Molecular Methods for Detecting and Discriminating Shigella Associated with Foods and Human Clinical Infections — A Review

Abstract

There are four species of the genus Shigella: S. dysenteriae, S. flexneri, S. sonneii, and S. boydii. It is extremely difficult if not impossible to utilize the PCR for distinguishing between shigellae and the five diarrheagenic pathotypes of Escherichia coli—enterohemolytic (EHEC), enteropathogenic (EPEC), engteroinvasive (EIEC), enterotoxigenic (ETEC), and enteroaggresive (EAEC)—due to the extremely close genetic relationship between Shigella spp. and E. coli. This problem is magnified by the presence of common virulence plasmids in these two microbial groups with virulence genes of extremely high homology. Although gastrointestinal infections by shigellae and EICC occur most frequently in developing nations, the responsible E. coli strains are indistinguishable from shigellae by PCR. In addition, with the exception of S. boydii, the other three species of Shigella are completely indistinguishable from one another by the PCR. The most fruitful approach to date in attempting to use the PCR for distinguishing the four species of Shigella from one another has been to first use immuno-capture based on somatic O antigenicity immediately prior to the use of the PCR to impart species selectivity to a coupled ELISA-PCR assay system. Most PCR assays involving the detection of shigellae have been designed to detect shigellae plus the E. coli pathotype EIEC as a collective and indistinguishable group of enteroinvasive organisms or to distinguish Shigella from pathogenic members of other pathogenic genera.