To improve the sensitivity of detection, various methods based
on the genetic rather than phenotypic attributes of Shigella have
been developed (Levin, 2009). Polymerase chain reaction (PCR)
assays to diagnose Shigella rely mainly on the amplification of DNA
fragments from the 220-kbp invasive plasmid shared by Shigella
and certain serotypes of Escherichia coli, known as enteroinvasive
E. coli (EIEC). This virulence plasmid can be spontaneously lost
during growth outside the host, so popular PCR protocols use
primers directed against the invasion plasmid antigen ipaH, which
is present in multiple copies in Shigella and EIEC bacterial genomes
(Sethabutr et al., 1993).