title: Dynamics of insertion sequence element IS629 inactivation of verotoxin 2 genes in Escherichia coli O157:H7.
abstract: There are several anecdotal reports of insertion sequence (IS) element inactivation of verotoxin genes among enterohaemorrhagic Escherichia coli of the serotype O157:H7, a pathogen causing severe gastrointestinal disease in infected humans. These insertions can be expected to drastically reduce the virulence of the bacteria. IS element inactivation has been shown to be reversible in model systems, suggesting the possibility of spontaneous restoration of virulence. In this study, traditional and high-throughput sequencing was used to characterise three patterns of IS629 inactivation of verotoxin 2 genes in EHEC O157:H7, caused by insertion or insertion followed by partial deletion. At least one of the patterns of inactivation appears to have persisted several years among cattle O157:H7, indicating it has no major effect on fitness in the animal reservoir. Digital PCR was used to directly quantify the reversal rates of the insertional inactivation of a selected isolate under laboratory conditions. Inserts were found to be absent from in the order of 1/105 of individual genomes, with significantly higher loss frequencies observed in cultures under nutrient-poor conditions. We conclude that strains with this type of inactivation found in food or animal samples should be considered a threat to human health, and may pose a challenge for PCR-based detection methods.
authors.affiliation: Department of Microbiology, National Veterinary Institute (SVA), 75189 Uppsala, Sweden.
authors.forename: Heiður
authors.initials: H
authors.lastname: Loftsdóttir
authors.affiliation: Department of Microbiology, National Veterinary Institute (SVA), 75189 Uppsala, Sweden.|SLU Global Bioinformatics Centre, Swedish University of Agricultural Sciences, 75007 Uppsala, Sweden.
authors.forename: Robert
authors.initials: R
authors.lastname: Söderlund
authors.affiliation: Department of Microbiology, National Veterinary Institute (SVA), 75189 Uppsala, Sweden.
authors.forename: Tomas
authors.initials: T
authors.lastname: Jinnerot
authors.affiliation: Department of Microbiology, National Veterinary Institute (SVA), 75189 Uppsala, Sweden.
authors.forename: Erik
authors.initials: E
authors.lastname: Eriksson
authors.affiliation: SLU Global Bioinformatics Centre, Swedish University of Agricultural Sciences, 75007 Uppsala, Sweden.
authors.forename: Erik
authors.initials: E
authors.lastname: Bongcam-Rudloff
authors.affiliation: Department of Microbiology, National Veterinary Institute (SVA), 75189 Uppsala, Sweden.
authors.forename: Anna
authors.initials: A
authors.lastname: Aspán
mesh_terms: D000818:Animals; D002417:Cattle; D004251:DNA Transposable Elements; D004927:Escherichia coli Infections / Q000382:microbiology; D019453:Escherichia coli O157 / Q000235:genetics* / Q000472:pathogenicity; D005243:Feces / Q000382:microbiology; D020868:Gene Silencing; D006801:Humans; D016133:Polymerase Chain Reaction; D022641:Shiga Toxin 2 / Q000235:genetics*; D014774:Virulence
chemical_list: D004251:DNA Transposable Elements; D022641:Shiga Toxin 2; C430449:verocytotoxin 2
keywords: IS629; O157:H7; STEC; VTEC; digital PCR; verotoxin