Gene expression induced in Escherichia coli O157:H7 upon exposure to model apple juice

Mechanisms of Pathogenic Escherichia coli Attachment to Meat

Escherichia coli are present in the human and animal microbiome as facultative anaerobes and are viewed as an integral part of the whole gastrointestinal environment. In certain circumstances, some species can also become opportunistic pathogens responsible for severe infections in humans. These infections are caused by the enterotoxinogenic E. coli, enteroinvasive E. coli, enteropathogenic E. coli and the enterohemorrhagic E. coli species, frequently present in food products and on food matrices. Severe human infections can be caused by consumption of meat contaminated upon exposure to animal feces, and as such, farm animals are considered to be a natural reservoir. The mechanisms by which these four major species of E. coli adhere and persist in meat postslaughter are of major interest to public health and food processors given their frequent involvement in foodborne outbreaks. This review aims to structure and provide an update on the mechanistic roles of environmental factors, curli, type I and type IV pili on E. coli adherence/interaction with meat postslaughter. Furthermore, we emphasize on the importance of bacterial surface structures, which can be used in designing interventions to enhance food safety and protect public health by reducing the burden of foodborne illnesses.

Atypical diarrhoeagenic Escherichia coli in milk related to a large foodborne outbreak

A foodborne outbreak related to milk cartons served in school lunches occurred in June 2021, which involved more than 1,800 cases from 25 schools. The major symptoms were abdominal pain, diarrhoea, vomiting, and fever. Although major foodborne toxins and pathogens were not detected, a specific Escherichia coli strain, serotype OUT (OgGp9):H18, was predominantly isolated from milk samples related to the outbreak and most patients tested. The strains from milk and patient stool samples were identified as the same clone by core genome multilocus sequence typing and single-nucleotide polymorphism analysis. The strain was detected in milk samples served for two days related to the foodborne outbreak at a rate of 69.6% and levels of less than ten most probable number/100 mL but not on days unrelated to the outbreak. The acid tolerance of the strain for survival in the stomach was similar to that of enterohaemorrhagic E. coli O157:H7, and the same inserts in the chu gene cluster in the acid fitness island were genetically revealed. The pathogenicity of the strain was not clear; however, it was indicated that the causative pathogen was atypical diarrhoeagenic E. coli OUT (OgGp9):H18.

Comparative Effects of Carum copticum Essential Oil on Bacterial Growth and Shiga-Toxin Gene Expression of Escherichia coli O157:H7 at Abused Refrigerated Temperatures

Abused refrigerated temperatures are described as unacceptable deviations from the optimal temperature, occurring frequently during transportation of food products. Escherichia coli O157:H7 is a serious contaminant of meats and meat products due to its ability to grow at abused temperatures (> 10 °C). The aim of this study was to evaluate the antibacterial activity of Carum copticum essential oil for the control of Escherichia coli O157:H7 using laboratory media and minced beef at severe abused refrigerated temperature (15 °C). A comparative quantitative reverse transcription real-time PCR was used to assess effects of temperature and Carum copticum essential oil at sub-minimum inhibitory concentrations on bacterial growth and Shiga-toxin gene (stx1A and stx2A) expression. Results indicated that Carum copticum essential oil inhibited growth of E. coli O157:H7 in tryptone soy broth (TSB) media at all sub-MIC values until Hour 48. However, bacterial population increased progressively until Hour 72 at essential oil concentration of 0.75% (ml g^-1) and reached 8.6 log CFU g^-1 in minced beef. The essential oil at concentration of 0.005% (ml g^-1) increased stx gene expression at all times, but increased stx gene expression (0.015%) at Hour 24 in TSB media. The expression rate of stx1A in minced beef decreased progressively (10.39 and 7.67 folds for 0.5 and 0.75%, respectively) and expression of stx2A was variable in minced beef during storage. In conclusion, results from this study have shown that effects of Carum copticum essential oil on growth and virulence gene expression are not necessarily correlated and temperature, essential oil concentration, investigated gene type, and bacterial growth environment (in vivo or in vitro) are effective as well.

