144 / 453
CONFIDENTIAL INFORMATION
Page 2 of 13
1.0 OBJECTIVE
The objective of this study was to validate the
3M™
Molecular Detection Assay
(MDA) for detection of
Escherichia coli
O157 in a frozen blueberry food matrix.
The Food Safety Net Services (FSNS) San Antonio Laboratory worked to
validate the MDA system,
considered here as the ‘candidate method’,
using a 25
g sample size for frozen blueberries. The study followed the format for the
validation of a candidate method using the experimental setup and Probability of
Detection (POD) statistical analysis that are provided in the 2012 version of the
AOAC International Methods Committee Guidelines for Validation of
Microbiological Methods for Food and Environmental Surfaces document (1). For
the purposes of this study, the candidate method results were compared with the
analysis of a 25 g sample size for frozen blueberries, performed by reference
method outlined via the 2014 version of the U.S. Food and Drug Administration
Bacteriological Analytical Manual (FDA BAM) (2).
2.0 METHODS AND MATERIALS
2.1 Acquisition and Initial Analysis of Food Matrices
As mentioned in Section 1.0, the food matrix of frozen blueberries were tested in
the validation of the MDA. The FSNS San Antonio Laboratory acquired blueberries
from various sources, to be mixed prior to use. The frozen blueberries were
thawed at refrigeration temperatures prior to inoculation. Prior to inoculation, a 25
g sample was removed and analyzed for
E. coli
O157 according to the FDA BAM
for
E. coli
O157 (2, 3), as outlined in Section 2.4.
2.2 Preparation of
E. coli
O157:H7 Inoculum
As stated, the objective of this study was to determine the ability of the MDA to
detect the presence of
E. coli
O157:H7 in frozen blueberries. The inoculum used
for this validation study consisted of the following strain of
E. coli
O157:H7:
Escherichia coli
serotype O157:H7 ATCC 700599
Prior to each day of the experiment, an individual culture was prepared by
streaking loopfuls of culture from -
80˚C freezer stocks onto Tryptic Soy Agar (TSA;
Becton, Dickinson and Company, Franklin Lakes, NJ) and incubating aerobically
for 24 h at 35 ± 2°C. One isolated colony from the TSA plate was then transferred
into Tryptic Soy Broth (TSB; Becton, Dickinson and Company) and incubated
aerobically at 35 ± 2°C for 18-24 h to achieve stationary phase growth. A 1:9
dilution of the TSB culture was made in B
utterfield’s Phosphate Buffer (BPB; Made
In-House) and the transmittance at 560 nm was measured using a Spectronic
TM
200 Spectrophotometer (Thermo Fisher Scientific, Inc., Waltham, MA). This
suspension served as the inoculum for
E. coli
O157:H7 and the concentrations of