A Fast, Reliable, and Sensitive Method for Detection and Quantification of Listeria monocytogenes and Escherichia coli O157:H7 in Ready-to-Eat Fresh-Cut Products by MPN-qPCR
Table 2
Quantification of L. monocytogenes and E. coli O157:H7 in artificially inoculated fresh-cut salads by MPN and confirmation of positive tubes by conventional methods and qPCR.
Theoretical inoculum (CFU g−1)
3-tube dilution1 (mL homogenized)
Positive tubes (MPN)
True positive2
MPN (g−1)3 (95% c.i.)
Positive (qPCR)
L. monocytogenes
Control
10/1/0.1
0/0/0
n.d.
n.d.
n.d.
1
10/1/0.1
3/2/0
3/2/0
0.93 (0.23–3.80)
3/2/0
10
1/0.1/0.01
3/1/0
3/1/0
4.3 (0.90–18)
3/1/0
100
0.1/0.01/0.001
3/0/2
3/0/2
64 (17–180)
3/0/2
1000
0.01/0.001/0.001
3/1/1
3/1/1
750 (170–2000)
3/1/1
E. coli O157:H7
Control
10/1/0.1
3/3/3
n.d.
n.d.
n.d.
1
10/1/0.1
3/3/3
3/1/2
>11.0
3/1/2
10
1/0.1/0.01
3/3/1
3/2/0
46 (9–200)
3/2/0
100
0.1/0.01/0.001
3/2/1
3/1/1
150 (37–420)
3/1/1
1000
0.01/0.001/0.001
3/2/0
3/2/0
930 (180–4200)
3/2/0
The reported dilution is referred to as the set of tubes considered for MPN enumeration.
2Confirmed by biochemical and immunological methods.
3Calculated on the basis of the true positive samples.