Genotypic Tests For Diphtheria Toxin

A genotypic test is designed to detect the nucleotide sequence for a particular gene. Unlike an immunoassay for a protein, a genotypic test does not require that the gene product be produced. Therefore a genotypic test for tox is one step more remote from a bioassay for diphtheria toxin than an immunoassay. Because many tests for specific DNA sequences have high sensitivity, it is possible to develop tests for tox that will have few false-negative results. However, the converse is not true, because nontoxigenic isolates of C. diphtheriae may contain an inactive allele, a single fragment, or multiple fragments of tox. In such isolates, a positive test for the tox gene would not agree with the negative result of a bioassay for diphtheria toxin.

Several investigators developed polymerase chain reaction (PCR) tests to detect tox in DNA isolated from pure cultures of C. diphtheriae.[17,18] The presence of nucleic acid sequences encoding the A fragment of diphtheria toxin agreed well with positive Elek tests for diphtheria toxin production, and the sensitivity of the PCR tests was approximately 50-500 colony forming units (CFU) of a tox+ isolate of C. diphtheriae. A PCR test for detecting tox in DNA isolated directly from throat or nasopharyngeal swabs was also developed. It eliminated the time required to culture C. diphtheriae and gave results within hours of collecting the initial clinical specimens.[19] Subsequently, a real-time fluorescence PCR assay decreased the limit of detection for tox to approximately 2 CFU/sample.[20] This real-time PCR test detected the coding regions for diphtheria toxin fragment A and fragment B in all of 23 tox+ clinical isolates and 2 tox+ reference strains of C. diphtheriae, and it was superior to routine PCR tests in detecting tox in DNA extracted directly from clinical specimens.

The events in Russia and the Newly Independent States during the 1990s provided an opportunity to apply molecular epidemiological methods to an evolving epidemic of diphtheria and showed that most patients

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.

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