Initially, each primer set (RB87 and RB88 for ipaH, RB89 and
RB90 for virB, and RB110 and RB111 for mxiC) was tested separately
to ensure that they gave a single product corresponding to the
predicted molecular size for each amplicon (data not shown).
Genome analysis of the 15 Shigella and EIEC genome and virulence
plasmid sequences available in public databases showed that the
RB87 and RB88 ipaH primers can recognize the five to seven copies
and the four to five copies of ipaH located on the chromosome and
the invasion plasmid, respectively, depending on the bacterial
isolate. The multiplex assay was therefore designed to contain four
times more primers directed against the virB and mxiC single-copy
genes than primers directed against the multicopy ipaH gene. The
assay was still sensitive enough to detect less than 10 Shigella cells
in pure culture (Fig.1A) in 1 h and 40min fromstart to finish (a one-
hour PCR reaction using the fast-cycling PCR kit on the DNA Tetrad
thermocycler followed by a 40-min agarose gel electrophoresis on
the RunOne system). The wild-type S. flexneri strain showed three
bands at the predicted molecular size while the mutant 2457M
showed only 2 bands for ipaH and virB, whether the HotStar Taq
Plus enzyme was used with the Master Mix kit (data not shown) or
with the fast cycling PCR kit (Fig. 1A; Table 1). A band at w1kb
corresponding to the mxiC::kan amplicon was seen only with
2457M when the multiplex reactions were performed with the
Qiagen Multiplex PCR mix (Fig. 1B).