4. AOACRIMicroMethods-2018Awards

170

438  C rowley et al . : J ournal of AOAC I nternational V ol . 97, N o . 2, 2014

LPT. The confidence intervals obtained for dLPOD C indicated no significant difference between the two methods. A dLPOD CP of 0.01 (–0.10, 0.13) was obtained between presumptive and confirmed VIDAS LPT results for both confirmation procedures. The confidence intervals obtained for dLPOD CP indicated no significant difference between the presumptive and confirmed results. For the high-level inoculum, a dLPOD C value of 0.00 (–0.02, 0.02) was obtained between AOAC 993.12 and VIDAS LPT. The confidence intervals obtained for dLPOD C indicated no significant difference between the two methods. A dLPOD CP of 0.00 (–0.02, 0.02) was obtained between presumptive and confirmed VIDAS LPT results. The confidence intervals obtained for dLPOD CP indicated no significant difference between the presumptive and confirmed results. Results of the POD statistical analysis are presented in Table  2013.10A , Tables 2A–B, and Figures 1A–D, as supplemental data on the J. AOAC Int. website. Queso fresco test portions were inoculated at a low and high level and analyzed for the detection of Listeria spp. (Table 4). 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 test portions) that produced non- L. monocytogenes profiles, with five of the test portions producing doubtful 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 confirmed 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 LPT 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 the VIDAS LPT method with all test portions confirming positive. For the low level, 70 out of 144 test portions were reported as positive by the VIDAS LPT method with all 70 test portions confirming positive. For the uninoculated controls, 0 out of 144 samples produced a presumptive positive result by the VIDAS LPT method and no samples confirming positive. For test portions analyzed by AOAC 993.12 , 144 out of 144 high inoculum and 69 out of 144 low inoculum test Queso Fresco (125 g Test Portions)

Table 2. Heat-stress injury

CFU/OXA (selective agar) 5.3 × 10 8

CFU/TSA (nonselective agar)

Degree injury, %

Matrix

Test organism

1.3 × 10 9

59

Queso fresco

L. innocua

ATCC a 33091

LPT – 25 g

2.9 × 10 8

9.0 × 10 8

Queso fresco

68

L. monocytogenes

LPT – 125 g ATCC 19115 a ATCC = American Type Culture Collection.

laboratory are presented in Tables 2A–D, and Figures 1A–D and 2A–D as supplemental data on J. AOAC Int. website.

Queso Fresco (25 g Test Portions)

Queso fresco test portions, inoculated at a low and high levels, were analyzed for the detection of Listeria spp. (Table 3). Uninoculated controls were included in each sample set. Fourteen laboratories participated in the analysis of this matrix, and the results of 13 laboratories were included in the statistical analysis. Laboratory 11 reported data for eight reference method test portions (including seven uninoculated control test portions) that produced doubtful profiles of L. grayi . Colonies on these plates were also reported as beta-hemolytic, a characteristic not associated with L. grayi. The selective agar plates for these test portions were sent to the coordinating laboratory for further examination. Colonies present on the plates did not possess characteristics typical of Listeria spp. Colonies were identified as Gram-positive rods containing spores with morphology typical of Bacillus species. 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 results for this laboratory were excluded from statistical analysis. The MPNs obtained for this matrix, with 95% confidence intervals, were 0.63 CFU/test portion (0.49, 0.79) for the low-inoculum level and 5.48 CFU/test portion (3.60, 8.36) for the high-inoculum level. For VIDAS LPT test portions, no differences were observed between confirmation of samples using the proprietary chromogenic ALOA and the reference method agar. For the high-inoculum level, 156 out of 156 test portions were reported as positive by the VIDAS LPT method with all test portions confirming positive. For the low-inoculum level, 80 out of 156 test portions were reported as positive by the VIDAS LPT method with 78 test portions confirming positive, indicating two false-positive results. For the uninoculated controls, 1 out of 156 samples produced a presumptive positive result by the VIDAS LPT method with no samples confirming positive. All three false-positive samples were obtained from the same laboratory. For test portions analyzed by AOAC 993.12 , 156 out of 156 high-inoculum test portions and 76 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 dLPOD C value of 0.01 (–0.10, 0.13) was obtained between AOAC 993.12 and VIDAS 03/10/2019