Ehec Serotypespecific Marker

Production of the O157 antigen is a distinguishing characteristic of O157:H7 serotype; however, serotyping is problematic because of the cross-reactivity of anti-O157 polyclonal sera with strains of Citrobacter freundii, Escherichia hermanii, and others. However, serotype-specific genetic markers are specific and provide a viable alternative to serotyping. One such marker is the rfbE gene, which encodes for perosamine synthethase that is essential for O157 antigen expression.1-17-1 PCR primers to the rfbE locus have been designed that detect O157 strains; however, these primers are not O157:H7-specific and detect other O157 strains that do not produce H7. Similarly, PCR primers specific to the serotype O111[18] and O113[19] rfb locus have been designed that detect these serotypes, some of which are EHEC and have been implicated in causing illness.

Production of the flagellar H7 antigen is another trait phenotype of O157:H7 strains. However, as in the case of the O157 antigen, other E. coli serotypes also express the H7 antigen. Flagellin is encoded by the fliC gene, which genetic analyses have shown to contain polymorphisms that can be distinguished by RFLP analysis of H7 fliC-derived PCR amplicons.[20] All O157:H7 and its Stx-producing nonmotile variants have identical H7 fliC RFLP profiles that are distinct from that of other H7-producing serotypes, except the EPEC O55:H7 strain, which has not only an identical H7 fliC RFLP profile,[20] but also has the H7a,c serological subtype.[21] Genetic studies show that O55:H7 is an ancestral strain of O157:H7, which is closely related and belongs to the same clonal group.[6] Although different, at present PCR primers specific for the H7 fliC gene detect all H7-producing strains regardless of its O serotype,[22] and none are specific for the H7 fliC allele exclusively found in the O157:H7 and O55:H7 strains.

Getting Started With Dumbbells

Getting Started With Dumbbells

The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.

Get My Free Ebook

Post a comment