Toxigenic properties and stx phage characterization of Escherichia coli O157 isolated from animal sources in a developing country setting

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.

Investigation of the Causes of Shigatoxigenic Escherichia coli PCR Positive and Culture Negative Samples

“…Different studies have reported the detection of STEC by PCR or qPCR from different matrices [35][36][37][38] and the challenges of subsequently obtaining cultured STEC isolates. This significant discrepancy may be related to dead and injured cells, cells in a viable but non-culturable (VBNC) state, the presence of free DNA and free Stx bacteriophages in the samples [10,[39][40][41][42][43][44]. In addition, the microbial communities can have an important influence on the survival capabilities of STEC in environmental substrates and consequential cultural isolation [24,45].…”

Comparable stx2a expression and phage production levels between Shiga toxin‐producing Escherichia coli strains from human and bovine origin

“…This lysogen and another one obtained in a previous study (Krüger et al, ) showed similar levels of stx 2a expression, infective phages and extracellular Stx production, in comparison with the corresponding wild‐type strains. Likewise, Rahman, Nabi, Asadulghani, Faruque, and Islam () obtained the same high Stx titre both in a lysogen and its wild‐type strain when studied at basal conditions, and Muniesa, Blanco, et al () have reported a high spontaneous phage release and high inducibility after mitomycin C treatment in lysogens obtained with E. coli DH5α.…”

“…Rahman et al () attributed the low incidence of human infection with E. coli O157 in many Asian countries to the low frequency of strains that carry inducible phages and produce Stx2. Among O157 STEC isolates from food and food‐producing animals, they only found inducible Stx phages among those isolates that produced Stx2.…”

Biological and molecular characterization of six Shiga toxin-producingEscherichia coli(STEC) strains encoding the stx2gene in Colombia

“…Perhaps, q gene detection in those strains may be due to the presence of defective phages that lack stx genes. Regarding the q21 allele, none strain was found carrying this allele, which has been identified principally in STEC strains with low or null Stx production frequently isolated from beef, animal origin, and the environment in Asian countries, where these are widely distributed [32][33][34]10].…”