Everybody loves cheese: crosslink between persistence and virulence of Shiga-toxin Escherichia coli

General cheese manufacturing involves high temperatures, fermentation and ripening steps that function as hurdles to microbial growth. On the other hand, the application of several different formulations and manufacturing techniques may create a bacterial protective environment. In cheese, the persistent behavior of Shiga toxin-producing Escherichia coli (STEC) relies on complex mechanisms that enable bacteria to respond to stressful conditions found in cheese matrix. In this review, we discuss how STEC manages to survive to high and low temperatures, hyperosmotic conditions, exposure to weak organic acids, and pH decreasing related to cheese manufacturing, the cheese matrix itself and storage. Moreover, we discuss how these stress responses interact with each other by enhancing adaptation and consequently, the persistence of STEC in cheese. Further, we show how virulence genes eae and tir are affected by stress response mechanisms, increasing either cell adherence or virulence factors production, which leads to a selection of more resistant and virulent pathogens in the cheese industry, leading to a public health issue.

Comparative Effects of Carum copticum Essential Oil on Bacterial Growth and Shiga-Toxin Gene Expression of Escherichia coli O157:H7 at Abused Refrigerated Temperatures

Abused refrigerated temperatures are described as unacceptable deviations from the optimal temperature, occurring frequently during transportation of food products. Escherichia coli O157:H7 is a serious contaminant of meats and meat products due to its ability to grow at abused temperatures (> 10 °C). The aim of this study was to evaluate the antibacterial activity of Carum copticum essential oil for the control of Escherichia coli O157:H7 using laboratory media and minced beef at severe abused refrigerated temperature (15 °C). A comparative quantitative reverse transcription real-time PCR was used to assess effects of temperature and Carum copticum essential oil at sub-minimum inhibitory concentrations on bacterial growth and Shiga-toxin gene (stx1A and stx2A) expression. Results indicated that Carum copticum essential oil inhibited growth of E. coli O157:H7 in tryptone soy broth (TSB) media at all sub-MIC values until Hour 48. However, bacterial population increased progressively until Hour 72 at essential oil concentration of 0.75% (ml g^-1) and reached 8.6 log CFU g^-1 in minced beef. The essential oil at concentration of 0.005% (ml g^-1) increased stx gene expression at all times, but increased stx gene expression (0.015%) at Hour 24 in TSB media. The expression rate of stx1A in minced beef decreased progressively (10.39 and 7.67 folds for 0.5 and 0.75%, respectively) and expression of stx2A was variable in minced beef during storage. In conclusion, results from this study have shown that effects of Carum copticum essential oil on growth and virulence gene expression are not necessarily correlated and temperature, essential oil concentration, investigated gene type, and bacterial growth environment (in vivo or in vitro) are effective as well.

Inactivation of Escherichia coli O157:H7 in apple cider by resveratrol and naringenin

The study investigated the efficacy of two GRAS-status phytochemicals, mega-resveratrol (RV) and naringenin (NG) to inactivate Escherichia coli O157:H7 (EHEC) in apple cider. A five-strain mixture of EHEC (∼7 log CFU/ml) was inoculated into cider, followed by the addition of RV (8.7 mM and 13.0 mM) or NG (7.3 mM and 11.0 mM). The cider samples were stored at 4 °C for 14 days and EHEC was enumerated on days 0,1,5,7 and 14. The deleterious effects of RV and NG on EHEC cells were visualized by scanning electron microscopy (SEM), and RT-qPCR was done to determine the effect of phytochemicals on three known acid resistance (AR) systems of EHEC. NG was more effective than RV and reduced EHEC counts by ∼4.5 log CFU/ml by day 14, whereas RV reduced counts by ∼2.5 log CFU/ml compared to controls (P < 0.05). SEM showed that RV and NG resulted in the destruction of EHEC cells, and surviving bacteria appeared 'lemon shaped'. RT-qPCR results revealed that RV and NG downregulated the transcription of AR associated genes in EHEC (P < 0.05). Results suggest the potential use of RV and NG as natural antimicrobial additives to enhance the microbiological safety of apple cider. However, sensory analysis studies are warranted.

Transcriptome sequencing reveals genes and adaptation pathways in Salmonella Typhimurium inoculated in four low water activity foods

Salmonella enterica serotypes have been reported as the agent of various outbreaks occurred after the consumption of low water activity (a_w) foods. When the pathogen encounters harsh conditions, several regulatory networks are activated through dynamic differential gene expression that lead to cell survival for prolonged periods. In this work, the transcriptome of S. enterica serovar Typhimurium using RNA-Seq, after cells' inoculation in four distinct types of low a_w foods (milk chocolate, powdered milk, black pepper, and dried pet food), following storage at 25 °C per 24 and 72 h was studied. The findings of this study suggest that gene regulation is influenced by the food composition mainly in the first 24 h post-inoculum, proceeded by the induction of similar genes shared among all samples. It was possible to evaluate the differences on each type of food matrix regarding the bacteria adaptation, as well as the similarities provoked by low a_w. The results reveal genes that may play key roles in response to desiccation in Salmonella, as well as the pathways in which they are involved.

