Induction of acid resistance of Salmonella typhimurium by exposure to short-chain fatty acids

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.

Salmonella in chicken: current and developing strategies to reduce contamination at farm level

Salmonella is a human pathogen that frequently infects poultry flocks. Consumption of raw or undercooked contaminated poultry products can induce acute gastroenteritis in humans. Faced with the public health concerns associated with salmonellosis, the European Union has established a European regulation forcing member states to implement control programs aimed at reducing Salmonella prevalence in poultry production, especially at the primary production level. The purpose of the present review article is to summarize the current research and to suggest future developments in the area of Salmonella control in poultry, which may be of value to the industry in the coming years. The review will focus especially on preventive strategies that have been developed and that aim at reducing the incidence of Salmonella colonization in broiler chickens at the farm level. In addition to the usual preventive hygienic measures, other strategies have been investigated, such as feed and drinking water acidification with organic acids and immune strategies based on passive and active immunity. Modification of the diet by changing ingredients and nutrient composition with the intent of reducing a bird's susceptibility to Salmonella infection also has been examined. Because in ovo feeding accelerates small intestine development and enhances epithelial cell function, this approach could be an efficient tool for controlling enteric pathogens. Feed additives such as antibiotics, prebiotics, probiotics, and synbiotics that modify the intestinal microflora are part of another field of investigation, and their success depends on the additive used. Other control methods such as the use of chlorate products and bacteriophages also are under study.

Chemical treatment of animal feed and water for the control of Salmonella

The control of Salmonella in animal feedstuffs is important, principally to protect the human food chain from contamination by Salmonella derived from infected animals. The transmission of Salmonella from animal feeds to animals, and onward to human food products, has been convincingly documented. This is especially important for chicken breeding and laying flocks and pigs, in view of the consequences of recent or imminent control legislation in the European Union. Animal feed ingredients, particularly animal and plant-derived protein meals, are frequently contaminated with Salmonella either from source or from processing plant, and recontamination in compounding mills is an additional problem. Several complementary strategies have been used to control this feed contamination, and these include a range of chemical treatments. The principal agents used are as follows: organic acids and their salts, formaldehyde, and bacterial membrane disruptors such as terpenes and essential oils. Experimental agents include chlorate compounds. Many products use blends of agents from the same or different chemical groups to achieve synergistic or combination effects. The present review draws upon published and company data to describe the various modes of action and efficacies of different chemical agents delivered in feed or in drinking water against Salmonella occurring in feed or in livestock environments. Reasons for the failure of protection are explored, along with problems in usage such as corrosion and reduced palatability. Given the wide array of products available with contrasting modes of action, the need for standardized tests of efficacy is also discussed.

Differential effects of short-chain fatty acids and iron on expression of iha in Shiga-toxigenic Escherichia coli

Shiga-toxigenic Escherichia coli (STEC) colonizing the bowel are exposed to a variety of short-chain fatty acids (SCFAs), including acetate, propionate and butyrate, produced by gut microflora. However, the total concentrations and relative amounts of SCFAs in the lumen vary with intestinal niche. Here we report that conditions simulating SCFA concentrations present in the human gut trigger expression of the iha gene, which encodes an adherence-conferring outer-membrane protein of pathogenic E. coli. We show that growth under conditions simulating colonic, but not ileal, SCFA concentrations increases iha expression in three tested STEC strains, with the strongest expression detected in LEE-negative STEC O113:H21 strain 98NK2. Expression of iha is known to be subject to Fur-mediated iron repression in O157:H7 STEC, and the same occurs in 98NK2. However, exogenous iron did not repress iha expression in the presence of colonic SCFAs in either 98NK2 or the O157:H7 strain EDL933. Moreover, exposure to the iron chelator 2,2′-dipyridyl caused no further enhancement of iha expression over that induced by colonic SCFAs. These findings indicate that SCFAs regulate iha expression in STEC independently of iron. Increased expression of iha under colonic but not ileal SCFA conditions possibly may contribute to preferential colonization of the human colon by STEC.

