Inactivation of Escherichia coli O157:H7 in simulated human gastric fluid

Bacterial envelope stress responses: Essential adaptors and attractive targets

Millions of deaths a year across the globe are linked to antimicrobial resistant infections. The need to develop new treatments and repurpose of existing antibiotics grows more pressing as the growing antimicrobial resistance pandemic advances. In this review article, we propose that envelope stress responses, the signaling pathways bacteria use to recognize and adapt to damage to the most vulnerable outer compartments of the microbial cell, are attractive targets. Envelope stress responses (ESRs) support colonization and infection by responding to a plethora of toxic envelope stresses encountered throughout the body; they have been co-opted into virulence networks where they work like global positioning systems to coordinate adhesion, invasion, microbial warfare, and biofilm formation. We highlight progress in the development of therapeutic strategies that target ESR signaling proteins and adaptive networks and posit that further characterization of the molecular mechanisms governing these essential niche adaptation machineries will be important for sparking new therapeutic approaches aimed at short-circuiting bacterial adaptation.

Experimentally observed Campylobacter jejuni survival kinetics in chicken meat products during model gastric digestion tended to be lower than model predictions

Campylobacter jejuni-related foodborne diseases are mainly attributed to the consumption of undercooked chicken meat and cross-contaminated produce. This study aimed to develop a survival kinetics model, based on the Weibull model, for predicting foodborne C. jejuni survival during gastric digestion in a model stomach. We previously confirmed that C. jejuni can survive temperatures up to 62 °C; therefore, certain types of grilled chicken skewers (yakitori) were examined for C. jejuni survival during simulated gastric digestion. C. jejuni survival on a chicken thigh following grilling was examined to confirm the foods for digestion experiments. Further, C. jejuni survival during model digestion was investigated through simultaneous digestion of raw chicken and cross-contaminated iceberg lettuce. The model stomach pH increased from 1.5 to 6.0 immediately after yakitori ingestion and did not decrease below 4.0 within 3 h of digestion. Gastric digestion did not significantly contribute to C. jejuni inactivation (<1.5 log reduction after 3 h digestion). Our model could predict C. jejuni survival kinetics in simulated gastric fluid under varying pH during model digestion. This approach can be used to predict C. jejuni survival rates following digestion to improve food safety and reduce Campylobacter-related disease outbreaks.

Membrane modification as a survival mechanism through gastric fluid in non-acid adapted enteropathogenic Escherichia coli (EPEC)

In bacterial cells, the cytoplasmic membrane forms a barrier between the environment and the cell's cytoplasm. This barrier regulates which substances (and the amount) that leave and enter the cell, to maintain homeostasis between the cytoplasm and the external environment. One of the mechanisms employed to maintain structure and functionality during exposure to environmental stress is adaptation of the membrane lipids. The aim of this study was to investigate membrane alteration as a possible survival method of non-acid adapted enteropathogenic Escherichia coli (E. coli) (EPEC) (as could be found in contaminated water or unprocessed food) through simulated gastric fluid (SGF). Enteropathogenic E. coli was grown in nutrient-rich media and then exposed to SGF of various pH (1.5, 2.5, 3.5, or 4.5) for 180 min. Flow cytometry was utilised to examine membrane integrity; and morphological changes were investigated using transmission electron microscopy (TEM). Gas chromatography-mass spectrometry (GC-MS) was used to assess the membrane lipid composition. The results of this study showed that SGF treatment caused membrane damage, as well as cell wall thickening and irregular plasma membranes. The morphological changes were accompanied by membrane lipid changes indicative of decreased membrane fluidity and increased rigidity. The findings suggest that non-acid adapted EPEC can perceive pH change in the environment and adapt accordingly.

Survival of a Norovirus surrogate on surfaces in synthetic gastric fluid

BACKGROUND

Norovirus (NoV) can survive in buffer at a range of pH values. However, when variations to pH are combined with the enzymatic conditions typical in gastric fluid, NoV survival is less predictable. Added to this, the material on which expelled vomitus fluid lands e.g. safety vinyl or carpet, might also affect the level of NoV survival in the environment.

