Progress 02/01/10 to 01/31/13
Outputs
OUTPUTS: We conducted thermal challenge studies using various heating temperatures (e.g. 71oC, 90oC and 126oC) to evaluate the survival and death rates and relative heat resistance of different Salmonella cocktails and individual Salmonella serotypes and strains (i.e. Typhimurium, Enteritidis, Tennessee) before and after the exposure to desiccation and rehydration conditions in peanut butter. Using statistical models, we evaluated the influences of different rehydration time, water activities, heating temperatures and peanut butter formulations on the heat resistance of different Salmonella strains and cocktails. We conducted deep RNA sequencing analysis and explored the molecular mechanisms underlying Salmonella thermal resistance in low water activity food. Genes that potentially mediate Salmonella survival under the desiccation stress were selected for deletion mutagenesis in a fully sequenced Enteritidis strain P125109. We sequenced the complete genome of Salmonella enterica serotype Tennessee strain CDC07-0191 implicated in the 2006-2007 multi-state foodborne outbreak linked to peanut butter in the United States. Using scanning electron microscopy, we observed the morphological alterations of different Salmonella strains during the desiccation and rehydration processes. Six graduate students were trained and participated in this project. Current findings have been disseminated to the key stakeholders and general scientific community through peer-reviewed publications and presentations at the IAFP annual meeting and the USDA NIFA project directors meeting. PARTICIPANTS: PI: Wei Zhang; Co-PI: Mary Lou Tortorello, Haiping Li; Graduate students: Xiangyu Deng, Yingshu He, Zengxin Li, Joelle Salazar, Dongjing Guo, Ye Li. TARGET AUDIENCES: Academic and governmental researchers and food industry. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
Salmonella enterica is a frequent food contaminant and the leading cause of foodborne bacterial illnesses in the United States. We demonstrated that S. enterica displayed increased heat resistance in peanut butter of low water activity (aw). We compared the relative heat resistance of three individual strains of S. enterica representing serotypes Typhimurium, Enteritidis and Tennessee and the 3-strain cocktail treated at both 90 oC and 126 oC in two different peanut butter formulations with varied fat and carbohydrate contents and adjusted water activities (aw from 0.2 to 0.8). We identified effective rehydration time, water activity, and heating temperatures that were adequate to inactivate desiccated, heat-resistant Salmonella strains in peanut butter. Using scanning electron microscopy, we observed morphological responses of different S. enterica strains during desiccation and rehydration processes. Using RNA-seq technology, we explored the transcriptomic profiles of S. Enteritidis under low moisture stress. The RNA-seq results suggested that S. enterica cells under low aw stress are likely present in a metabolically dormant state, expressing only about 6% of the genome. Genes involved in heat and cold shock response, DNA protection, and regulatory functions likely played roles in protecting S. enterica from low aw stress, starvation, and possibly other environmental stresses such as heat. We analyzed the complete genome sequence of a heat resistant Salmonella Tennessee strain which caused a major foodborne outbreak linked to peanut butter in the U.S. Results from this project clearly demonstrated that water activity plays a critical role in determining S. enterica heat resistance in peanut butter. In addition, we reported the variability that existed among the heat resistance of different S. enterica serotypes in different peanut butter formulations. Results from this project shed new light on the physiology of Salmonella under desiccation stress and specific molecular mechanisms of its survival and persistence in low water activity foods. Data generated from the thermal challenge studies may prove useful for developing risk assessment plans and validating effective intervention and mitigation strategies to reduce the risks of Salmonella contamination in peanut butter production.
Publications
- Deng X, Salazar JK, Frezet S, MacCannell D, Ribot EM, Fields PI, Fricke WF, Zhang W. (2013) Genome sequence of Salmonella enterica serotype Tennessee strain CDC07-0191 implicated in the 2006-2007 multi-state foodborne outbreak linked to peanut butter in the United States. Genome Announc. submitted.
- He Y, Li Y, Salazar JK, Yang J, Tortorello ML, Zhang W. (2013) Increased water activities reduces thermal resistance of Salmonella enterica in peanut butter. Appl Environ Microbiol. submitted.
