in lieu of 20,214 or 18,895 bp, respectively, of the bacteriophage central region. A single-nucleotide polymorphism in the latter variant suggests that recombination in that element extended beyond the inserted octamer. An stx(2) bacteriophage usually occupies wrbA in stx(1)(+)/stx(2)(+) E. coli O157:H7, but wrbA is unexpectedly unoccupied in most stx(1)-negative/stx(2)(+) E. coli O157:H7 strains, the presumed progenitors of stx(1)(+)/stx(2)(+) E. coli O157:H7. Trimethoprim-sulfamethoxazole promotes the excision of all, and ciprofloxacin and fosfomycin significantly promote the excision of a subset of complete and truncated stx bacteriophages from the E. coli O157:H7 strains tested; bile salts usually attenuate excision. These data demonstrate the unexpected diversity of the chromosomal architecture of E. coli O157:H7 (with novel truncated bacteriophages and multiple stx(2) bacteriophage insertion sites), suggest that stx(1) acquisition might be a multistep process, and compel the consideration of multiple exogenous factors, including antibiotics and bile, when chromosome stability is examined.

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Figures

FIG. 1.

FIG. 1.

Amplicons for investigation of the yehV and wrbA integration sites in E. coli O157:H7. Bacterial strains used, loci examined, and lengths of resulting amplicons are listed across the top and to the left and right of the rows of amplicons, respectively. LJ, left junction; RJ, right junction.

FIG. 2.

FIG. 2.

Structures of two forms of truncated bacteriophages of _stx1_-negative E. coli O157:H7. Inserted GTT and TGACTGTT sequences replace segments that are found in truncated stx1 bacteriophages in two sequenced strains. ORF borders, proportions, and designations and nucleotide positions relate to those in reference . An A→G SNP 108 nucleotides 5′ to the octamer in both sequenced Δ18,895 bacteriophages and primer locations used to generate data pertaining to stx1 bacteriophages are noted. A, E, I, K, L, J, F, and B are primers.

FIG. 3.

FIG. 3.

Amplicons elicited across integration sites, in response to antibiotics and bile salts. E. coli O157:H7 strain 87-20 was grown in LB without (lanes 1 and 2) or with (lanes 3 and 4) TMP-SMX, ciprofloxacin (lanes 5 and 6), or ampicillin (lanes 7 and 8) or in LB with G-6-P without (lanes 9 and 10) or with (lanes 11 and 12) fosfomycin. Samples in even-number lanes were grown in bile salts. Loci examined and lengths of resulting amplicons are listed to the right and left of the rows. LJ, left junction; RJ, right junction.

FIG. 4.

FIG. 4.

Evolutionary scenarios. Serotypes, phenotypes, genotypes, critical events, and postulated intermediate form (circle) in three different scenarios leading to the five STEC 1 forms known to exist today (boxes). One asterisk indicates that the sequence of listed events is not known but is presumed to have occurred at different times during evolution. This result would obviate the need for postulated lineage B or C to produce such an organism. Two asterisks indicate that organisms in this box would not exist if lineage A exists.

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