Survival and Thermal resistance of Desiccated Salmonella

Some Enterobacteriaceae develop a mechanism to survive in environments containing low moisture. Salmonella is the most frequent cause of enteric infections. Salmonella enteric causes “high morbidity and economic losses worldwide” (Gruzdev, 2011). The intent of this research is to investigate the survival and thermal resistance of desiccated Salmonella in low moisture foods and dry processing environments, such as peanut butter, using microbiological and molecular approaches to identify extrinsic and intrinsic factors that regulate Salmonella resistance and survival under long-term starvation and desiccation stresses. 

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Food born illnesses are normally associated with the ingestion of contaminated foods; specifically, wet foods such as milk. Non-typhoidal human salmonellosis is identified by an “onset of fever, abdominal pain, nausea, diarrhea, and sometimes vomiting” (Álvarez-Ordóñez, 2011). The disease typically does not last more than a few days. Although in some instances, it may last for a longer period especially in “immunocompromised individuals, pregnant women, the elderly and children” (Álvarez-Ordóñez, 2011). A decade ago, salmonellosis was second on the list of reported zoonotic diseases in humans. There were 108,614 confirmed cases in Europe; Salmonella enterica serovar Enteritidis and S. enterica serovar Typhimurium were the two most common (Álvarez-Ordóñez, 2011). The most common host of Salmonella is the intestinal track of wild and domestic animals which leads to food as the driving force of infection. Human S. Enteritidis cases are normally associated with the ingestion of contaminated egg, poultry meat and raw milk or products (Álvarez-Ordóñez, 2011).
The reduction of moisture in foods has been a process used in foods to control the growth of bacteria in food production (Gruzdev, 2011). However, there have been outbreaks of salmonellosis from the intake of dried foods. This indicates that Salmonella can live in harsh environments in the absence of moisture for extended periods of time (Gruzdev, 2011). Desiccated salmonella possesses a high tolerance to heat and ethanol. In a study conducted by Nadia Gruzdev et al., dried Salmonella enterica serotype Typhimurium cells were introduced to various stresses to examine if dehydration contributes to tolerance to other stressors. Results indicate that desiccated S.Typhimurium possesses a greater tolerance to various stressors than non-desiccated cells. These cells were more resistant to stressors including ethanol, sodium hypochlorite, didecyl dimethyl ammonium chloride, hydrogen peroxide, NaCl, bile salts, dry heat, and UV radiation (Gruzdev, 2011). In the presence of acetic acid and citric acid, the rate of survival reduced in the desiccated calls in comparison the non-desiccated cells. There are three other serotypes in S. enterica that demonstrated stress responses such like S.Typhimurium; S. Newport, S. Infantis, and S. Enteritidis (Gruzdev, 2011). In contrast, S. Hadar was more vulnerable and could only tolerate a small number of stressors. This concluded that dehydration causes cross-tolerance to several stresses in S. enterica, indicating the restriction of treatments used by the food industry to deactivate food-borne pathogens (Gruzdev, 2011).  
The first outbreak of salmonellosis related to the ingestion of peanut butter was reported in 1996. A study by Burnett et al. was conducted to determine characteristics of survival of high and low inoculation of a five-serotype blend of Salmonella in five commercial peanut butters and 2 commercial peanut butter spreads. A similar study conducted by Park et al. was performed on peanut butter in Tennessee. Park et al. used an inoculation of a 3-strain blend of 5 commercial brands of peanut butter. Both studies found that the products initially inoculated with Salmonella were found positive for the pathogen after storing the product at 4-5°C for 24 weeks; except for natural peanut butter. However, products stored at 21-22°C did not show traces of Salmonella after storage following 24 weeks (Burnett, 2000). Results of this study indicate that Salmonella survives in peanut butter and peanut butter spread for a minimum of 24 weeks at 5°C and potentially 21-22°C; or for the duration of their shelf life (Park, 2008). Salmonella in the products can be caused by post-process contamination while repackaging or use as an ingredient in other food product which are not protected against contamination from the pathogen. This can lead to its existence at the time of ingestion (Burnett, 2000).  