Whole-Transcriptome Analysis of Verocytotoxigenic Escherichia coli O157:H7 (Sakai) Suggests Plant-Species-Specific Metabolic Responses on Exposure to Spinach and Lettuce Extracts

Verocytotoxigenic Escherichia coli (VTEC) can contaminate crop plants, potentially using them as secondary hosts, which can lead to food-borne infection. Currently, little is known about the influence of the specific plant species on the success of bacterial colonization. As such, we compared the ability of the VTEC strain, E. coli O157:H7 'Sakai,' to colonize the roots and leaves of four leafy vegetables: spinach (Spinacia oleracea), lettuce (Lactuca sativa), vining green pea (Pisum sativum), and prickly lettuce (Lactuca serriola), a wild relative of domesticated lettuce. Also, to determine the drivers of the initial response on interaction with plant tissue, the whole transcriptome of E. coli O157:H7 Sakai was analyzed following exposure to plant extracts of varying complexity (spinach leaf lysates or root exudates, and leaf cell wall polysaccharides from spinach or lettuce). Plant extracts were used to reduce heterogeneity inherent in plant-microbe interactions and remove the effect of plant immunity. This dual approach provided information on the initial adaptive response of E. coli O157:H7 Sakai to the plant environment together with the influence of the living plant during bacterial establishment and colonization. Results showed that both the plant tissue type and the plant species strongly influence the short-term (1 h) transcriptional response to extracts as well as longer-term (10 days) plant colonization or persistence. We show that propagation temperature (37 vs. 18°C) has a major impact on the expression profile and therefore pre-adaptation of bacteria to a plant-relevant temperature is necessary to avoid misleading temperature-dependent wholescale gene-expression changes in response to plant material. For each of the plant extracts tested, the largest group of (annotated) differentially regulated genes were associated with metabolism. However, large-scale differences in the metabolic and biosynthetic pathways between treatment types indicate specificity in substrate utilization. Induction of stress-response genes reflected the apparent physiological status of the bacterial genes in each extract, as a result of glutamate-dependent acid resistance, nutrient stress, or translational stalling. A large proportion of differentially regulated genes are uncharacterized (annotated as hypothetical), which could indicate yet to be described functional roles associated with plant interaction for E. coli O157:H7 Sakai.

Application of a Nonlinear Model to Transcript Levels of Upregulated Stress Response Gene ibpA in Stationary-Phase Salmonella enterica Subjected to Sublethal Heat Stress

Sublethal heating, which can occur during slow cooking of meat products, is known to induce increased thermal resistance in Salmonella. However, very few studies have addressed the kinetics of this response. Although several recent studies have reported improved thermal inactivation models that include the effect of prior sublethal history on subsequent thermal resistance, none of these models were based on cellular-level responses to sublethal thermal stress. The goal of this study was to determine whether a nonlinear model could accurately portray the response of Salmonella to heat stress induced by prolonged exposure to sublethal temperatures. To accomplish this, stationary-phase Salmonella Montevideo cultures were subjected to various heating profiles (held at either 40 or 45°C for 0, 5, 10, 15, 30, 60, 90, 180, or 240 min) using a PCR thermal cycler. Differential plating on selective and nonselective media was used to confirm the presence of cellular injury. Reverse transcription quantitative PCR was used to screen the transcript levels of six heat stress-related genes to find candidate genes for nonlinear modeling. Injury was detected in populations of Salmonella held at 45°C for 30, 60, and 90 min and at 40°C for 0, 5, and 90 min (P < 0.05), whereas no significant injury was found at 180 and 240 min (P > 0.05). The transcript levels of ibpA, which codes for a small heat shock protein associated with the ClpB and DnaK-DnaJ-GrpE chaperone systems, showed the greatest increase relative to the transcript levels at 0 min, which was significant at 5, 10, 15, 30, 60, 90, and 180 min at 45°C and at 5, 10, 15, 30, 60, and 90 min at 40°C (P < 0.05). Using ibpA transcript levels as an indicator of adaptation to thermal stress, a nonlinear model for sublethal injury is proposed. The use of variables indicating the physiological state of the pathogen during stress has the potential to increase the accuracy of thermal inactivation models that must account for prolonged exposure to sublethal temperatures.

