During adverse environmental conditions in nature or food processing environments bacteria may remain in a long-term-survival (LTS) phase for months or years. In this phase, pathogens may become stress-hardened and resist antimicrobial interventions.
Purpose: Studies were performed to determine the influence of desiccation on survival and dry-heat resistance of LTS phase Salmonella Typhimurium and Salmonella PT 30 on paper discs and raw almonds.
Methods: LTS Salmonella cells were developed in tryptic soy broth with 0.6% yeast extract (TSBYE) at 35°C for 14 days. Exponential- and stationary-phase cells grown in TSBYE (35°C) for 4 h and 24 h, respectively, served as controls. Cells (107 CFU/mL) from each physiological state were desiccated on paper discs and raw almonds at 22 ± 1°C for 24 hours. Heat (100°C, 30 min) resistance of desiccated Salmonellaon raw almonds was evaluated. Survivors were enumerated by plating diluted (10-fold) samples on tryptic soy agar with 0.6% yeast extract and counting bacterial colonies after incubation (35°C, 24 h).
Results: LTS Salmonella exhibited the highest desiccation and heat resistance (P < 0.05). Log reductions of exponential, stationary, and LTS cells on paper discs were 6.34, 4.39 and 2.46 for S. Typhimurium, and 6.5, 3.75, and 2.62 for S. PT30, respectively (P < 0.05). After desiccation on almonds, log reductions were 4.24 (exponential), 2.54 (stationary) and 1.97 (LTS) for S. Typhimurium; and 3.03, 2.58, and 1.76 for S. PT 30. Heating eliminated exponential phase survivors and decreased stationary and LTS cells, respectively, by ~1.82 and 1.58 log (S. Typhimurium) and ~2.28 and 1.85 log (S. PT30).
Significance: Results demonstrate that the LTS state cross-protects S. Typhimurium and S. PT30 against desiccation and heat, and should be considered when developing dry-heat processes for destruction of those pathogens on raw almonds.