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This study aimed to elucidate determinants of heat resistance in Escherichia coli by comparing the composition of membrane lipids, as well as gene expression, in heat-resistant E. coli AW1.7 and heat-sensitive E. coli GGG10 with or without heat shock. The survival of E. coli AW1.7 at late exponential phase was 100-fold higher than that of E. coli GGG10 after incubation at 60°C for 15 min. The cytoplasmic membrane of E. coli AW1.7 contained a higher proportion of saturated and cyclopropane fatty acids than that of E. coli GGG10. Microarray hybridization of cDNA libraries obtained from exponentially growing or heat-shocked cultures was performed to compare gene expression in these two strains. Expression of selected genes from different functional groups was quantified by quantitative PCR. DnaK and 30S and 50S ribosomal subunits were overexpressed in E. coli GGG10 relative to E. coli AW1.7 upon heat shock at 50°C, indicating improved ribosome stability. The outer membrane porin NmpC and several transport proteins were overexpressed in exponentially growing E. coli AW1.7. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of membrane properties confirmed that NmpC is present in the outer membrane of E. coli AW1.7 but not in that of E. coli GGG10. Expression of NmpC in E. coli GGG10 increased survival at 60°C 50-to 1,000-fold. In conclusion, the outer membrane porin NmpC contributes to heat resistance in E. coli AW1.7, but the heat resistance of this strain is dependent on additional factors, which likely include the composition of membrane lipids, as well as solute transport proteins.Escherichia coli is a common contaminant of the food supply. The majority of the strains of this species are not pathogenic; however, their relatively high resistance to environmental insults and the occurrence of virotypes with a low infectious dose, particularly enterohemorrhagic E. coli, make E. coli an organism of major concern in the production of minimally processed foods, particularly produce and fresh beef (11).Most strains of E. coli have a D 60 value (the duration of heat treatment at 60°C required to reduce the number of microorganisms to 1/10 of the initial value) of less than 1 min. However, the heat resistance of E. coli is highly variable among different strains and individual strains exhibit D 60 values of up to 6.5 min (8,21,25,37). The heat resistance of individual strains of E. coli relates to their ability adapt to heat stress by the homoviscous adaptation of the plasma membrane, as well as the synthesis of heat shock proteins (20, 43). The 32 -induced expression of heat shock proteins after sublethal thermal stress increases resistance to lethal heat treatment (43; for a review, see reference 6). Increased basal expression of the heat shock proteins DnaK, Lon, and ClpX was linked to the increased heat resistance of E. coli mutants LMM1010, LMM1020, and LMM 1030 (1, 21). The S -mediated general stress response additionally contributes to acid, heat, pressure, and salt resistance in E. coli (2,14,22,3...

Significant food-borne disease outbreaks have occurred from consumption of ready-to-eat foods, including produce, contaminated with Listeria monocytogenes. Challenging food matrices (e.g., cantaloupe, sprouts) with limited processing steps postharvest to reduce pathogen loads have underscored a need for new mitigation strategies. Chlorine dioxide (ClO 2 ) is increasingly being used in produce and other food systems to reduce food-borne pathogen levels. The goal of this study was to characterize the transcriptional response and survival of L. monocytogenes 10403S exposed to ClO 2 . The transcriptional profile of log-phase cells exposed to 300 mg/liter ClO 2 for 15 min was defined by whole-genome microarray. A total of 340 genes were significantly differentially expressed. Among the differentially expressed genes, 223 were upregulated (fold change > 1.5; adjusted P value < 0.05) in role categories responsible for protein fate, cellular processes, and energy metabolism. There were 113 and 16 genes differentially expressed belonging to regulatory networks of B and CtsR, respectively. We assessed L. monocytogenes 10403S survival after exposure to 100, 300, and 500 mg/liter aqueous ClO 2 in brain heart infusion (BHI) broth; there was a significant difference between cells exposed to 500 mg/liter ClO 2 and those exposed to all other conditions over time (P value < 0.05). Isogenic ⌬sigB and ⌬ctsR mutants exposed to 300 mg/liter ClO 2 were more sensitive to ClO 2 than the wild type under the same conditions. These results provide an initial insight into the mechanisms that L. monocytogenes employs to survive sublethal ClO 2 and further our understanding of the inactivation mechanisms of this increasingly used sanitizer.

Developing novel, engaging courses in food safety is necessary to train professionals in this discipline. Courses that are interactive and case-based encourage development of critical thinking skills necessary for identifying and preventing foodborne disease outbreaks. The purpose of this study was to assess the efficacy of a case study module-based course for upper division undergraduate and graduate students interested in food safety. Four independent case study modules were developed and implemented by 4 universities working in tandem over a semester course. All case studies incorporated molecular and epidemiological methods employed by professionals in food safety in outbreak investigations. Each case study was based on a (i) suspect foodborne pathogen, (ii) identification tools including biochemical test variations and pulsed field gel electrophoresis, multiplex PCR and/or whole genome sequencing, and (iii) suspect location and company type from farm to production site to restaurant. Pre-and postinstruction evaluations revealed significant increases in understanding of the concepts introduced through each module as demonstrated by overall mean normalized gain of 0.32 ± 0.35 (15% ± 19%; n = 60). Institution, role of instructor, prescore, and learning environment all played a significant role in the effects of overall learning. This study provides a successful model for a case study-based course in food safety. The guidelines and materials developed by our group are available for use by other institutions.

Maintaining current, relevant curriculum in undergraduate Food Microbiology courses is essential for training future experts in food quality and safety. Having an understanding of the fundamental techniques (for example, polymerase chain reaction [PCR]) that are used in the food industry and regulatory agencies is critical for students entering the workforce. The purpose of this study was to assess the efficacy of integrating molecular methods into an undergraduate Food Microbiology course in both lecture and laboratory settings. Modules on PCR and pulsed-field gel electrophoresis (PFGE), both of which are currently used by government agencies and the food industry to investigate the presence and persistence of foodborne pathogens, were developed, introduced, and evaluated among 269 students over 4 y. Multiple teaching and learning styles were incorporated through (i) traditional lecture format on the basics of PCR and PFGE; (ii) hands-on group activities to build upon the lecture instruction; (iii) performing PCR and PFGE in the laboratory; and (iv) group discussions to analyze results from laboratory exercises. Pre-and postinstruction evaluations revealed significant increases in understanding and application of both methods in lecture and laboratory settings as demonstrated by 0.60 and 0.51 mean normalized gains for respective PCR and PFGE lectures and 0.50 and 0.56 mean normalized gains in respective labs. Academic year significantly impacted score improvement, potentially due to hidden factors, such as previous exposure to material and student aptitude. This study provides the platform for successful introduction of molecular techniques in an undergraduate Food Microbiology course. The guidelines and materials developed by our group are publicly available for use by other institutions.