Antibacterial Activity of Carum copticum Essential Oil Against Escherichia Coli O157:H7 in Meat: Stx Genes Expression

This work were aimed to (a) determination of minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of Carum copticum essential oil (EO) against Escherichia. coli O157:H7 in vitro Trypticase Soy Broth, (TSB) and in ground beef; (b) evaluation of the effect of sub-inhibitory concentrations (sub-MICs) of EO on the growth of bacterium in TSB over 72 h (at 35 °C) and ground beef over 9 days (at 4 °C); and (c) investigation of gene expression involved in Shiga toxins production using relative quantitative real-time PCR method. The MIC in broth and ground beef medium were determined as 0.05 (v/v) and 1.75 % (v/w), respectively. In comparison with control cultures, the EO concentration of 0.03 % in broth caused reduction of colony counting as 1.93, 1.79, and 2.62 log10 CFU ml(-1) after 24, 48, and 72 h at 35 °C, and similarly EO (0.75 %) in ground beef resulted to reduction of colony counting as 1.03, 0.92, 1.48, and 2.12 log10 CFU g (-1) after 2, 5, 7, and 9 days at 4 °C, respectively. An increase and decrease in gene expression were observed as result of EO addition (0.03 %) to broth and (0.5 %) to ground beef was noticed, respectively.

Folding Optimization In Vivo Uncovers New Chaperones

By employing a genetic selection that forces the cell to fold an unstable, aggregation-prone test protein in order to survive, we have generated bacterial strains with enhanced periplasmic folding capacity. These strains enhance the soluble steady-state level of the test protein. Most of the bacterial variants we isolated were found to overexpress one or more periplasmic proteins including OsmY, Ivy, DppA, OppA, and HdeB. Of these proteins, only HdeB has convincingly been previously shown to function as chaperone in vivo. By giving bacteria the stark choice between death and stabilizing a poorly folded protein, we have now generated designer bacteria selected for their ability to stabilize specific proteins.

First step in using molecular data for microbial food safety risk assessment; hazard identification of Escherichia coli O157:H7 by coupling genomic data with in vitro adherence to human epithelial cells

The potential for using whole genome sequencing (WGS) data in microbiological risk assessment (MRA) has been discussed on several occasions since the beginning of this century. Still, the proposed heuristic approaches have never been applied in a practical framework. This is due to the non-trivial problem of mapping microbial information consisting of thousands of loci onto a probabilistic scale for risks. The paradigm change for MRA involves translation of multidimensional microbial genotypic information to much reduced (integrated) phenotypic information and onwards to a single measure of human risk (i.e. probability of illness).

In this paper a first approach in methodology development is described for the application of WGS data in MRA; this is supported by a practical example. That is, combining genetic data (single nucleotide polymorphisms; SNPs) for Shiga toxin-producing Escherichia coli (STEC) O157 with phenotypic data (in vitro adherence to epithelial cells as a proxy for virulence) leads to hazard identification in a Genome Wide Association Study (GWAS).

This application revealed practical implications when using SNP data for MRA. These can be summarized by considering the following main issues: optimum sample size for valid inference on population level, correction for population structure, quantification and calibration of results, reproducibility of the analysis, links with epidemiological data, anchoring and integration of results into a systems biology approach for the translation of molecular studies to human health risk.

Future developments in genetic data analysis for MRA should aim at resolving the mapping problem of processing genetic sequences to come to a quantitative description of risk. The development of a clustering scheme focusing on biologically relevant information of the microbe involved would be a useful approach in molecular data reduction for risk assessment.