AIM

This study assessed survival of the NoV surrogate Feline calicivirus (FCV) after application onto four material types in the presence of synthetic gastric fluid of varying pH values.

METHODS

Synthetic gastric fluid of varying pH values (1.5, 2.5, 3.5 and 4.5) containing FCV (1×106 PFU/100 μL of gastric fluid) was seeded (100 μL) onto four material types: Formica, safety vinyl, cotton sheet and carpet tiles (2 cm × 2 cm) and left for 0, 30 and 60 minutes before recovering and identifying any viable viruses via plaque assay.

FINDINGS

FCV survived in synthetic gastric fluid at pH values as low as 2.5 and in some cases as low as 1.5 for at least 30 min when associated with certain materials. Greater average numbers of viable virus were recovered from the more absorbent materials of cotton (1900 PFU/sample) and carpet (1600 PFU/sample) compared with Formica (360 PFU/sample) and safety vinyl (380 PFU/sample).

CONCLUSION

This study showed that FCV can survive in synthetic gastric fluids of low pH values for significant time periods especially on absorbent materials. This emphasises the importance of ensuring effective cleaning and decontamination procedures, particularly of soft furnishings and laundry, for infection prevention and control of NoV outbreaks.

Enterohemorrhagic Escherichia coli pathogenesis: role of Long polar fimbriae in Peyer's patches interactions

Enterohemorrhagic Escherichia coli (EHEC) are major food-borne pathogens whose survival and virulence in the human digestive tract remain unclear owing to paucity of relevant models. EHEC interact with the follicle-associated epithelium of Peyer’s patches of the distal ileum and translocate across the intestinal epithelium via M-cells, but the underlying molecular mechanisms are still unknown. Here, we investigated the involvement of Long polar fimbriae (Lpf) in EHEC pathogenesis. Of the 236 strains tested, a significant association was observed between the presence of lpf operons and pathogenicity. In sophisticated in vitro models of the human gastro-intestinal tract, lpf expression was induced during transit through the simulated stomach and small intestine, but not in the colonic compartment. To investigate the involvement of Lpf in EHEC pathogenesis, lpf isogenic mutants and their relative trans-complemented strains were generated. Translocation across M-cells, interactions with murine ileal biopsies containing Peyer’s patches and the number of hemorrhagic lesions were significantly reduced with the lpf mutants compared to the wild-type strain. Complementation of lpf mutants fully restored the wild-type phenotypes. Our results indicate that (i) EHEC might colonize the terminal ileum at the early stages of infection, (ii) Lpf are an important player in the interactions with Peyer’s patches and M-cells, and could contribute to intestinal colonization.

Gastric Mixing During Food Digestion: Mechanisms and Applications

Gastric mixing is a complex process that is governed by meal properties, such as food buffering capacity, physical properties, and the rate of breakdown as well as physiological factors, such as the rate of gastric secretions, gastric emptying, and gastric motility. Gastric mixing processes have been studied through the use of experimental and computational methods. Gastric mixing impacts the intragastric pH distribution and residence time in the stomach for ingested materials. Development of a fundamental understanding of the advective and diffusion processes and their roles in gastric mixing will be important in furthering our understanding of food breakdown, microbial survival, and drug dissolution during gastric digestion.

Increased EHEC survival and virulence gene expression indicate an enhanced pathogenicity upon simulated pediatric gastrointestinal conditions

BACKGROUND

Enterohemorrhagic Escherichia coli (EHEC) are major foodborne pathogens that constitute a serious public health threat, mainly in young children. Shiga toxins (Stx) are the main virulence determinants of EHEC pathogenesis but adhesins like intimin (eae) and Long polar fimbriae (Lpf) also contribute to infection. The TNO GastroIntestinal Model (TIM) was used for a comparative study of EHEC O157:H7 survival and virulence under adult and child digestive conditions.

METHODS

Survival kinetics in the in vitro digestive tract were determined by plating while bacterial viability was assessed by flow cytometry analysis. Expression of stx, eae, and lpf genes was followed by reverse transcriptase-quantitative PCR (RT-qPCR) and Stx production was measured by ELISA (enzyme-linked immunosorbent assay).

