Impact of bacterial stress and biofilm forming ability on transfer of surface-dried listeria monocytogenes during slicing of delicatessen meats

Authors: Keskinen, Lindsey; TODD, EWEN - MICHIGAN STATE UNIVERSITY; RYSER, ELLIOT - MICHIGAN STATE UNIVERSITY

Submitted to: International Journal of Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/18/2008
Publication Date: 12/1/2008
Citation: Keskinen, L.A., Todd, E., Ryser, E. 2008. Impact of bacterial stress and biofilm forming ability on transfer of surface-dried listeria monocytogenes during slicing of delicatessen meats. International Journal of Food Microbiology. 127:298-304.

Interpretive Summary: Listeria monocytogenes has been responsible for a large number of recalls of ready to eat meat products, with at least 35 recalls involving more than 44.7 million pounds of product recalled over the past 6 years. These contaminated products have the potential to contaminate other surfaces and previously uncontaminated products in a delicatessen. Therefore, even once a recalled product is taken off of the market, the possibility exists that the recalled product has contaminated other products within the delicatessen case. Six isolates of L. monocytogenes that were identified as being strong (3 isolates) or weak (3 isolates) at forming communities (biofilms) on contact surfaces were combined into inoculum cocktails. The cocktails (healthy, cold-stressed or chlorine-injured) were used to inoculate stainless steel delicatessen slicer blades (over 1 million bacteria per blade). The contaminated blades were held at room temperature for 6 and 24 h, and then were used to generate 30 slices from chubs of roast turkey breast or Genoa salami. Biofilm-forming ability, length of incubation on stainless steel, and prior injury had a significant effect on transfer of L. monocytogenes cells from the blade to the luncheon meat. Overall, significantly greater cumulative transfer to turkey (cumulative transfer = 15849 bacteria to the 30 slices of meat) than salami (cumulative transfer = 3162 bacteria to the 30 slices of meat) was observed. Under all conditions, L. monocytogenes was still present on the slicer after slicing of product was completed. These results indicate that one contaminated product can be a source of contamination to other surfaces and foods in the delicatessen environment.

Technical Abstract: Listeria monocytogenes contamination of delicatessen slicer blades can lead to cross-contamination of luncheon meats. A cocktail of 3 strong or 3 weak biofilm-forming strains of L. monocytogenes suspended in turkey slurry were used to inoculate stainless steel delicatessen slicer blades to a final concentration of 6 log CFU/blade. Cells were used with or without injury (cold-shocked at 4C for 2h, or chlorine-injured at 100 ppm for 1 min). Inoculated blades were held at 22C and 78 +/- 2% relative humidity for 6 and 24 h, before being used to generate 30 slices from chubs of roast turkey breast or Genoa salami. Slices (25 g) were diluted 1:5 in University of Vermont Medium, stomached, and pour-plated into Tryptose Phosphate Agar containing esculin and ferric ammonium citrate. More strong biofilm-formers transferred on average to the 30 slices (3.62 log CFU) than weak biofilm-formers (3.12 log CFU), and significantly greater transfer to turkey (3.61 log CFU) than to salami (3.12 log CFU) was observed. Cold-shock significantly increased subsequent L. monocytogenes transfer (3.69 log CFU) compared to healthy (3.30 log CFU) and chlorine-injured cells (3.12 log CFU). Significantly greater numbers of L. monocytogenes transferred from blades that were held for 6 h than blades held for 24 h. Differences product composition and survival of L. monocytogenes as observed via viability staining are the likely cause of differences in transfer.