To determine the assay’s limit of detection (LOD),
10-fold serial dilutions of cloned DNA ranging
from100 pg/ml to 0.1 fg/ml were made and the assays were
carried out according to optimized protocols. The LOD of
the SEA and SEB assay was 0.5 fg of plasmid DNA (12
copies) per PCR reaction (Fig. 1). The electrochemical
signals obtained with 0.5 fg of plasmid DNA (23 replicates)
were significantly higher (p!0.001) than those obtained
with the negative controls (23 replicates) for both assays.
Regression analysis of electrochemical signals obtained
with the SEA and SEB assays showed a linear correlation
with DNA concentrations spanning seven orders of
magnitude (r
2
R93%) indicating the utility of the assays
as quantitative tests (Fig. 1).
When a 10-fold dilution series of S. aureus genomic DNA
was used, the LOD of each assay was 10 fg/ml (50 fg per PCR
reaction) or 16 copies of template. Each assay was tested by
colorimetric EIA detection and compared with the electrochemical
detection assay. Similar sensitivity levels were
obtained with the colorimetric assays (data not shown). Tests
with spiked DNA showed that addition of non-enterotoxigenic
staphylococcal DNA or human DNA to a cloned DNA
had no effect on the amplification and detection limits. This
indicates the absence of PCR inhibition and interference with
ELISA detection (data not shown) and that the performance
of our hand held electrochemical detection system was
comparable to the colorimetric PCR-EIA methods.