Perturbation of the small intestine microbial ecology by streptomycin alters pathology in a Salmonella enterica serovar typhimurium murine model of infection

The small intestine is an important site of infection for many enteric bacterial pathogens, and murine models, including the streptomycin-treated mouse model of infection, are frequently used to study these infections. The environment of the mouse small intestine and the microbiota with which enteric pathogens are likely to interact, however, have not been well described. Therefore, we compared the microbiota and the concentrations of short-chain fatty acids (SCFAs) present in the ileum and cecum of streptomycin-treated mice and untreated controls. We found that the microbiota in the ileum of untreated mice differed greatly from that of the cecum of the same mice, primarily among families of the phylum Firmicutes. Upon treatment with streptomycin, substantial changes in the microbial composition occurred, with a marked loss of population complexity. Characterization of the metabolic products of the microbiota, the SCFAs, showed that formate was present in the ileum but low or not detectable in the cecum while butyrate was present in the cecum but not the ileum. Treatment with streptomycin altered the SCFAs in the cecum, significantly decreasing the concentration of acetate, propionate, and butyrate. In this work, we also characterized the pathology of Salmonella infection in the ileum. Infection of streptomycin-treated mice with Salmonella was characterized by a significant increase in the relative and absolute levels of the pathogen and was associated with more severe ileal inflammation and pathology. Together these results provide a better understanding of the ileal environment in the mouse and the changes that occur upon streptomycin treatment.

Foodborne Salmonella ecology in the avian gastrointestinal tract

Foodborne Salmonella continues to be a major cause of salmonellosis with Salmonella Enteritidis and S. Typhimurium considered to be responsible for most of the infections. Investigation of outbreaks and sporadic cases has indicated that food vehicles such as poultry and poultry by-products including raw and uncooked eggs are among the most common sources of Salmonella infections. The dissemination and infection of the avian intestinal tract remain somewhat unclear. In vitro incubation of Salmonella with mammalian tissue culture cells has shown that invasion into epithelial cells is complex and involves several genetic loci and host factors. Several genes are required for the intestinal phase of Salmonella invasion and are located on Salmonella pathogenicity island 1 (SPI 1). Salmonella pathogenesis in the gastrointestinal (GI) tract and the effects of environmental stimuli on gene expression influence bacterial colonization and invasion. Furthermore, significant parameters of Salmonella including growth physiology, nutrient availability, pH, and energy status are considered contributing factors in the GI tract ecology. Approaches for limiting Salmonella colonization have been primarily based on the microbial ecology of the intestinal tract. In vitro studies have shown that the toxic effects of short chain fatty acids (SCFA) to some Enterobacteriaceae, including Salmonella, have resulted in a reduction in population. In addition, it has been established that native intestinal microorganisms such as Lactobacilli provide protective mechanisms against Salmonella in the ceca. A clear understanding of the key factors involved in Salmonella colonization in the avian GI tract has the potential to lead to better approach for more effective control of this foodborne pathogen.

Probiotic alternatives to reduce gastrointestinal infections: the poultry experience

The intestinal mucosa represents the most active defense barrier against the continuous challenge of food antigens and pathogenic microorganisms present in the intestinal lumen. Protection against harmful agents is conferred by factors such as gastric acid, peristalsis, mucus, intestinal proteolysis, and the intestinal biota. The establishment of beneficial bacterial communities and metabolites from these complex ecosystems has varying consequences for host health. This hypothesis has led to the introduction of novel therapeutic interventions based on the consumption of beneficial bacterial cultures. Mechanisms by which probiotic bacteria affect the microecology of the gastrointestinal tract are not well understood, but at least three mechanisms of action have been proposed: production/presence of antibacterial substances (e.g., bacteriocins or colicins), modulation of immune responses and specific competition for adhesion receptors to intestinal epithelium. The rapid establishment of bacterial communities has been thought to be essential for the prevention of colonization by pathogenic bacteria. Some animal models suggest that the reduction in bacterial translocation in neonatal animals could be associated with an increase in intestinal bacterial communities and bacteriocin-like inhibitory substances produced by these species. This review emphasizes the role of the intestinal microbiota in the reduction of the gastrointestinal infections and draws heavily on studies in poultry.