- Deng X, Li Z, Zhang W. (2012) Transcriptome sequencing of Salmonella enterica Enteritidis under desiccation and starvation stress in peanut oil. Food Microbiol, 30, 311-315.
- He Y, Yang J, Guo D, Tortorello ML, Zhang W. (2011) Survival and thermal resistance of Salmonella enterica and Escherichia coli O157:H7 in peanut butters. Appl Environ Microbiol, 77, 8434-8438.
- He Y, Ye L, Tortorello ML, Zhang W. (2013) Increased water activities reduce the thermal resistance of Salmonella enterica in peanut butter. International Association for Food Protection Annual Meeting, Charlotte, North Carolina.
- Zhang W. (2011) Salmonella thermal resistance during desiccation and rehydration in low water activity foods. USDA NIFA Project Directors Meeting. Milwaukee, Wisconsin.
- Deng X, Li Z, Zhang W. (2011) Transcriptomic analysis of Salmonella under desiccation stress in peanut oil. International Association for Food Protection Annual Meeting. Milwaukee, Wisconsin.
- He Y, Guo D, Zhang W. (2011) Salmonella survival and thermal resistance in peanut butters. International Association for Food Protection Annual Meeting. Milwaukee, Wisconsin.
|
Progress 02/01/11 to 01/31/12
Outputs
OUTPUTS: We conducted thermal challenge studies using various heating temperatures to evaluate the survival and death rates and relative heat resistance of five-strain Salmonella cocktails and individual Salmonella serotypes (i.e. Typhimurium, Enteritidis, Tennessee) before and after the exposure to desiccation and rehydration conditions in peanut butter. Using statistical models, we predicted combinations of rehydration time, water activities, and heating temperatures that would allow >2 log reductions of heat-resistant Salmonella strains in different commercial peanut butter formulations. We conducted deep RNA sequencing analysis and explored the molecular mechanisms underlying Salmonella thermal resistance in low water activity food such as peanut oil. Genes that potentially mediate Salmonella survival under the desiccation stress were selected for deletion mutagenesis in a fully sequenced Enteritidis strain P125109. Additional experiments are underway to sequence the full genome of a representative heat resistant strain and to evaluate the correlation(s) between the observed Salmonella morphological alterations under low water stress and the potential impact on Salmonella virulence and pathogenicity. Four graduate students and one undergraduate student were trained and participated in this project. Current findings have been disseminated to the key stakeholders and general scientific community through peer-reviewed publications and presentations at the ASM annual meeting, the IAFP annual meeting, and the USDA NIFA project directors meeting. PARTICIPANTS: PI: Wei Zhang; Co-PI: Mary Lou Tortorello, Haiping Li; Graduate students: Xiangyu Deng, Yingshu He, Zengxin Li, Joelle Salazar, Christina Megalis. TARGET AUDIENCES: Academic and governmental researchers and food industry. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
Salmonella has been implicated in numerous foodborne disease outbreaks associated with low water activity foods, including peanut butters. Using deep RNA sequencing, we reported the specific transcriptomic response of S. Enteritidis to low water activity using peanut oil as a food model. Our results suggested that low water "stressed" Salmonella cells in peanut oil transit to a metabolically dormant state, which is distinct from the generally-assumed "viable but non-culturable" (VBNC) state or the rdar morphotype. Minor morphological alterations were observed in "stressed" Salmonella cells, including the loss of motility and increased reductive division and cell aggregation. Results from our ongoing thermal challenge studies indicated that Salmonella heat resistance varies significantly among different serovars and is affected by peanut butter formulations, storage time, heating methods, and final water activities in the finished products. Collectively, results from this project will improve our fundamental understanding of the Salmonella stress response and heat resistance in low water activity foods, in particular, in peanut butter. Such knowledge may lead to developing a framework of more effective processing and intervention strategies to reduce Salmonella contamination in low water activity foods.
Publications
- Xiangyu Deng, Zengxin Li, Wei Zhang (2012) Transcriptome sequencing of Salmonella enterica Enteritidis under desiccation and starvation stress in peanut oil. Food Microbiology. 30:311-315.
