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C rowley et al.: J ournal of AOAC International Vol. 97, No. 2, 2014 449
the participating laboratory and were not able to identify any
Listeria species. Laboratory 11 also reported one uninoculated
control portion positive for L. monocytogenes. Testing at the
coordinating laboratory verified this result, which indicated cross
contamination with that sample. The results from this laboratory
were excluded from statistical analysis. The MPN obtained for
this matrix, with 95% confidence intervals, were 0.55 CFU/test
portion (0.43, 0.70) for the low inoculum level and 3.81 CFU/test
portion (3.06, 5.48) for the high inoculum level. For VIDAS LMX
test portions, no difference was observed between confirmation
of samples using the proprietary chromogenic ALOA agar and
OXA required by the reference method.
For the high inoculum level, 156 out of 156 test portions were
reported as positive by the VIDAS LMX method with all test
portions confirming positive. For the low inoculum level, 77 out
of 156 test portions were reported as positive by the VIDAS LMX
method with 75 test portions confirming positive, indicating
two false-positive results. For the uninoculated controls, 0 out
of 156 samples produced a presumptive positive result by the
VIDAS LMX method with no samples confirming positive. For
test portions analyzed by AOAC 993.12, 153 out of 156 high
inoculum test portions and 69 out of 156 low inoculum test
portions confirmed positive. For the uninoculated controls, 0 out
of 156 test portions confirmed positive.
For the low-level inoculum, a dLPODC value of 0.04
(–0.08, 0.15) was obtained between AOAC 993.12 and VIDAS
LMX. The confidence intervals obtained for dLPODC indicated
no significant difference between the two methods. A dLPOD CP
of 0.01 (–0.10, 0.13) was obtained between presumptive and
confirmedVIDAS LMX results for both confirmation procedures.
The confidence intervals obtained for dLPODCP indicated no
significant difference between the presumptive and confirmed
results.
For the high-level inoculum, a dLPODC value of 0.02 (–0.01,
0.06) was obtained betweenAOAC 993.12andVIDASLMX. The
confidence intervals obtained for dLPODC indicated no significant
difference between the two methods. A dLPODCP of 0.00 (–0.02,
0.02) was obtained between presumptive and confirmed VIDAS
LMX results. The confidence intervals obtained for dLPODCP
indicated no significant difference between the presumptive and
confirmed results. Results of the POD statistical analysis are
presented in Table 2013.11A , Tables 2A–B, and Figures 1A–D as
supplemental data on the J. AOAC Int. website.
Queso Fresco (125 g Test Portions)
Queso fresco test portions were inoculated at a low
and high level and analyzed (Table 4) for the detection of
L. monocytogenes. Uninoculated controls were included in each
sample set. Fourteen laboratories participated in the analysis of
this matrix and the results of 12 laboratories were included in the
statistical analysis. Laboratory 2 did not report any data for this
matrix. Laboratory 11 reported 10 reference method test portions
(including five uninoculated control replicates) that produced
non-L. monocytogenes profiles, with five of the test portions
producing questionable API profiles of L. grayi. Colonies on
these plates contained one or more of the following biochemical
reactions not typically associated with L. monocytogenes: Gram-
negative, non-beta-hemolytic, and catalase negative. Based on
the preliminary biochemical tests conducted, the test portions
should not have been carried through for final biochemical
identification on API Listeria strips which resulted in the
misidentification of the test portion as Listeria spp. The selective
agar plates for these test portions were sent to the coordinating
laboratory for further examination. The coordinating laboratory
verified the supplementary results (Gram stain, hemolysis,
and catalase reaction) reported by the participating laboratory
and were not able to identify any Listeria species. The results
from this laboratory were excluded from statistical analysis.
The MPN levels obtained for this test portion, with 95%
confidence intervals, were 0.59 CFU/test portion (0.46, 0.74)
for the low level and 5.41 CFU/test portion (3.53, 8.30) for the
high level. For VIDAS LMX test portions, no differences were
observed between confirmation of samples using the proprietary
chromogenic ALOA and the reference method agar.
For the high-level, 144 out of 144 test portions were reported
as positive by VIDAS LMX with all test portions confirming
positive. For the low level, 70 out of 144 test portions were
reported as positive by VIDAS LMX with all 70 test portions
confirming positive. For the uninoculated controls, 0 out of
144 samples produced a presumptive positive result by VIDAS
LMX and no samples confirmed positive. For test portions
analyzed by AOAC 993.12, 144 out of 144 high inoculum and
69 out of 144 low inoculum test portions confirmed positive. For
the uninoculated controls, 0 out of 144 test portions confirmed
positive.
For the low-level inoculum, dLPODC values of 0.01
(–0.10, 0.13) were obtained between AOAC 993.12 and VIDAS
LMX. The confidence intervals obtained for dLPODC indicated
no significant difference between the two methods. dLPOD CP
values of 0.00 (–0.12, 0.12) were obtained between presumptive
and confirmed VIDAS LMX results. The confidence intervals
obtained for dLPODCP indicated no significant difference
between the presumptive and confirmed results using either
confirmation process.
For the high-level inoculum, dLPODC values of 0.00
(–0.03, 0.03) were obtained between AOAC 993.12 and VIDAS
LMX. The confidence intervals obtained for dLPODC indicated
no significant difference between the two methods. dLPOD CP
values of 0.00 (–0.03, 0.03) were obtained between presumptive
and confirmed VIDAS LMX results. The confidence intervals
obtained for dLPODCP indicated no significant difference
between the presumptive and confirmed results. Detailed results
of the POD statistical analysis are presented in Table2013.11B,
Table 2. Heat-stress injury results
Matrix (LMX test portion size) Test organisma CFU/OXA (selective agar) CFU/TSA (nonselective agar) Degree injury, %
Queso fresco LMX – 25 g L. monocytogenes ATCC 19115 5.0 × 108 1.3 × 109 62
Queso fresco LMX – 125 g L. monocytogenesATCC 19115 2.9 × 108 9.0 × 108 68
aATCC = American Type Culture Collection.
Candidates for 2016 Method of the Year
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