Salmonella's Tolerance to Desiccation
Salmonella's Tolerance to Desiccation and Thermal Resistance under Dry Conditions
Center for Food Safety
Salmonellosis infections due to dry foods have been recognized as a serious public health emerging issue. Research was conducted to identify the genetic components of Salmonella's ability to survive dry conditions and to characterize the factors influencing tolerance when subjected to high temperatures.
Low moisture foods do not allow the growth of microorganisms and have traditionally been considered safe from contamination from pathogens such as Salmonella. However, recent outbreaks of salmonellosis linked to low water content foods (e.g., chocolate, dried milk, almonds, peanut butter, peanut products, toasted oats cereal and dried spices) have demonstrated that this bacterium survives dry conditions very well. Furthermore, Salmonella cells subjected to dry conditions can even became more heat tolerant. This response has clear implications for food processing when heating is used to kill foodborne pathogens making Salmonella an important emerging problem for this food sector. The cellular and metabolic components that allow Salmonella to survive dry conditions have yet to be fully elucidated, but modern molecular techniques can be instrumental to advance this field. Dry toasted oats cereal has been implicated in at least two salmonellosis outbreaks, but there is little information on how Salmonella survives in this type of food. Advances that lead to the elimination of this pathogenic bacterium will lead to the prevention of more infections and the reduction of costly recalls for the food industry.
To determine the genetic components involved in Salmonella's cellular response to desiccation, we performed a global transcriptomic analysis of S. enterica serovar Typhimurium cells equilibrated to low water activity (aw 0.11). The analysis revealed that 719 genes were differentially regulated and most of these genes were involved in metabolic pathways, transporter regulation, DNA replication/repair, transcription and translation, and, more importantly, virulence genes. Among these, we decided to focus on the role of sopD and sseD. Deletion mutants were created and their ability to survive desiccation and exposure to aw 0.11 was studied. The mutants exhibited significant cell viability reductions of 2.5 and 1.3 Log (CFU/g), respectively, compared to the natural strain after desiccation for 4 days.
In other experiments, we observed that Salmonella enterica serovars were able to survive on extruded toasted oat cereals for up to 90 days without any significant decrease in viable cell counts. Results on thermal treatment with Salmonella Tennessee outbreak strain (2007 peanut butter) in low aw food toasted oat cereals have shown that a temperature of 95°C must be sustained for at least 10 min to kill 99% of the Salmonella population. When comparing three different Salmonella enterica serovars isolated from three low-moisture food outbreaks, a consistent reduction in the rate of killing was observed as the oat cereals were dried even further (from 0.53 to 0.11 aw).
These findings suggested that two genes that have been previously known to be involved in its pathogenicity are required for Salmonella's survival during desiccation. The elucidation of the genetic components of Salmonella's unique ability to remain alive and infectious in dry foods will be critical to find intervention strategies and prevent further diseases. In addition, the understanding that further reduction in water content increases its resistant to heating will help food processors to design processing steps that will advance the control of food contamination and protection of the consumer.