Abstract
Shiga toxin-producing Escherichia coli (STEC) is a notorious foodborne pathogen containing stx genes located in the sequence region of Shiga toxin (Stx) prophages. Stx prophages, as one of the mobile elements, are involved in the transfer of virulence genes to other strains. However, little is known about the diversity of prophages among STEC strains. The objectives of this study were to predict various prophages from different STEC genomes and to evaluate the effect of different stress factors on Stx prophage induction. Forty bacterial whole-genome sequences of STEC strains obtained from National Center for Biotechnology Information (NCBI) were used for the prophage prediction using PHASTER webserver. Eight of the STEC strains from different serotypes were subsequently selected to quantify the induction of Stx prophages by various treatments, including antibiotics, temperature, irradiation, and antimicrobial agents. After induction, Stx1-converting phage Lys8385Vzw and Stx2-converting phage Lys12581Vzw were isolated and further confirmed for the presence of stx genes using conventional PCR. Phage morphology was observed by transmission electron microscopy. The prediction results showed an average of 8–22 prophages, with one or more encoding stx, were predicted from each STEC genome obtained in this study. Additionally, the phylogenetic analysis revealed high genetic diversity of Stx prophages among the 40 STEC genomes. However, the sequences of Stx prophages in the genomes of STEC O45, O111, and O121 strains, in general, shared higher genetic homology than those in other serotypes. Interestingly, most STEC strains with two or more stx genes carried at least one each of Stx1 and Stx2 prophages. The induction results indicated EDTA and UV were the most effective inducers of Stx1 and Stx2 prophages of the 8 selected STECs, respectively. Additionally, both Stx-converting phages could infect non-pathogenic E. coli (WG5, DH5α, and MG1655) and form new lysogens. The findings of this study confirm that Stx prophages can be induced by environmental stress, such as exposure to solar radiation, and lysogenize other commensal E. coli strains.
Highlights
Shiga toxin-producing Escherichia coli (STEC) has been associated with numerous foodborne outbreaks around the world and causes severe illnesses, such as hemorrhagic colitis, bloody diarrhea, and hemolytic-uremic syndrome (NüeschInderbinen et al, 2018)
The results of the study showed that most prophages predicted from the selected STEC strains belonged to the Siphoviridae family
The 40 complete whole-genome sequences of STEC strains generated by next-generation sequencing (NGS) technology and obtained from the National Center for Biotechnology Information (NCBI) database were used to understand the presence of different prophages within STEC genomes
Summary
Shiga toxin-producing Escherichia coli (STEC) has been associated with numerous foodborne outbreaks around the world and causes severe illnesses, such as hemorrhagic colitis, bloody diarrhea, and hemolytic-uremic syndrome (NüeschInderbinen et al, 2018). The number of infections caused by the top 6 non-O157 STEC strains – the serotypes O26, O45, O103, O111, O121, and O145 – has recently significantly increased (Marder et al, 2018) These non-O157 STEC strains can cause human illnesses as severe as the illnesses caused by E. coli O157:H7 and have been associated with numerous foodborne outbreaks around the world (Muniesa et al, 2004; Thomas et al, 2017; Nüesch-Inderbinen et al, 2018). The contamination of these pathogens, such as the serotypes O103, O26, and O121, has been associated with different types of food products, including meat, produce, and flour (CDC, 2019)
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