Transfer of Listeria monocytogenes during mechanical slicing of turkey breast, bologna, and salami.

A commercial delicatessen slicer was used as the vector for sequential quantitative transfer of Listeria monocytogenes (i) from an inoculated slicer blade (approximately 10(8), 10(5), or 10(3) CFU per blade) to 30 slices of uninoculated delicatessen turkey, bologna, and salami, (ii) from inoculated product (approximately 10(8) CFU/cm2) to the slicer, and (iii) from inoculated product (10(8), 10(5), or 10(3) CFU/cm2) to 30 slices of uninoculated product via the slicer blade. Cutting force and product composition also were assessed for their impact on L. monocytogenes transfer. Five product contact areas on the slicer, which were identified from residue of product bathed in Glow-Germ, were also sampled using a 1-ply composite tissue technique after inoculated product had been sliced. After being sliced with inoculated blades, each product slice was surface or pour plated on modified Oxford agar and/ or enriched in University of Vermont medium. Greater transfer (P < 0.05) occurred from inoculated turkey (10(8) CFU/cm2) to the five slicer contact areas from an application force of 4.5 kg as compared with 0 kg. On uninoculated product sliced with blades inoculated at 10(8) CFU per blade, L. monocytogenes populations decreased logarithmically to 10(2) CFU per slice after 30 slices. Findings for the inoculated slicer blade and product (10(5) CFU per blade or cm2) were similar; L. monocytogenes concentrations were 102 CFU per slice after 5 slices and enriched samples were generally negative for L. monocytogenes after 27 slices. For uninoculated product sliced with blades inoculated at 10(3) CFU per blade, the first 5 slices typically produced L. monocytogenes at approximately 10 CFU per slice by direct plating, and enrichments were negative for L. monocytogenes after 15 slices. The higher fat and lower moisture content of salami compared with turkey and bologna resulted in a visible fat layer on the blade that likely prolonged L. monocytogenes transfer. As a result of cross-contamination, those delicatessen-sliced meats that allow growth of L. monocytogenes during prolonged refrigerated storage likely pose an increased public health risk for certain consumers.

Improved quantitative recovery of Listeria monocytogenes from stainless steel surfaces using a one-ply composite tissue.

Four sampling devices, a sterile environmental sponge (ES), a sterile cotton-tipped swab (CS), a sterile calcium alginate fiber-tipped swab (CAS), and a one-ply composite tissue (CT), were evaluated for quantitative recovery of Listeria monocytogenes from a food-grade stainless steel surface. Sterile 304-grade stainless steel plates (6 by 6 cm) were inoculated with approximately 106 CFU/cm2 L. monocytogenes strain Scott A and dried for 1 h. The ES and CT sampling devices were rehydrated in phosphate buffer solution. After plate swabbing, ES and CT were placed in 40 ml of phosphate buffer solution, stomached for 1 min and hand massaged for 30 s. Each CS and CAS device was rehydrated in 0.1% peptone before swabbing. After swabbing, CS and CAS were vortexed in 0.1% peptone for 1 min. Samples were spiral plated on modified Oxford agar with modified Oxford agar Rodac Contact plates used to recover any remaining cells from the stainless steel surface. Potential inhibition from CT was examined in both phosphate buffer solution and in a modified disc-diffusion assay. Recovery was 2.70, 1.34, and 0.62 log greater using CT compared with ES, CS, and CAS, respectively, with these differences statistically significant (P < 0.001) for ES and CT and for CAS, CS, and CT (P < 0.05). Rodac plates were typically overgrown following ES, positive after CS and CAS, and negative after CT sampling. CT was noninhibitory in both phosphate buffer solution and the modified disc-diffusion assay. Using scanning electron microscopy, Listeria cells were observed on stainless steel plates sampled with each sampling device except CT. The CT device, which is inexpensive and easy to use, represents a major improvement over other methods in quantifying L. monocytogenes on stainless steel surfaces and is likely applicable to enrichment of environmental samples.