There have been several recalls of ready-to-eat (RTE) foods due to contamination with Listeria monocytogenes. Efforts to prevent contamination of food products with Listeria monocytogenes must be conducted at all levels of production. This is a difficult task given the fact that the bacteria is so widespread in the environment. Focusing efforts in your process where contamination risk is of greatest concern to the consumer is important. There is solid evidence that commercially prepared foods that have been contaminated with Listeria monocytogenes has occurred after the food product has been subjected to an initial lethality treatment. The product may be exposed in this area as a result of slicing, peeling, packing, re-bagging, cooling, or other procedures that may expose the product to potential contamination.

Listeria monocytogenes survives extremely well in food processing and retail food preparation environments. It may be introduced into your facility through a variety of routes, including:

  • Raw materials
  • Employees’ shoes or clothes
  • Equipment (boxes, crates, carts)
  • Controlling traffic flow into critical areas of the process can help reduce the chances of introducing and spreading the organism.

Once Listeria is introduced into the nonsterile environment, retail and factory conditions that promote its growth increase the risk of post-processing contamination. Several factors, including moisture, nutrients, temperature, competitive microflora and pH, affect the growth of Listeria in the food preparation and processing environment. Moisture is the most crucial factor, as it is essential for microbial growth and is the most easily controlled of the factors.

Listeria tends to form a biofilm to enhance its survival when resident populations become established in the food prep/processing environment. The resident populations that are referred to as “persistent” are not easily eliminated by general cleaning and sanitizing procedures. Biofilm penetration is necessary for removal and inactivation of Listeria. The correct blend of chemical, contact time and agitation will aid in the removal. This combination dissolves the biofilm and the organic material to which it adheres, allowing the sanitizer to inactivate the released, sensitive cells.

Biofilm removal is important, because persistent L. monocytogenes can be dispersed from a biofilm into the environment and onto food processing equipment, and non-food contact and food-contact surfaces. Passive dispersal of Listeria can occur by aerosolization from high-pressure hoses or brushing; once aerosolized, Listeria can contaminate other growth niches in the food handling/processing area, eventually contaminating food contact surfaces and food. Another form of passive dispersal is the movement of processing equipment. If a biofilm is present, cells can be released by the movement or vibration of the equipment.

Inactivation of L. monocytogenes in biofilms is an important part of a Listeria control program. Understanding this face prompted our team to perform research with the University of Georgia using a mixed culture biofilm formed by Pseudomonas putida and L. monocytogenes to evaluate the ability of Chemstar’s foaming sanitizer to inactivate L. monocytogenes present in biofilms under realistic use conditions. The results revealed that it provides for a greater than four-log reduction.1

Identifying Listeria in the environment and eliminating the resident populations can reduce the risk of secondary contamination. Once these procedures are established, employee training and environmental monitoring are vital. An effective Listeria control program requires that employees understand their role in mitigating the spread of Listeria, and management must relay those expectations. Control strategies are not likely to be effective if employees won’t cooperate, or don’t understand what they are expected to do, or why it is important, and that expected procedures or behavior will be monitored.

To conclude, the  five key elements in building an effective Listeria Control Program are:

  • Specific Sanitation Controls for Listeria
  • Training of Personnel (they need to understand their role in the program)
  • Traffic Control
  • Targeted Environmental Monitoring and Testing
  • Control Water Introduced into the Process Environment