UVM Theses and Dissertations
Format:
Print
Author:
Nyachuba, David Getuma
Dept./Program:
Animal Sciences
Year:
2007
Degree:
PhD
Abstract:
Listeria monocytogenes is a foodborne pathogen that causes a deadly foodborne infection, listeriosis. During the past few decades, this pathogen has been implicated in several large listeriosis outbreaks, all of which were associated with ready-to-eat (RTE) meat, poultry and seafood products. L. monocytogenes is also a major problem for food processors as product contamination with this pathogen results in serious economic consequences in terms of costly product recalls and lawsuits. Injured L. monocytogenes cells should be accounted for as they may repair themselves, grow, multiply to high levels, and even regain pathogenicity during extended refrigerated storage.
This dissertation focuses on (1) the impact of nitrite (NaNO₂)-induced injury on detection and recovery of Listeria from smoked salmon, smoked ham, beef frankfurters, and beef bologna; (2) comparison of the 3M Petrifilm Environmental Listeria (EL) Plate method vs. conventional methods in recovering Listeria from a variety of environmental surfaces; and (3) evaluation of the efficacy of 14 different sanitizers against Listeria on 7 different types of overshoe material (footwear) used in the food processing environment. The resiliency of the 7 overshoe materials to these sanitizers and to high temperature sterilization (121°C for 15 min) was also evaluated.
Nitrite-containing (NC; 100-200 ppm NaNO₂) or nitrite-free (NF) foods were inoculated with a 5 strain cocktail of L. monocytogenes by immersion into Butterfield's buffer solution containing 5.4-7.4 x 10³ CFUIml, vacuum-packed, and stored at 5°C. Samples were analyzed weekly for presence of L. monocytogenes for up to 8 weeks using 5 different methods - the genetic-based BAX® System, the U.S. Food and Drug Administration (FDA) method using Listeria enrichment broth (LEB), the U.S. Department of Agriculture/Food Safety and Inspection Service (USDNFSIS) method using University of Vermont broth (UVM), the USDNFSIS method using Listeria repair broth (LRB), and the modified USDAFSIS method using a combination of UVM & LRB (mUSDA). NaNO₂ at concentrations of 100-200 ppm resulted in 83-99% injury. The BAX system and mUSDA were consistently superior in recovering Listeria from both NC and NF samples.
M Petrifilm EL Plate may allow detection of low levels and injured Listeria cells from environmental surfaces. The mUSDA method was superior to the rest of the methods and demonstrates the need for consideration of sublethal injury when recovering epidemiologically significant Listeria from environmental surfaces. Data indicate that the Petrifilm EL Plate no enrichment method recovered both low levels and injured Listeria from surfaces while saving up to 91 h compared to standard culture methods.
We demonstrated that the 7 overshoe materials were compatible with the 14 tested cleaning and sanitizing chemicals and high temperature sterilization and therefore overshoes constructed of these materials can be easily cleaned and sanitized compared to conventional industrial footwear. This type of footwear would assist in the control of spread and proliferation oflisteria in the food processing environments.
In summary, our results indicate that sublethal injury has negative implications on recovery of L. monocytogenes from food and environmental samples. LRB can recover sublethally-injured Listeria and is generally more efficacious than most regulatory enrichment broths and its superiory can be further enhanced through dual enrichment with UVM.
This dissertation focuses on (1) the impact of nitrite (NaNO₂)-induced injury on detection and recovery of Listeria from smoked salmon, smoked ham, beef frankfurters, and beef bologna; (2) comparison of the 3M Petrifilm Environmental Listeria (EL) Plate method vs. conventional methods in recovering Listeria from a variety of environmental surfaces; and (3) evaluation of the efficacy of 14 different sanitizers against Listeria on 7 different types of overshoe material (footwear) used in the food processing environment. The resiliency of the 7 overshoe materials to these sanitizers and to high temperature sterilization (121°C for 15 min) was also evaluated.
Nitrite-containing (NC; 100-200 ppm NaNO₂) or nitrite-free (NF) foods were inoculated with a 5 strain cocktail of L. monocytogenes by immersion into Butterfield's buffer solution containing 5.4-7.4 x 10³ CFUIml, vacuum-packed, and stored at 5°C. Samples were analyzed weekly for presence of L. monocytogenes for up to 8 weeks using 5 different methods - the genetic-based BAX® System, the U.S. Food and Drug Administration (FDA) method using Listeria enrichment broth (LEB), the U.S. Department of Agriculture/Food Safety and Inspection Service (USDNFSIS) method using University of Vermont broth (UVM), the USDNFSIS method using Listeria repair broth (LRB), and the modified USDAFSIS method using a combination of UVM & LRB (mUSDA). NaNO₂ at concentrations of 100-200 ppm resulted in 83-99% injury. The BAX system and mUSDA were consistently superior in recovering Listeria from both NC and NF samples.
M Petrifilm EL Plate may allow detection of low levels and injured Listeria cells from environmental surfaces. The mUSDA method was superior to the rest of the methods and demonstrates the need for consideration of sublethal injury when recovering epidemiologically significant Listeria from environmental surfaces. Data indicate that the Petrifilm EL Plate no enrichment method recovered both low levels and injured Listeria from surfaces while saving up to 91 h compared to standard culture methods.
We demonstrated that the 7 overshoe materials were compatible with the 14 tested cleaning and sanitizing chemicals and high temperature sterilization and therefore overshoes constructed of these materials can be easily cleaned and sanitized compared to conventional industrial footwear. This type of footwear would assist in the control of spread and proliferation oflisteria in the food processing environments.
In summary, our results indicate that sublethal injury has negative implications on recovery of L. monocytogenes from food and environmental samples. LRB can recover sublethally-injured Listeria and is generally more efficacious than most regulatory enrichment broths and its superiory can be further enhanced through dual enrichment with UVM.