- Yingshu He, Dongjing Guo, Jingyun Yang, Mary Lou Tortorello, Wei Zhang (2011) Survival and heat resistance of Salmonella enterica and Escherichia coli O157:H7 in peanut butter. Applied and Environmental Microbiology. 77:8434-8438.
|
Progress 02/01/10 to 01/31/11
Outputs
OUTPUTS: In the first year of this project, we compared the population dynamics, survival rates and thermal resistance profiles for five Salmonella enterica strains and five Escherichia coli O157:H7 strains in four different commercial peanut butter products with variable fat, sodium, proteins, and carbohydrate contents. Bacterial survival rates were monitored over 30 days of incubation at 4oC or 25oC in peanut butters. Immediately after the 30-day incubation, the thermal resistance of pre-desiccated bacterial cells was evaluated by heating the inoculated peanut butter samples at 72oC or 90oC with 10 min increments for up to an hour. To further explore the molecular mechanisms underlying Salmonella response to desiccation and cross-resistance to other stresses such as heat, we used the high-throughput RNA sequencing approach to compare the transcriptomes of a Salmonella Enteritidis strain after inoculation in peanut oil (water activity 0.30) for 72hr, 216hr and 528hr to those grown in LB broth for 12hr and 312hr. We generated 27 to 33 million 36-bp reads per sample, and total over 5 Gb of sequencing data. Findings from this study are disseminated through scientific publications, reports to USDA NIFA, presentations at professional meetings, and through the website and newsletters of the National Center for Food Safety and Technology. PARTICIPANTS: Nothing significant to report during this reporting period. TARGET AUDIENCES: Nothing significant to report during this reporting period. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
Salmonella can survive long-term starvation and desiccation stresses and contaminate foods that have intermediate to extremely low water activities. Salmonella has been implicated in numerous foodborne disease outbreaks associated with dried and low water activity foods such as peanut butters. However, little is known about the physiology of Salmonella under desiccation stress and specific molecular mechanisms of its survival and persistence in low water activity foods. Our studies showed that S. enterica survived significantly better than E. coli O157:H7 (p<0.05) in peanut butters when aw was lower than 0.4. High fat and carbohydrate contents in peanut butters and low storage temperatures significantly increased the thermal resistance of both S. enterica and E. coli O157:H7. Thermal treatment at 72 oC for an hour only achieved 1 to 2 log reduction of pre-desiccated bacterial cells in peanut butters, compared to 4 to 7 log reduction at 90 oC. Our research so far provided some important data on the comparative analysis of the survival rates and heat resistance of S. enterica and E. coli O157:H7 in different commercial peanut butters, which may prove useful for designing new validation studies. We also first reported the use of RNA seq technology in characterizing the bacterial transcriptomes in real food matrices. Our RNA seq results suggested that desiccated Salmonella cells in peanut oil are likely present in a physiologically dormant state by expressing only 6% of its genome compared to 78% in LB broth. Among the few detected transcripts under such status, genes involved in heat and cold shock response, DNA protection and regulatory functions likely play roles in cross protecting Salmonella from desiccation, thermal and other stresses. In addition, non-coding RNAs may also play important roles in Salmonella desiccation/starvation stress response. We demonstrated the advantages of RNA seq in analyzing bacterial RNA samples directly extracted from food items, where detection sensitivity is critical for the identification of important transcripts at low expression levels. Knowledge gained from this part of study improved our basic understanding of how Salmonella manages to survive in low water activity foods, which may lead to developing a framework of more effective processing and intervention to reduce Salmonella contamination in such foods.
Publications
- Two manuscripts are prepared for publication in Applied and Environmental Microbiology. Two oral/poster presentations are invited for the International Association for Food Protection annual meeting in 2011:
- 1. Deng X, Li Z, Zhang W. 2011. Transcriptomic analysis of Salmonella under desiccation stress in peanut oil. International Association for Food Protection Annual Meeting. Milwaukee, Wisconsin.
- 2. He Y, Guo D, Zhang W. 2011. Salmonella survival and thermal resistance in peanut butters. International Association for Food Protection Annual Meeting. Milwaukee, Wisconsin.
|
|