Purpose: This study assessed the impact of two substrates and three temperatures on Salmonella attachment and biofilm formation on tomatoes as well as stainless steel and HDPE surfaces found in processing facilities.
Methods: Changes in viability, morphology, surface hydrophobicity and surface charge were assessed in diluted trypticase soy broth + yeast extract (pH 4.6 or 7.0) that was inoculated to contain avirulent Salmonella Typhimurium LT2 at ~9 log CFU/ml and held for 8 days at 4, 10, and 23°C. Attachment and biofilm formation as determined by direct plating and/or Confocal Scanning Laser Microscopy (CSLM) was also assessed on three surfaces of different hydrophobicities (contact angles) – tomatoes (100), high-density polyethylene (HDPE) (63.1), and stainless steel (35.6) that were inoculated with two substrates – water or 10% (w/v) blended tomatoes containing ~8 log CFU/ml Salmonella and then held for 6 days at 4, 10 and 23°C.
Results: Salmonella viability remained constant with cell elongation observed after 8 days of incubation at 4°C. Salmonella surface charge was higher at pH 4.6 with hydrophobicity inversely related to temperature. Significantly greater (P < 0.05) attachment to tomatoes was seen at 4°C using the blended tomato substrate. Biofilm formation significantly decreased (P < 0.05) with temperature for both substrates. Water as a substrate led to greater (P < 0.05) attachment and biofilm formation on HDPE at 23°C and on stainless steel at 23 and 10°C. Confocal imaging showed more uniform distribution of Salmonella on stainless steel as opposed to HDPE and tomatoes.
Significance: Based on these findings, tomato storage < 10°C and processing in water having a minimal organic load and an acidic pH is recommended to decrease Salmonella attachment and biofilm formation, with sanitizer efficacy also enhanced under these same conditions.