Abstract
BackgroundIn many Asian countries including Bangladesh E. coli O157 are prevalent in animal reservoirs and in the food chain, but the incidence of human infection due to E. coli O157 is rare. One of the reasons could be inability of the organism from animal origin to produce sufficient amount of Shiga toxin (Stx), which is the main virulence factor associated with the severe sequelae of infection. This study aimed to fill out this knowledge gap by investigating the toxigenic properties and characteristics of stx phage of E. coli O157 isolated from animal sources in Bangladesh.ResultsWe analysed 47 stx2 positive E. coli O157 of food/animal origin for stx2 gene variants, Shiga toxin production, presence of other virulence genes, stx phage insertion sites, presence of genes associated with functionality of stx phages (Q933 and Q21) and stx2 upstream region. Of the 47 isolates, 46 were positive for both stx2a and stx2d while the remaining isolate was positive for stx2d only. Reverse Passive Latex Agglutination assay (RPLA) showed that 42/47 isolates produced little or no toxin, while 5 isolates produced a high titre of toxin (64 to 128). 39/47 isolates were positive for the Toxin Non-Producing (TNP) specific regions in the stx2 promoter. Additionally, all isolates were negative for antiterminator Q933while a majority of isolates were positive for Q21 gene suggesting the presence of defective stx phage. Of the yehV and wrbA phage insertion sites, yehV was found occupied in 11 isolates while wrbA site was intact in all the isolates. None of the isolates was positive for the virulence gene, cdt but all were positive for hlyA, katP, etpD and eae genes. Isolates that produced high titre Stx (n = 5) produced complete phage particles capable of infecting multiple bacterial hosts. One of these phages was shown to produce stable lysogens in host strains rendering the Stx2 producing ability.ConclusionDespite low frequency in the tested isolates, E. coli O157 isolates in Bangladesh carry inducible stx phages and have the capacity to produce Stx2, indicating a potential risk of E. coli O157 infection in humans.
Highlights
In many Asian countries including Bangladesh E. coli O157 are prevalent in animal reservoirs and in the food chain, but the incidence of human infection due to E. coli O157 is rare
Epidemiological data suggest that Shiga toxin 2 (Stx2)-producing E. coli O157 isolates are more commonly associated with serious diseases than isolates producing Shiga toxin 1 (Stx1) [6]. stx phages are temperate lambdoid bacteriophages and Shiga toxin genes are located in the late–phase region, downstream of Q homologue and upstream of genes S, R and Rz, required for the release of phage particles [7]
Majority of E. coli O157 isolates are non-toxigenic despite carrying virulence genes Despite being positive for stx2 gene, 42 out of 47 E. coli O157 (89%) produced no (Stx2 titre: < 2) or lower level of toxin (Stx2 titre: 8 to 32) and the remaining 5 isolates (11%) produced high titre of Stx2 toxin (Stx2 titre: 64 to 128) in VTEC Reverse Passive Latex Agglutination assay (RPLA) assay [32]
Summary
In many Asian countries including Bangladesh E. coli O157 are prevalent in animal reservoirs and in the food chain, but the incidence of human infection due to E. coli O157 is rare. Shiga toxins (Stx and Stx2) are the important virulence factors of E. coli O157 that play a crucial role in the progression of the most severe form of the disease in human. Induction of lysogenized phages eventually leads to the lysis of host bacterial cell and deliverance of free infectious phage particles [10]. These free phage particles play an important role in the transduction of stx genes to susceptible bacteria and can lead to the emergence of new Shiga toxin-producing E. coli (STEC) pathogens by horizontal gene transfer [11]
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