RESULTS

Upon gastrointestinal passage, a higher amount of viable cells was found in the simulated ileal effluents of children compared to that of adults (with 34 and 6% of viable cells, respectively). Expression levels of virulence genes were up to 125-fold higher in children. Stx was detected only in child ileal effluents.

CONCLUSION

Differences in digestive physicochemical parameters may partially explain why children are more susceptible to EHEC infection than adults. Such data are essential for a full understanding of EHEC pathogenesis and would help in designing novel therapeutic approaches.

Surviving the acid barrier: responses of pathogenic Vibrio cholerae to simulated gastric fluid

When bacteria are subjected to low acidic pHs of the gastric environment, they may enter the viable but nonculturable (VBNC) state of survival. In this state, bacteria cannot be cultured on solid media, still exhibit signs of metabolic activity (viability). In this study, the response of pathogenic Vibrio cholerae O1 and O139 to low pH-simulated environments of the human stomach was evaluated for their survival by culturability (plate count) and viability (flow cytometry-FC) assays. Bacteria were acid challenged with simulated gastric fluid (SGF) at pH 1.5, 2.5, 3.5 and 4.5 over a period of 180 min. Exposure to SGF up to 120 min increased acid tolerance of the Vibrios up to pH 3.5 with acid challenge occurring at pH 4.5. Bacteria were culturable from pH 2.5 to 4.5 up to 60 min SGF exposure. The stationary-phase cultures of Vibrio were able to survive SGF at all pHs in an 'injured' state with FC. This could possibly mean that the bacteria have entered the VBNC stage of survival. This is a worrying public health concern due to the fact that once favourable conditions arise (intestines), these Vibrios can change back to an infectious state and cause disease.

Persistence of human norovirus RT-qPCR signals in simulated gastric fluid

Human noroviruses (HuNoV) are a leading cause of foodborne disease and are known to be environmentally persistent. Foods usually become contaminated by contact with fecal material, both on hands and on surfaces. Emerging evidence suggests that HuNoVs are also shed and potentially aerosolized during projectile vomiting, resulting in another source of contamination. The purpose of this study was to compare the persistence of HuNoV in vomitus-like material (simulated gastric fluid, SGF, pH 2.5) to that in a pH neutral buffer (phosphate buffered saline, PBS, pH 7.4) in suspension and on surfaces. Human fecal suspensions containing two HuNoV strains (GI.1 and GII.4) were suspended in SGF and PBS. Suspension and surface samples were held at room temperature, and subsamples were collected from both samples for a period up to 42 days. Subsamples were subjected to RNA isolation, with and without inclusion of an RNase pre-treatment, followed by RT-qPCR amplification. In suspension assays, the genome copy number of HuNoV GII.4 decreased by ≤1.0-1.3 log10 over 42 days, irrespective of suspension buffer. On stainless steel, there was virtually no reduction in HuNoV GII.4 RT-qPCR signal over the 42-days experimental period, regardless of suspension buffer. Overall, the GI.1 RT-qPCR signal dropped more precipitously. In most cases, there were no statistically significant differences (p > 0.05) between persistence in solution or on surfaces when comparing RT-qPCR assays with and without prior RNase treatment. This study suggests that HuNoV suspended in vomitus-like material can persist for long periods, a likely contributor to foodborne transmission.

The Escherichia coli Acid Stress Response and Its Significance for Pathogenesis

Escherichia coli has a remarkable ability to survive low pH and possesses a number of different genetic systems that enable it to do this. These may be expressed constitutively, typically in stationary phase, or induced by growth under a variety of conditions. The activities of these systems have been implicated in the ability of E. coli to pass the acidic barrier of the stomach and to become established in the gastrointestinal tract, something causing serious infections. However, much of the work characterizing these systems has been done on standard laboratory strains of E. coli and under conditions which do not closely resemble those found in the human gut. Here we review what is known about acid resistance in E. coli as a model laboratory organism and in the context of its lifestyle as an inhabitant-sometimes an unwelcome one-of the human gut.

