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Lineage and Host Source Are Both Correlated with Levels of Shiga Toxin 2 Production by Escherichia coli O157:H7 Strains

Zhang, Yongxiang, Laing, Chad, Zhang, Zhengzhong, Hallewell, Jennyka, You, Chunping, Ziebell, Kim, Johnson, Roger P., Kropinski, Andrew M., Thomas, James E., Karmali, Mohamed, Gannon, Victor P.J.
Applied and environmental microbiology 2010 v.76 no.2 pp. 474-482
Escherichia coli O157, Shiga-like toxin 2, cattle, enzyme-linked immunosorbent assay, genes, genotype, hosts, human diseases, humans, messenger RNA, pathogens, polymerase chain reaction, sequence analysis, serine, threonine, virulence
Escherichia coli O157:H7 strains fall into three major genetic lineages that differ in their distribution among humans and cattle. Several recent studies have reported differences in the expression of virulence factors between E. coli O157:H7 strains from these two host species. In this study, we wished to determine if important virulence-associated "mobile genetic elements" such as Shiga toxin 2 (Stx2)-encoding prophage are lineage restricted or are host source related and acquired independently of the pathogen genotype. DNA sequencing of the stx₂ flanking region from a lineage II (LII) strain, EC970520, revealed that the transcriptional activator gene Q in LI strain EDL933 (upstream of stx₂) is replaced by a pphA (serine/threonine phosphatase) homologue and an altered Q gene in this and all other LII strains tested. In addition, nearly all LI strains carried stx₂, whereas all LII strains carried variant stx₂c and 4 of 14 LI/II strains had copies of both stx₂ and variant stx₂c. Real-time PCR (RT-PCR) and enzyme-linked immunosorbent assay (ELISA) demonstrated that LI and LI/II strains produce significantly more stx₂ mRNA and Stx2 than LII strains. However, among LI strains significantly more Stx2 is also produced by strains from humans than from cattle. Therefore, lineage-associated differences among E. coli O157:H7 strains such as prophage content, toxin type, and toxin expression may contribute to host isolation bias. However, the level of Stx2 production alone may also play an important role in the within-lineage association of E. coli O157:H7 strains with human clinical disease.