Purpose: To quantify the effect of physiological state (planktonic versus biofilm) and prior exposure to desiccation on Salmonella survival after passage through an in-vitrod igestion model.
Methods: Salmonella enterica Tennessee was grown as planktonic cells or as biofilms on glass beads and subsequently dried at room temperature and stored in dried milk powder (aw = 0.3) for up to 30 days. Salmonella survival was quantified by serial dilution onto brilliant green agar before desiccation, after desiccation, after 1-day storage and after 30-day storage. At each sampling both lifestyles were tested for survival through a simulated gastrointestinal system.
Results: The physiological state and length of storage both affected the survival of Salmonella within the desiccated milk powder environment and after passage through an in-vitro digestion system (P < 0.01). Cells in a biofilm state persisted in statistically greater numbers pre-digestion (0.2-2.7 log CFU/g) and after simulated gastric digestion (0.5-4.1 log CFU/ml) compared to planktonic cells (P < 0.01). Recovery of culturable Salmonella was reduced after prolonged exposure to desiccation for 30 days (compared to 1-day desiccation) for biofilm (2.1-log reduction) and planktonic cells (3.3-log reduction). All cells, regardless of prior lifestyle grew within the simulated small intestinal phase, achieving final yields greater than 5 logs CFU/ml. This suggests more cells are alive but in a viable-but-non-culturable state when stressed by prolonged desiccation and low pH.
Significance: The ability to survive long-term storage as well as grow exponentially within the intestinal tract further suggests that industrial processes may influence the virulence of Salmonella, suggesting the need to identify different strategies for controlling Salmonella within dry food processing environments.