Survival of pathogenic and lactobacilli species of fermented olives during simulated human digestion

The present survey uses a dynamic gastric and small intestinal model to assess the survival of one pathogenic (Escherichia coli O157:H7 EDL 933) and three lactobacilli bacteria with probiotic potential (Lactobacillus rhamnosus GG, L. pentosus TOMC-LAB2, and L. pentosus TOMC-LAB4) during their passage through the human gastrointestinal tract using fermented olives as the food matrix. The data showed that the survival of the E. coli strain in the stomach and duodenum was very low, while its transit through the distal parts (jejunum and ileum) resulted in an increase in the pathogen population. The production of Shiga toxins by this enterohemorrhagic microorganism in the ileal effluents of the in vitro system was too low to be detected by ELISA assays. On the contrary, the three lactobacilli species assayed showed a considerable resistance to the gastric digestion, but not to the intestinal one, which affected their survival, and was especially evident in the case of both L. pentosus strains. In spite of this, high population levels for all assayed microorganisms were recovered at the end of the gastrointestinal passage. The results obtained in the present study show the potential use of table olives as a vehicle of beneficial microorganisms to the human body, as well as the need for good hygienic practices on the part of olive manufacturers in order to avoid the possibility of contamination by food-borne pathogens.

Membrane-directed high bactericidal activity of (gold nanoparticle)-polythiophene composite for niche applications against pathogenic bacteria

The use of nanoscale materials as bactericidal agents represents a novel paradigm in the development of therapeutics against drug-resistant pathogenic bacteria. In this paper the antimicrobial activity of a water soluble (gold nanoparticle)-polythiophene (AuNP-PTh) composite against common bacterial pathogens is reported. The nanocomposite is broad-spectrum in its bactericidal activity and exhibits a membrane-directed mode of action on target pathogens. The therapeutic potency of AuNP-PTh is demonstrated by experiments which reveal that the nanocomposite can breach the outer membrane defense barrier of Gram-negative pathogens for subsequent killing by a hydrophobic antibiotic, inhibit the growth of model gastrointestinal pathogens in simulated gastric fluid, and significantly eradicate bacterial biofilms. The high bacterial selectivity and lack of cytotoxicity on human cells augers well for future therapeutic application of the nanocomposite against clinically relevant pathogenic bacteria.

Modeling of pathogen survival during simulated gastric digestion

The objective of the present study was to develop a mathematical model of pathogenic bacterial inactivation kinetics in a gastric environment in order to further understand a part of the infectious dose-response mechanism. The major bacterial pathogens Listeria monocytogenes, Escherichia coli O157:H7, and Salmonella spp. were examined by using simulated gastric fluid adjusted to various pH values. To correspond to the various pHs in a stomach during digestion, a modified logistic differential equation model and the Weibull differential equation model were examined. The specific inactivation rate for each pathogen was successfully described by a square-root model as a function of pH. The square-root models were combined with the modified logistic differential equation to obtain a complete inactivation curve. Both the modified logistic and Weibull models provided a highly accurate fitting of the static pH conditions for every pathogen. However, while the residuals plots of the modified logistic model indicated no systematic bias and/or regional prediction problems, the residuals plots of the Weibull model showed a systematic bias. The modified logistic model appropriately predicted the pathogen behavior in the simulated gastric digestion process with actual food, including cut lettuce, minced tuna, hamburger, and scrambled egg. Although the developed model enabled us to predict pathogen inactivation during gastric digestion, its results also suggested that the ingested bacteria in the stomach would barely be inactivated in the real digestion process. The results of this study will provide important information on a part of the dose-response mechanism of bacterial pathogens.

Effect of a new probiotic Saccharomyces cerevisiae strain on survival of Escherichia coli O157:H7 in a dynamic gastrointestinal model

Survival of Escherichia coli O157:H7 was investigated using a dynamic gastrointestinal model. A high bacterial mortality was observed in the stomach and duodenum. In contrast, bacteria grew in the distal parts of the small intestine. The coadministration of Saccharomyces cerevisiae CNCM I-3856 led to a significant reduction of bacterial resumption, maybe through ethanol production.

