IMAGE ANALYSIS OF A FLUORESCENT PHYSICAL SURROGATE FOR QUANTIFYING LISTERIA MONOCYTOGENES TRANSFER BETWEEN DELICATESSEN MEATS AND PRODUCT CONTACT SURFACES

摘要

Listeria monocytogenes is the primary pathogen of concern for microbial safety of delicatessen meats sliced at retail. A fluorescent powder (Glo Germ (TM), GGP) has been used previously to qualitatively assess cleaning and hand washing practices in foodservice. However, quantitative use of this material as a physical surrogate for L. monocytogenes transfer would enable direct evaluation of training intervention programs and validation of existing risk models. The objective was to develop a method to quantify the spread of GGP between delicatessen meats (turkey and ham) and product contact surfaces [stainless steel (SS) and high density polyethylene (HDPE)], then mathematically relate these findings to the rate of L. monocytogenes transfer under equivalent conditions. A six-strain L. monocytogenes cocktail was inoculated onto 8 x 8 cm pieces of delicatessen roast turkey (T), ham (H), grade 304 SS, or HDPE, at similar to 10(9) CFU/cm(2), and allowed to surface dry. Inoculated T, H, SS, or HPDE was placed in contact with an uninoculated surface of an opposite type for 3 s at room temperature, with additional normal force, for 12 sequential contact events on 12 separate surfaces. This same procedure was repeated using a suspension of GGP in 70% ethanol (1:10). Listeria populations were quantified by plating stomached (T, H) or sonicated (SS, HDPE) dilutions on modified Oxford agar. GGP-contacted surfaces were photographed under controlled UV-light and quantitatively analyzed via image processing software. Results showed a logarithmic transfer response for both L. monocytogenes and GGP. The GGP data for each transfer scenario were fit to the corresponding L. monocytogenes data by minimizing the root mean squared error (RMSE) between the two curves using an offset and multiplier. Because the parameters for T, H, SS, and HPDE differed significantly (P < 0.05), a general algorithm was not feasible. However, for known recipient surfaces, image analysis of GGP transfer among contact surfaces was a reasonable surrogate to rapidly quantify simulated L. monocytogenes cross-contamination.