Purpose: The purpose of this AOAC OMA Collaborative Study was to compare the two rapid methods to the AOAC OMA 993.12 for Soft Mexican Cheese (125g).
Methods: The two rapid methods were compared in a multi-laboratory collaborative study to the AOAC OMA 993.12 Listeria monocytogenes in Milk and Dairy Products reference method. A total of 14 laboratories participated, representing government and industry, throughout the United States. Soft Mexican cheese was analyzed using 125-gram sample size. Each replicate was artificially contaminated with Listeria monocytogenes ATCC 19115 at 3 levels, an un-inoculated control level (0 CFU/125 g), a low inoculum level (0.2-2 CFU/125 g) and a high inoculum level (2-5 CFU/125 g). All test portions were confirmed using Oxford agar, as prescribed by the reference method, as well as ALOA chromogenic agar for the identification and differentiation of L. monocytogenes and Listeria species.
Results: For this evaluation, 900 unpaired replicate test portions (125 g) were analyzed by either the candidate or reference methods. Each inoculation level was analyzed using the Probability of Detection (POD) statistical model. For both VIDAS LPT and LMX low level inoculated test portions, dLPOD values of 0.01, (-0.10, 0.13), with 95% confidence intervals, were obtained. The range of the confidence intervals for dLPOD values contain the point 0.0 indicating no statistically significant difference in the number of positive samples detected between the new methods and the AOAC reference method. Additionally, no differences were observed between the two selective agars used for confirmation.
Significance: The collaborative study of these two novel methods, along with the optional ALOA agar confirmation method, demonstrated their robustness and high level of reproducibility in the detection of Listeria spp in 125-gram food samples. The harmonized 125g enrichment protocol for LPT and LMX enables the detection of Listeria spp and L. monocytogenes from the same sample.