Microbial experimental evolution

Microbes have been widely used in experimental evolutionary studies because they possess a variety of valuable traits that facilitate large-scale experimentation. Many replicated populations can be cultured in the laboratory simultaneously along with appropriate controls. Short generation times and large population sizes make microbes ideal experimental subjects, ensuring that many spontaneous mutations occur every generation and that adaptive variants can spread rapidly through a population. Another highly useful experimental feature is the ability to preserve and store ancestral and evolutionarily derived clones. These can be revived in parallel to allow the direct measurement of the competitive fitness of a descendant compared with its ancestor. The extent of adaptation can thereby be measured quantitatively and compared statistically by direct competition among derived groups and with the ancestor. Thus, fitness and adaptation need not be matters of qualitative speculation, but are quantitatively measurable variables in these systems. Replication allows the quantification of heterogeneity in responses to imposed selection and thereby statistical distinction between changes that are systematic responses to the selective regimen and those that are specific to individual populations.

Variation in acid resistance among enterohaemorrhagic Escherichia coli in a simulated gastric environment

AIMS

To compare survival of enterohaemorrhagic Escherichia coli (EHEC) strains of two clonal groups in a simulated gastric environment and to quantify the effect of storage in an acidic food, apple juice, on subsequent survival of EHEC in the simulated gastric environment.

METHODS AND RESULTS

To characterize acid resistance of EHEC under conditions simulating the gastric environment, survival was measured in a model stomach system (MSS) for two clonal groups of EHEC: 14 EHEC 1 strains of serotype O157:H7 and 12 EHEC 2 strains of serotypes O26:H11 and O111:H8. There were significant differences between the two EHEC groups, with the average survival rate of O157 strains in the MSS twice as great as the O26/O111 strains. Strains of the two groups also differed in the quantity of injured cells in MSS and in the transcript levels of the glutamate decarboxylase genes (measured by quantitative PCR) in stationary phase before cultures entered the MSS.

CONCLUSIONS

The results indicate that E. coli O157:H7 strains have superior ability to survive simulated gastric acidity compared with the non-O157 EHEC.

SIGNIFICANCE AND IMPACT OF THE STUDY

E. coli O157:H7 becomes acid resistant rapidly upon entry into stationary phase, which may underlie the low infectious dose of this pathogen.

Adaptation of Salmonella spp. in juice stored under refrigerated and room temperature enhances acid resistance to simulated gastric fluid

The objective of this study was to evaluate the acid resistance of Salmonella spp. adapted in juices stored under refrigeration and room temperatures to simulated gastric fluid (SGF, pH 1.5). Five Salmonella serovars, Agona, Gaminara, Michigan, Montevideo, and Poona were used in this study. Apple, orange, and tomato juices inoculated with five serovars were stored at refrigeration (7 degrees C) and room temperature (20 degrees C) for 24 h for adaptation. Acid resistances of serovars adapted in juice were determined in SGF at 37 degrees C. All acid-adapted Salmonella serovars in juices displayed enhanced survival time compared to non-adapted controls. Among serovars, S. Poona adapted in apple at 20 degrees C and orange juices at 7 and 20 degrees C showed >2.0 log cfu/ml survivors, while the other serovars decreased to non-detectable level or <2.0 log cfu/ml for 100 s in SGF. Unlike apple and orange juices, all serovars adapted in tomato juice survived with >2.0 log cfu/ml for 100 s. For D-values, all Salmonella serovars adapted in apple and tomato juice enhanced their acid resistances compared to orange juices. S. Agona adapted in tomato juice at 7 degrees C and S. Poona in apple juice at 20 degrees C had the highest D-values with 82.9 and 82.5s, respectively. Results showed that the adaptation in juice increased acid resistance in SGF and varied by serovar, juice type, and adaptation temperature. Therefore, this study indicates that the introduction of Salmonella spp. to an acidic juice environment during processing can enhance their ability to survive in a human stomach, possibly increasing the risk of a Salmonella outbreak by juice.