Before 1970s, eggs that were not handled in a sterile fashion often corresponded to food borne illnesses (Jones, 1995). It was not until the passage of the Egg Product Sanitation Act in 1970 that contaminated egg production decreased (Jones, 1995). A study conducted by Jones et al. was done is to examine which egg, egg contents, and egg production facilities are contaminated with Salmonella. Egg samples were taken from different stages of an egg processing operation as well as from the production facility. Salmonella was recovered from ventilation fan, egg belt and flush water. Salmonella was detected on 7 put of the 90 eggshells sampled prior to processing and 1 out of 90 eggshells sampled following processing. There was only 1 eggshell found positive for Salmonella which was likely due to a low pH of the water sample. However, Salmonella was not found in the 180 eggs examined for internal contamination after processing. The 60 isolates from the production facilities included Salmonella  serotypes: S. agona, S.typhimurium, S. infantis, S.derby, S. heidelberg, S. california, S. montevideo, S. mbandaka, and untypable (Jones, 1995). This report suggests that Salmonella is a common inhabitant of egg production facilities (Jones, 1995). In order to determine the true percentage of eggs infected with Salmonella, more eggs should be examined. However, based on the data, only a small percentage of eggs contain liquid contents contaminated with  Salmonella. 
In a study to test non-thermal inactivation of Salmonella enteritidis in a food model system supplemented with a natural antimicrobial, taramasalad, a popular Greek appetizer, was used. It was first inoculated with S. enteritidis in addition to different concentrations of oregano essential oil and stored at different temperatures (5, 10, 15, 20°C). Lemon juice was added to adjust the pH from 4.3 to 5.3. In order to estimate the kinetic parameters of the pathogen, bacterial counts were “modeled as a function of time (Koutsoumanis, 1999).” Two models were used for comparison. Depletion of Salmonella was evident in all cases and pH, storage temperature, and oil concentration were the determinants for the death rate (Koutsoumanis, 1999). The three factors were used to predict the death of S. enteritidis in homemade taramasalad.   
Products that are have low-water activity such as peanut butter, infant formula, chocolate, cereal products, and dried milk do not typically support the development of vegetative pathogens such as Salmonella. (Podolak, 2010) Instead, contamination of Salmonella in these products are from “poor sanitation practices, poor equipment design and poor ingredient control” (Podolak, 2010). Reduction of Salmonella is affected by factors such as storage temperature and formulation of the product (Podolak, 2010). Heat resistant Salmonella is dependent on the serotypes tested, growth and storage conditions prior to testing, media used to test, media used for the recovery of heat-damaged cells and chemical and physical composition. Reduction of moisture results in the increase of Salmonella resistance to heat (Podolak, 2010). 
A study conducted by Jung et al. was performed to determine if a four-strain blend of multidrug-resistant Salmonella typhimurium definitive type 104 cells and a four-strain blend of S. typhimurium non-DT104 cells varied in survival mechanism in whole egg powder. Corn syrup solids and salt, egg yolk powder was also added to the egg powder. Items were stored in 13 or 37°C for 8 weeks. Results indicate that there was no significant difference in the rates of inactivation of DT104 and non-DT104 cells. Both DT104 and non-DT104 cell types were found in egg white powder containing 4.9% moisture while in 54°C for one week. Heating at 82°C did not eradicate S. typhimurium in white powder (Jung, 1999). 
A research conducted by Finn et al. was done to examine the phenotypic characterization of Salmonella isolated from food production environments associated with low-water activity food. An examination of Salmonella growth in 18 serotypes in NaCl, KCl, and glycerol determined that “glycerol was the least inhibitory” amongst the three humectants (Finn, 2013). This proved that there is a link between the ability to subsist in KCl and biofilm formation. This information is significant especially for food safety as well as the safety of the public (Finn, 2013). 