Expression of stress and virulence genes in Escherichia coli O157:H7 heat shocked in fresh dairy compost

The purpose of this study was to determine the gene expression of Escherichia coli O157:H7 heat shocked in dairy compost. A two-step real-time PCR assay was used to evaluate the expression of stress and virulence genes in E. coli O157:H7 heat shocked in compost at 47.5°C for 10 min. Heat-shocked E. coli O157:H7 in compost was isolated by using an immunomagnetic bead separation method, followed by total RNA extraction, which was then converted to cDNA by using a commercial kit. E. coli O157:H7 heat shocked in broth served as the media control. In compost, heat shock genes (clpB, dnaK, and groEL) and the alternative sigma factor (rpoH) of E. coli O157:H7 were upregulated (P < 0.05), whereas the expression of trehalose synthesis genes did not change. Virulence genes, such as stx1 and fliC, were upregulated, while genes stx2, eaeA, and hlyA were downregulated. In the toxin-antitoxin (TA) system, toxin genes, mazF, hipA, and yafQ were upregulated, whereas among antitoxin genes, only dinJ was upregulated (P < 0.05). In tryptic soy broth, all heat shock genes (rpoH, clpB, dnaK, and groEL) were upregulated (P < 0.05), and most virulence genes (stx1, stx2, hlyA, and fliC) and TA genes (mazF-mazE, hipA-hipB, and yafQ-dinJ and toxin gene chpS) were down-regulated. Our results revealed various gene expression patterns when E. coli O157:H7 inoculated in compost was exposed to a sublethal temperature. Clearly, induction of the heat shock response is one of the important protective mechanisms that prolongs the survival of pathogens during the composting process. In addition, other possible mechanisms (such as the TA system) operating along with heat shock response may be responsible for the extended survival of pathogens in compost.

The gluconeogenesis pathway is involved in maintenance of enterohaemorrhagic Escherichia coli O157:H7 in bovine intestinal content

Enterohaemorrhagic Escherichia coli (EHEC) are responsible for outbreaks of food- and water-borne illness. The bovine gastrointestinal tract (GIT) is thought to be the principle reservoir of EHEC. Knowledge of the nutrients essential for EHEC growth and survival in the bovine intestine may help in developing strategies to limit their shedding in bovine faeces thus reducing the risk of human illnesses. To identify specific metabolic pathways induced in the animal GIT, the transcriptome profiles of EHEC O157:H7 EDL933 during incubation in bovine small intestine contents (BSIC) and minimal medium supplemented with glucose were compared. The transcriptome analysis revealed that genes responsible for the assimilation of ethanolamine, urea, agmatine and amino acids (Asp, Thr, Gly, Ser and Trp) were strongly up-regulated suggesting that these compounds are the main nitrogen sources for EHEC in BSIC. A central role for the gluconeogenesis pathway and assimilation of gluconeogenic substrates was also pinpointed in EHEC incubated in BSIC. Our results suggested that three amino acids (Asp, Ser and Trp), glycerol, glycerol 3-phosphate, L-lactate and C4-dicarboxylates are important carbon sources for EHEC in BSIC. The ability to use gluconeogenic substrates as nitrogen sources (amino acids) and/or carbon sources (amino acids, glycerol and lactate) may provide a growth advantage to the bacteria in intestinal fluids. Accordingly, aspartate (2.4 mM), serine (1.9 mM), glycerol (5.8 mM) and lactate (3.6 mM) were present in BSIC and may represent the main gluconeogenic substrates potentially used by EHEC. A double mutant of E. coli EDL933 defective for phosphoenolpyruvate synthase (PpsA) and phosphoenolpyruvate carboxykinase (PckA), unable to utilize tricarboxylic acid (TCA) intermediates was constructed. Growth competition experiments between EHEC EDL933 and the isogenic mutant strain in BSIC clearly showed a significant competitive growth advantage of the wild-type strain further illustrating the importance of the gluconeogenesis pathway in maintaining EHEC in the bovine GIT.

Comparison of strand-specific transcriptomes of enterohemorrhagic Escherichia coli O157:H7 EDL933 (EHEC) under eleven different environmental conditions including radish sprouts and cattle feces

Background

Multiple infection sources for enterohemorrhagic Escherichia coli O157:H7 (EHEC) are known, including animal products, fruit and vegetables. The ecology of this pathogen outside its human host is largely unknown and one third of its annotated genes are still hypothetical. To identify genetic determinants expressed under a variety of environmental factors, we applied strand-specific RNA-sequencing, comparing the SOLiD and Illumina systems.

