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Chapter 14 : Evolutionary Emergence and Impact of Atypical Escherichia Coli O157:H7 Strains
This chapter reviews the findings on some of atypical strains, examines the genetic mutations underlying their atypical phenotypes, addresses the impact they have on public health, and discusses how they fit into the evolutionary model of Escherichia coli O157:H7. Almost all generic E. coli strains produce β-glucuronidase (GUD), so a popular assay to identify E. coli utilizes the fluorogenic GUD substrate 4-methylumbelliferyl-β-D-glucuronide. The definitive identification of O157:H7 strains is based on the serological presence of both the O and the H antigens. The authors found that the majority of Shiga toxin (Stxs)-producing O157:NM strains are actually phenotypic variants of O157:H7 and they can sometimes be induced to express the H7 antigen. More recent analysis by multilocus sequence typing (MLST) concurred with the multilocus enzyme electrophoresis (MLEE) results in that both TT12A and TT12B had ST66, the most common genotype for O157:H7 strains. The somatic (O) 157 and the flagellar (H) 7 antigens are key markers that are extensively used in diagnostics to identify the O157:H7 serotype. Many of the atypical O157:H7 variants have a public health impact, as they are often pathogenic and will cause illness but, due to the lack of trait marker(s), are not easily detected by assays routinely used to test for O157:H7. Although phenotypically distinct, many of the atypical O157:H7 variants were found to have identical multilocus genotypes (ST66) and belong to the A6 clonal group. Therefore, such strains represent newly identified O157:H7 variants that merely lost typical phenotypic features.