Reduction of L. monocytogenes in Biofilms and Floor Drains
Studies have revealed that some strains of Listeria monocytogenes can become well established in food processing facilities in locations such as floor drains and remain members of the resident microbial flora for months or years, with some becoming antimicrobial resistant. The formation and growth characteristics of microbial biofilms can be influenced by increased contact time of the cells with surfaces of stainless steel, polypropylene, or rubber, and conditions that increase the rate of bacterial growth, such as nutrients, pH, and temperature.
Although significant improvements in plant layout and equipment design and procedures for cleaning and sanitizing have been made throughout the food processing industry, it is believed that L. monocytogenes will continue to be introduced into the environment in which ready-to-eat (RTE) foods are exposed for further processing and packaging, and these bacterial cells in biofilms may be more resistant to sanitizing chemicals.
In a previous study, we observed that two competitive exclusion bacteria (CE), Lactococcus lactis subsp. lactis (#C-1-92) and Enterococcus durans (#152), substantially reduced L. monocytogenes on stainless steel coupons. The objective of the study presented here was to compare the efficacy of these CE to reduce L. monocytogenes populations in biofilm on stainless steel coupons and coupons comprised of plastic, rubber, glass, and silicone at 4 and 8°C, and in floor drains in a poultry processing plant. Biofilm characteristics were examined by scanning electron microscopy.
L. monocytogenes produced well-formed biofilms within 24 h at 37°C on coupon surfaces. Treating Listeria-laden biofilms with the CE isolates individually at either 4 or 8°C for 3 weeks substantially reduced or eliminated the bacteria. Treatment with L. lactis subsp. lactis (#C-1-92) and E. durans (#152) at 4°C for 3 weeks reduced the population of L. monocytogenes in biofilm from 7.1 - 7.7 to 3.0 - 4.5 and 3.1 - 5.2 log CFU/cm2, respectively. At 8°C for 3 weeks, L. monocytogenes was reduced from 7.5 - 8.3 to 2.4 - 3.5 and 3.8 - 5.2 log CFU/cm2, respectively, depending on the coupon composition.
The two CE isolates were combined and evaluated to control Listeria in floor drains of a RTE poultry processing plant. Results revealed that treating the floor drains with CE four times in 1 week eliminated detectable Listeria from five of six drains. Drains remained free of detectable Listeria for 13 weeks following the first treatments. Results of these studies indicate that CE can effectively reduce Listeria contamination in biofilms and in floor drains of a plant producing RTE poultry products.
These studies also showed that Listeria can persist for several months in floor drains in a RTE poultry processing plant and be a potential source of contamination of RTE food. Applying CE bacteria to floor drains provides a practical and environmental-friendly approach to mitigate Listeria contamination.