Improved Recovery of Stressed Listeria monocytogenes from Frozen Foods
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The Canadian reference method for the enumeration of Listeria monocytogenes (Health Canada method MFLP-74) was modified by the addition of a layer of non-selective tryptone soy agar (TSA) to the Oxford, Palcam, and (or) Rapid’L.mono selective agars, to improve the recovery of stressed L. monocytogenes from frozen foods. The performance of the standard selective agars versus the selective agars with an additional TSA agar layer (TAL agars) showed that for each of the frozen food items tested (vegetables, shrimp and meatballs), as well as for all data combined, L. monocytogenes counts were significantly higher (up to 20% higher) in TAL agars in relation to the standard agars (p < 0.0001, α = 0.05). A selectivity assessment showed that the 50 inclusivity L. monocytogenes strains tested produced true positive reactions on the TAL agars, and the 30 exclusivity bacterial strains tested produced true negative results on the TAL agars, indicating that the specificity of the selective agars was not altered by the additional non-selective TSA layer. In light of the 2016 Listeria outbreak linked to frozen fruits and vegetables in the USA, the results of this method investigation seem to be timely and particularly important for the regulatory testing of frozen foods.
KeywordsListeria monocytogenes Stress Enumeration Thin agar layer Microbiological methods RTE food products
The authors gratefully acknowledge Jacqueline Upham and Jodi Elliot for contributions made during planning of the preliminary experiments and Elizabeth Boutilier for providing technical assistance during the selectivity study.
Compliance with Ethical Standards
This work was funded by internal research grants provided by the Canadian Food Inspection Agency.
Conflict of Interest
Julie McLennon declares that she has no conflict of interest. Antonela Borza declares that she has no conflict of interest. Mikaela Eisebraun declares that she has no conflict of interest. Rafael Garduño declares that he has no conflict of interest.
This article does not contain any studies with human participants or animals performed by any of the authors.
Informed consent is not applicable in this study.
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