Research – Effect of Time, Temperature and Transport Media on the Recovery of Listeria monocytogenes from Environmental Swabs

Journal of Food Protection

Environmental monitoring for Listeria monocytogenes in food processing environments is key for ensuring the safety of ready-to-eat foods. For sampling, swabs are often hydrated with a wetting or transport medium which may contain neutralizers and other ingredients. After swabbing the environment, the swabs may then be transported or shipped cold to an off-site laboratory for testing, ideally within 48 h. Extended shipping times may subject the pathogen to increased temperatures in the presence of the wetting medium, organics, and other chemicals from the processing facility which may confound detection. This study evaluated growth and detection of L. monocytogenes on stainless steel exposed to either buffer or sodium hypochlorite prior to drying. Swabs were rehydrated with Butterfield’s Phosphate Buffer, Neutralizing Buffer, Letheen Broth or Dey-Engley Neutralizing Broth prior to swabbing. Swabs were stored in the presence of no added food, cheese whey or ice cream under both optimal (4°) and sub-optimal (15°C) temperatures for up to 72 h. Overall, there was no growth of L. monocytogenes at 4°C through 72 h storage, though enrichment from these swabs was dependent on the presence and type of food matrix. Pathogen growth during storage at 15°C was more variable and depended on both the food matrix and transport media used, with Dey-Engley and Letheen Broth allowing for the highest population increases. Overall, more enrichments resulting in L. monocytogenes detections were observed when using Letheen Broth and Neutralizing Buffer than Dey-Engley which resulted in fewer detections at 15°C. Logistic regression and Cochran-Mantel-Haenszel (CMH) analyses determined that storage temperature, transport media, and food matrix all significantly affected detection of L. monocytogenes , while storage time did not have a clear effect on recovery from swabs.

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