Results

Transcriptomes of EHEC were sequenced under 11 different biotic and abiotic conditions: LB medium at pH4, pH7, pH9, or at 15°C; LB with nitrite or trimethoprim-sulfamethoxazole; LB-agar surface, M9 minimal medium, spinach leaf juice, surface of living radish sprouts, and cattle feces. Of 5379 annotated genes in strain EDL933 (genome and plasmid), a surprising minority of only 144 had null sequencing reads under all conditions. We therefore developed a statistical method to distinguish weakly transcribed genes from background transcription. We find that 96% of all genes and 91.5% of the hypothetical genes exhibit a significant transcriptional signal under at least one condition. Comparing SOLiD and Illumina systems, we find a high correlation between both approaches for fold-changes of the induced or repressed genes. The pathogenicity island LEE showed highest transcriptional activity in LB medium, minimal medium, and after treatment with antibiotics. Unique sets of genes, including many hypothetical genes, are highly up-regulated on radish sprouts, cattle feces, or in the presence of antibiotics. Furthermore, we observed induction of the shiga-toxin carrying phages by antibiotics and confirmed active biofilm related genes on radish sprouts, in cattle feces, and on agar plates.

Conclusions

Since only a minority of genes (2.7%) were not active under any condition tested (null reads), we suggest that the assumption of significant genome over-annotations is wrong. Environmental transcriptomics uncovered hitherto unknown gene functions and unique regulatory patterns in EHEC. For instance, the environmental function of azoR had been elusive, but this gene is highly active on radish sprouts. Thus, NGS-transcriptomics is an appropriate technique to propose new roles of hypothetical genes and to guide future research.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-353) contains supplementary material, which is available to authorized users.

Rewiring global regulator cAMP receptor protein (CRP) to improve E. coli tolerance towards low pH

Bioprocesses such as production of organic acids or acid hydrolysis of bioresources during biofuel production often suffer limitations due to microbial sensitivity under acidic conditions. Approaches for improving the acid tolerance of these microbes have mainly focused on using metabolic engineering tools. Here, we tried to improve strain acidic tolerance from its transcription level, i.e. we adopted error-prone PCR method to engineer global regulator cAMP receptor protein (CRP) of Escherichia coli to improve its performance at low pH. The best mutant AcM1 was identified from random mutagenesis libraries based on its growth performance. AcM1 almost doubled (0.113h(-1)) the growth rate of the control (0.062h(-1)) at pH 4.24. It also demonstrated better thermotolerance than the control at 48°C, whose growth was completely inhibited at this temperature. Quantitative real time reverse transcription PCR results revealed a stress response overlap among low pH stress-, oxidative stress- and osmotic stress-related genes. The chief enzyme responsible for cell acid tolerance, glutamate decarboxylase, demonstrated over twofold activity in AcM1 compared to the control. Differential binding properties of AcM1 mutant CRP with Class-I, II, and III CRP-dependent promoters suggested that modifications to native CRP may lead to transcription profile changes. Hence, we believe that transcriptional engineering of global regulator CRP can provide a new strain engineering alternative for E. coli.

Selection of in vivo expressed genes of Escherichia coli O157:H7 strain EDL933 in ground meat under elevated temperature conditions

Enterohemorrhagic Escherichia coli O157:H7 strains are important foodborne pathogens that are often transmitted to humans by the ingestion of raw or undercooked meat of bovine origin. To investigate adaptation of this pathogen during persistence and growth in ground meat, we established an in vivo expression technology model to identify genes that are expressed during growth in this food matrix under elevated temperatures (42°C). To improve on the antibiotic-based selection method, we constructed the promoter trap vector pAK-1, containing a promoterless kanamycin resistance gene. A genomic library of E. coli O157:H7 strain EDL933 was constructed in pAK-1 and used for promoter selection in ground meat. The 20 in vivo expressed genes identified were associated with transport processes, metabolism, macromolecule synthesis, and stress response. For most of the identified genes, only hypothetical functions could be assigned. The results of our study provide the first insights into the complex response of E. coli O157:H7 to a ground meat environment under elevated temperatures and establish a suitable vector for promoter studies or selection of in vivo induced promoters in foods such as ground meat.

Evolutionary silence of the acid chaperone protein HdeB in enterohemorrhagic Escherichia coli O157:H7