To study the environmental conditions that affect Salmonella survival after dehydration and cold storage, Gruzdev et al. used a 96-well polystyrene plate model. It was determined that the SL 1344 strain displayed a higher level of survival in comparison with the other Typhimurium isolates and S. enterica serotypes (Gruzdev, 2012). Desiccation of this strain also suggested that “temperature, stationary-phase of the growth, solid medium, and the presence of increasing NaCl concentrations in growth medium enhanced desiccation tolerance” (Gruzdev, 2012).  Desiccated Salmonella was able to survive over 100 weeks at 4°C. When a viability staining was performed, it revealed a “50% reduction in viable cells” (Gruzdev, 2012). These findings suggest that bacteria transitions into a “viable-but-not-cultivable state (VBNC)” (Gruzdev, 2012). When chloramphenicol is added, it reduces survival in bacteria which may suggest that “adaptation to desiccation stress required de-novo protein synthesis” (Gruzdev, 2012).  This means that a shorter dehydration time equals a lower survival. This study focuses on how desiccated Salmonella in the food industry is impacted by the conditions in the environment.
Foods preserved in dry environments typically have a longer shelf life and have the ability to last for years. Foods that are low in moisture are considered to have a reduced “water activity” (Finn, 2013). The accessibility of moisture for biological reactions can be lowered by taking proper precautions such as freeing, spray drying, or by adding solutes such as sugar and salt (Finn, 2013). Reducing the aw is a great preserving method however, microorganisms can still survive this phase (Finn, 2013). Spores and vegetative cells can survive for months to years in dried foods. Once a dry food production environment is contaminated by a microorganism, it is difficult to remove the bacterium (Finn, 2013). An example of a method used in high moisture foods to reduce microorganisms but cannot be used in low moisture foods is mild heat treatment and high pressure (Finn, 2013). The same methods used for high moisture foods should be different from the methods used for low moisture foods. The contamination of Salmonella in a dry food production can spread in the presence of water. This allows the growth and spread of the organism causing contamination. It is advised that in processing environments, the use of wet cleaning should be limited because it can spread contamination unless it is necessary (Finn, 2013).
Works Cited
Álvarez-Ordóñez, A., Begley, M., Prieto, M., Messens, W., López, M., Bernardo, A., & Hill, C. (2011). Salmonella spp. survival strategies within the host gastrointestinal tract. Microbiology, 157(12), 3268–3281. doi: 10.1099/mic.0.050351-0
Burnett, S., Gehm, E., Weissinger, W., & Beuchat, L. (2000). Survival of Salmonella in peanut butter and peanut butter spread. Journal of Applied Microbiology, 89(3), 472–477. doi: 10.1046/j.1365-2672.2000.01138.x 
Finn, S., Condell, O., McClure, P., Amézquita, A., & Fanning, S. (2013). Mechanisms of survival, responses and sources of Salmonella in low-moisture environments. Frontiers in microbiology, 4, 331. doi:10.3389/fmicb.2013.00331 
Finn, S., Hinton, J. C. D., Mcclure, P., Amézquita, A., Martins, M., & Fanning, S. (2013). Phenotypic Characterization of Salmonella Isolated from Food Production Environments Associated with Low–Water Activity Foods. Journal of Food Protection, 76(9), 1488–1499. doi: 10.4315/0362-028x.jfp-13-088 
Gruzdev, N., Pinto, R., & Sela, S. (2011). Effect of Desiccation on Tolerance of Salmonella entericato Multiple Stresses. Applied and Environmental Microbiology, 77(5), 1667–1673. doi: 10.1128/aem.02156-10.  
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Jones, F. T., Rives, D. V., & Carey, J. B. (1995). Salmonella Contamination in Commercial Eggs and an Egg Production Facility. Poultry Science, 74(4), 753–757. doi: 10.3382/ps.0740753  
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Park, E.-J., Oh, S.-W., & Kang, D.-H. (2008). Fate of Salmonella Tennessee in Peanut Butter at 4 and 22 °C. Journal of Food Science, 73(2). doi: 10.1111/j.1750-3841.2007.00638.x 
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