Exposure to salt and organic acids increases the ability of Listeria monocytogenes to invade Caco-2 cells but decreases its ability to survive gastric stress

The effects of environmental stress exposure on Listeria monocytogenes growth and virulence-associated characteristics were investigated. Specifically, we measured the effects of temperature (7 or 37 degrees C), pH (5.5 or 7.4), the presence of salt and organic acids (375 mM NaCl, 8.45 mM sodium diacetate [SD], 275 mM sodium lactate [SL], or a combination of NaCl, SD, and SL), and deletion of sigB, which encodes a key stress response regulator, on the ability of L. monocytogenes to grow, invade Caco-2 cells, and survive exposure to synthetic gastric fluid (pH 2.5 or 4.5). Our results indicate that (i) L. monocytogenes log-phase generation times and maximum cell numbers are not dependent on the alternative sigma factor sigmaB in the presence of NaCl and organic acids at concentrations typically found in foods; (ii) growth inhibition of L. monocytogenes through the addition of organic acids is pH dependent; (iii) the ability of L. monocytogenes to invade Caco-2 cells is affected by growth phase, temperature, and the presence of salt and organic acids, with the highest relative invasion capabilities observed for cells grown with SL or NaCl at 37 degrees C and pH 7.4; (iv) growth of L. monocytogenes in the presence of NaCl, SD, or SL reduces its ability to survive exposure to gastric fluid; and (v) exposure of L. monocytogenes to gastric fluid reduces the enhanced invasiveness caused by growth in the presence of NaCl or SL. These findings suggest that virulence-associated characteristics that determine the L. monocytogenes infectious dose are likely to be affected by food-specific properties (e.g., pH or the presence of salt or organic acid).

Inactivation of enterohemorrhagic Escherichia coli in rumen content- or feces-contaminated drinking water for cattle

Cattle drinking water is a source of on-farm Escherichia coli O157:H7 transmission. The antimicrobial activities of disinfectants to control E. coli O157:H7 in on-farm drinking water are frequently neutralized by the presence of rumen content and manure that generally contaminate the drinking water. Different chemical treatments, including lactic acid, acidic calcium sulfate, chlorine, chlorine dioxide, hydrogen peroxide, caprylic acid, ozone, butyric acid, sodium benzoate, and competing E. coli, were tested individually or in combination for inactivation of E. coli O157:H7 in the presence of rumen content. Chlorine (5 ppm), ozone (22 to 24 ppm at 5 degrees C), and competing E. coli treatment of water had minimal effects (<1 log CFU/ml reduction) on killing E. coli O157:H7 in the presence of rumen content at water-to-rumen content ratios of 50:1 (vol/wt) and lower. Four chemical-treatment combinations, including (i) 0.1% lactic acid, 0.9% acidic calcium sulfate, and 0.05% caprylic acid (treatment A); (ii) 0.1% lactic acid, 0.9% acidic calcium sulfate, and 0.1% sodium benzoate (treatment B); (iii) 0.1% lactic acid, 0.9% acidic calcium sulfate, and 0.5% butyric acid (treatment C); and (iv) 0.1% lactic acid, 0.9% acidic calcium sulfate, and 100 ppm chlorine dioxide (treatment D); were highly effective (>3 log CFU/ml reduction) at 21 degrees C in killing E. coli O157:H7, O26:H11, and O111:NM in water heavily contaminated with rumen content (10:1 water/rumen content ratio [vol/wt]) or feces (20:1 water/feces ratio [vol/wt]). Among them, treatments A, B, and C killed >5 log CFU E. coli O157:H7, O26:H11, and O111:NM/ml within 30 min in water containing rumen content or feces, whereas treatment D inactivated approximately 3 to 4 log CFU/ml under the same conditions. Cattle given water containing treatment A or C or untreated water (control) ad libitum for two 7-day periods drank 15.2, 13.8, and 30.3 liters/day, respectively, and cattle given water containing 0.1% lactic acid plus 0.9% acidic calcium sulfate (pH 2.1) drank 18.6 liters/day. The amounts of water consumed for all water treatments were significantly different from that for the control, but there were no significant differences among the water treatments. Such treatments may best be applied periodically to drinking water troughs and then flushed, rather than being added continuously, to avoid reduced water consumption by cattle.