The efficiency of the multiplex assay was further confirmed
when we looked for Shigella in artificially contaminated food
samples. The sensitive detection of foodborne pathogens in
produce requires optimal cell lysis and efficient DNA purification
to remove associated PCR inhibitors (Schrader et al., 2012).
Boiling in PBS or water has been the DNA isolation method of
choice in the FDA Survey of Domestic and Import Fresh Produce
assignments for Shigella because its cost is low and it can easily
be used to process a large volume of samples, however there is
no step to remove eventual food matrix-associated PCR in-
hibitors (Warren et al., 2009). Such inhibitors would cause false
negative PCR results with potentially serious public health
consequences when a contaminated food is not removed from
circulation. We found that the reagents and methods used in
DNA extraction had some impact on the PCR outcome. The
presence of inhibitors produced false negative PCR results in
DNA prepared from three commercial DNA extraction kits, but
this effect was not seen with the boiling method, probably
because the DNA and DNA inhibitors were more diluted. Indeed,
we found that diluting the DNA template 1:10 before the assay
reduced the concentration of inhibitory compounds to a level
that did not affect DNA amplification and Shigella could be
detected. Nevertheless, as a protective measure, it may be useful
to add an internal amplification control as we did recently for the TaqMan-based multiplex real-time assay for Shigella and
EIEC (Deer and Lampel, 2010).