The Long-term Survival of Salmonella Cells Adhered to Stainless Steel under Various Environmental Conditions and Their Resistance to Disinfectants

Introduction: Low a_w foods (< 0.85 a_w) do not support pathogen growth, but have been associated with numerous outbreaks of Salmonella. Increasing concern over the safety of dry products is encouraging the industry to seek better manufacturing practices, primarily to prevent cross-contamination from the factory environment.

Purpose: The aim of this study was to investigate the persistence of Salmonella strains attached to surfaces under various environmental conditions, and their resistance to disinfectants.

Methods: Fifteen Salmonella isolates were dried onto stainless steel surfaces, placed in controlled conditions and their viability assessed at times from 1 hour to 30 days. Strains showing high and moderate survival were reassessed using various parameters: temperatures (37°C, 25°C and 15°C); relative humidity (33% and 53%); presence of food debris (infant formula, skimmed milk, milk chocolate and cocoa powders); inoculum levels (10⁴, 10⁵ and 10⁷CFU/surface); and periodic wetting (every 7 days). The strains attached to discs for 30 days were subjected to surface disinfection using sodium hypochlorite and propan-2-ol, following a modified protocol of the EU surface disinfectant test.

Results: The highest survival rate was associated with strains of S. Muenchen, S. Enteritidis, S. Typhimurium DT104 and S. Agona, showing, after 30 days, a reduction in viability of 1.5 to 2 log CFU/surface. Prolonged survival was not serotype or time related. Presence of food debris (0.0-1.0 log CFU/surface reduction) and exposure to water (<0.2 log CFU/surface reduction) favored the survival of all isolates over 30 days. Strains survived better at lower temperatures (1.0-1.4 log CFU/surface reduction) and lower humidity (1.5-2.1 log CFU/surface reduction). 30 day adhered strains were not resistant to disinfectants (500 ppm of sodium hypochlorite and 40 % of propan-2-ol), achieving the required standard 4 log reduction.

Significance: The results provide an insight into Salmonella survival in dry processing environments, can help to choose appropriate methods for their control and create a basis for current studies on physiological and molecular mechanisms of Salmonella survival.