The periplasmic chaperones HdeA and HdeB are known to be important for cell survival at low pH (pH < 3) in Escherichia coli and Shigella spp. Here we investigated the roles of HdeA and HdeB in the survival of various enterohemorrhagic E. coli (EHEC) following exposure to pH 2.0. Similar to K-12 strains, the acid protections conferred by HdeA and HdeB in EHEC O145 were significant: loss of HdeA and HdeB led to over 100- to 1,000-fold reductions in acid survival, depending on the growth condition of prechallenge cells. However, this protection was much less in E. coli O157:H7 strains. Deletion of hdeB did not affect the acid survival of cells, and deletion of hdeA led to less than a 5-fold decrease in survival. Sequence analysis of the hdeAB operon revealed a point mutation at the putative start codon of the hdeB gene in all 26 E. coli O157:H7 strains analyzed, which shifted the ATG start codon to ATA. This mutation correlated with the lack of HdeB in E. coli O157:H7; however, the plasmid-borne O157-hdeB was able to restore partially the acid resistance in an E. coli O145ΔhdeAB mutant, suggesting the potential function of O157-HdeB as an acid chaperone. We conclude that E. coli O157:H7 strains have evolved acid survival strategies independent of the HdeA/B chaperones and are more acid resistant than nonpathogenic K-12 for cells grown under nonfavorable culturing conditions such as in Luria-Bertani no-salt broth at 28°C. These results suggest a divergent evolution of acid resistance mechanisms within E. coli.

Transcriptome sequencing of Salmonella enterica serovar Enteritidis under desiccation and starvation stress in peanut oil

It is well recognized that Salmonella can survive long-term starvation and desiccation stresses and contaminate foods that have intermediate to low water activities; however, little is known about the specific molecular mechanisms underlying its survival and persistence in low water activity foods. In this study, we used the RNA-seq approach to compare the transcriptomes (27-33 million 36-bp reads per sample) of a Salmonella enterica subsp. enteric serovar Enteritidis strain ATCC BAA-1045 after inoculation in peanut oil (water activity 0.30) for 72 h, 216 h and 528 h to those grown in Luria-Bertani (LB) broth for 12 h and 312 h. Our results showed that desiccated Salmonella cells in peanut oil were in a physiologically dormant state with <5% of its genome being transcribed compared to 78% in LB broth. Among the few detected transcripts in peanut oil, genes involved in heat and cold shock response, DNA protection and regulatory functions likely play roles in cross protecting Salmonella from desiccation and starvation stresses. In addition, non-coding RNAs may also play roles in Salmonella desiccation stress response. This is the first report of using RNA-seq technology in characterizing bacterial transcriptomes in a food matrix.

Specific expression of adherence-related genes in Escherichia coli O157:H7 strain EDL933 after heat treatment in ground beef

In this study, the expression of particular stress- and virulence-associated genes of Escherichia coli O157:H7 strain EDL933 in ground beef was investigated using real-time PCR. Specific gene expression in the food matrix was found in combination with heat treatment. In contrast to a treatment at 37°C, treatment at 48°C for 10 min resulted in increased expression of the genes eae, hcpA, iha, lpfA, and toxB. Adherence to human intestinal HT-29 cells was enhanced in bacterial cells inoculated and heat treated in ground beef. The expression of gadE, which encodes a main regulator of the glutamate system of the acid response, was reduced under these conditions. However, expression of rpoS and recA, which are involved in the establishment of stress responses, and Shiga toxin genes was not significantly different under the same conditions.

Proteomic analysis of Salmonella enterica serovar Enteritidis following propionate adaptation

BACKGROUND

Salmonella Enteritidis is a highly prevalent and persistent foodborne pathogen and is therefore a leading cause of nontyphoidal gastrointestinal disease worldwide. A variety of stresses are endured throughout its infection cycle, including high concentrations of propionate (PA) within food processing systems and within the gut of infected hosts. Prolonged PA exposure experienced in such milieus may have a drastic effect on the proteome of Salmonella Enteritidis subjected to this stress.

RESULTS

In this study, we used 2 D gel electrophoresis to examine the proteomes of PA adapted and unadapted S. Enteritidis and have identified five proteins that are upregulated in PA adapted cultures using standard peptide mass fingerprinting by MALDI-TOF-MS and sequencing by MALDI LIFT-TOF/TOF tandem mass spectrometry. Of these five, two significant stress-related proteins (Dps and CpxR) were shown (via qRT-PCR analysis) to be upregulated at the transcriptional level as well. Unlike the wild type when adapted to PA (which demonstrates significant acid resistance), PA adapted S. Enteritidis ∆dps and S. Enteritidis ∆cpxR were at a clear disadvantage when challenged to a highly acidic environment. However, we found the acid resistance to be fully restorable after genetic complementation.

CONCLUSIONS

This work reveals a significant difference in the proteomes of PA adapted and unadapted S. Enteritidis and affirms the contribution of Dps and CpxR in PA induced acid resistance.