Resistance to Four Sanitizers of Different Strains of Salmonella and L. monocytogenes in Biofilms of Stainless Steel

Introduction: Biofilms on stainless steel surfaces of food processing are extremely important as they may lead to food spoilage and occurrence of foodborne disease outbreaks. The persistence phenomenon can be attributed to an increased tolerance of microorganisms to sanitizers and further adhesion ability to abiotic surfaces.

Purpose: This study aimed to assess the resistance to sanitizers (sodium hypochlorite, chlorine dioxide, peracetic acid and quaternary ammonium) of different strains of Salmonella and Listeria monocytogenes when in biofilms.

Methods: Four strains of each, Salmonella and L. monocytogenes, previously screened for sanitizer’s resistance were inoculated in a 12-well plate (1 strain/well) containing 3 ml of Luria Bertani broth and sterile stainless steel coupon (AISI #304, 10mm x 10mm x 1mm). The 12-well plates were further incubated at 37°C/24 h, following removal of coupons from wells and washing for 1 minute in 10 ml of PBS. Then, coupons were placed in contact with 3 ml of selected sanitizer (5, 10, 20, 25, 50 and 100 ppm) for 3 min. Next, the coupons were vortexed for 2 min to detach cells adhered to coupons, followed by drop plating in Muller Hinton Agar for enumeration of survivors (37°C/24 h).

Results: Quaternary ammonium (QAC) did not inactivate Salmonella in all tested concentrations, while chlorine dioxide (ClO₂) was the most efficient sanitizer. Reductions of adhered cells of L. monocytogenes 4b reached up to 7.1 log in treatment with ClO₂ at 20 ppm. Differences in terms of resistance in biofilms were clearly noticed among the different strains of L. monocytogenes. L. monocytogenes survivors ranged from 3-6.4 log reduction when a concentration of 10 ppm of ClO₂ was used. 

Significance: The resistance of different strains of Salmonella and L. monocytogenes when in biofilms to different sanitizers. These results may assist the development and application of sanitization strategies to eliminate biofilms from food processing environments.