| Presence of Listeria monocytogenes strains endemic to food processing environments is presumably related to biofilm formation. Following exposure to various environmental stresses, Listeria cells may be more prone to attach to surfaces. Due to concerns regarding the potential impact of biofilm formation on Listeria cross-contamination of ready-to-eat meats in delicatessens, a series of studies was conducted to: (1) determine the ability of L. monocytogenes to form biofilms under various temperatures and stress conditions present in food processing and retail environments, (2) determine the effects of biofilm-forming ability on direct and sequential transfer rates for L. monocytogenes from delicatessen slicers to ready-to-eat meats, (3) determine the effects of environmental stress on direct and sequential transfer rates for L. monocytogenes from delicatessen slicers to ready-to-eat meats, and (4) develop one or mathematical models that can be used to predict the transfer rates for L. monocytogenes during retail slicing of ready-to-eat meats. A total of 196 L. monocytogenes isolates were assessed for biofilm formation at 22 and 4°C in Modified Welshimer's Broth, as measured by optical density (OD) of stained biofilms, while a subset of 26 food, environmental and human clinical isolates were further assessed for biofilm formation after exposure to common environmental stressors (starvation, cold-shock, chlorine injury and acid injury). Only 5% of all isolates were strong biofilm-formers, forming biofilms with OD values two standard deviations above the mean, with 81% of strains failing to produce detectable biofilms at 4°C. Prior injury of L. monocytogenes by starvation and cold resulted in enhanced biofilm formation, while exposure to acid and chlorine diminished subsequent biofilm formation. Cold- and chlorine-shock produced statistically similar levels of injury, however the cultures were significantly different in their abilities to form biofilms (mean OD chlorine-shock = 0.309, mean OD cold-shock = 1.457), showing that non-oxidative stresses common in the environment increase likelihood of biofilm formation.; Thereafter, six of the identified strong and weak biofilm-forming strains were combined into two 3-strain cocktails. The cocktails (healthy, cold-shocked or chlorine-injured) were used to inoculate stainless steel delicatessen slicer blades (106 CFU/blade). After incubation for 6 and 24 h (22°C/∼78% RH), the inoculated blades were attached to a gravity-fed delicatessen slicer and used to generate 30 slices from retail chubs of roast turkey breast or Genoa salami. Biofilm-forming ability, length of incubation on stainless steel, and prior injury had no significant affect on transfer. Listeria was able to survive physiological stress and contaminate at statistically similar levels to healthy cells. Overall, significantly greater cumulative transfer to turkey (cumulative transfer = 4.2 log CFU) than salami (cumulative transfer = 3.5 log CFU) was observed. Under all conditions, L. monocytogenes was still present on the slicer after slicing.; These findings were then used to validate a predictive model in the form [CFU (X) = kaX] along with a program written in GWBasic. This model can be used if any two of the following three values are known: (a) initial inoculum, (b) total bacteria transfer, (c) bacteria fraction remaining on the blade after consecutive slicing, solving for each model parameter CFU (X), k, or a. The fit of the model ranged from R 2 = 0.65-0.94. |
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