The use of organic acids to combatSalmonellain poultry: a mechanistic explanation of the efficacy

Salmonella is a human pathogen that is commonly found in poultry products. It is possible to decrease chicken carcass and egg contaminations by adding organic acids to the feed or drinking water at appropriate times. Medium-chain fatty acids are more antibacterial against Salmonella than short-chain fatty acids. The antibacterial effect of these acids is species specific. Bacteria that are unable to decrease intracellular pH accumulate organic acid anions in accordance with the pH gradient across their cell membranes. The short-chain fatty acid butyrate specifically down-regulates expression of invasion genes in Salmonella spp. at low doses. Also medium-chain fatty acids and propionate decrease the ability of Salmonella spp. to invade epithelial cells, in contrast to acetic acid. Because not all bacteria are affected in a similar fashion by organic acids, it may be possible to use probiotic and prebiotic bacteria to achieve beneficial effects. If diets can be designed to stimulate organic acid production in the caecum, it may be possible to control Salmonella spp. via even easier and more cost-effective measures, compared with addition of acids to feed or drinking water.

The lactic acid-induced acid tolerance response in Salmonella enterica serovar Typhimurium induces sensitivity to hydrogen peroxide

Transcriptome analyses of Salmonella enterica serovar Typhimurium revealed that 15 genes were significantly up-regulated after 2 h of adaptation with lactic acid. cadB was the most highly up-regulated gene and was shown to be an essential component. Lactic acid-adapted cells exhibited sensitivity to hydrogen peroxide, likely due to down-regulation of the OxyR regulon.

The Acetic Acid Tolerance Response induces cross-protection to salt stress in Salmonella typhimurium

Salmonella typhimurium induces an Acid Tolerance Response (ATR) upon exposure to mildly acidic conditions in order to protect itself against severe acid shock. This response can also induce cross-protection to other stresses such as heat and salt. We investigated whether both the acetic acid induced and lactic acid induced ATR in S. typhimurium provided cross-protection to a salt stress at 20 degrees C. Acid-adapted cells were challenged with both a sodium chloride (NaCl) and potassium chloride (KCl) shock and their ability to survive ascertained. Acetic acid adaptation provided cells with protection against both NaCl and KCl stress. However, lactic acid adaptation did not protect against either osmotic stressor and rendered cells hypersensitive to NaCl. These results have implications for the food industry where hurdle technology means multiple sub-lethal stresses such as mild pH and low salt are commonly used in the preservation of products.

Growth inhibition by nitrocompounds of selected uric acid-utilizing microorganisms isolated from poultry manure

The overall objective of this study was to evaluate the potential ability of nitrocompounds to reduce ammonia volatilization by inhibiting uric acid-utilizing microorganisms. Experiment I was conducted to evaluate the effects of nitrocompounds on the growth of uric acid-utilizing microorganisms isolated from poultry manure during six-hour incubation. There were five treatments: (1) control, (2) 50 mM nitroethane, (3) 50 mM nitroethanol, (4) 50 mM nitropropanol, and (5) 50 mM nitropropionic acid. Optical density values of nitrocompounds were significantly lower than that of control at two, four, and six hours. Plate counts of uric acid-utilizing microorganisms after six-hour incubation exhibited that nitrocompounds greatly reduced the growth of these microorganisms except for the nitroethane (P < 0.05). The nitropropanol and nitropropionic acid treatments showed significantly higher inhibitory effects compared to the nitroethanol. Experiments II and III were conducted to evaluate inhibitory effects of nitrocompounds on growth of uric acid-utilizing microorganisms compared to non-nitrocompounds such as ethanol, propanol, and propionic acid. Experiments II and III consisted of seven treatments: (1) control, (2) nitroethanol, (3) nitropropanol, (4) nitropropionic acid, (5) ethanol, (6) propanol, and (7) propionic acid. The incubation times of Experiments II and III were 6 and 24 h, respectively. The nitrocompounds were significantly more successful in inhibiting growth of uric acid-utilizing microorganisms compared to those non-nitrocompounds. These results suggest that nitrocompounds exhibit potential to reduce ammonia volatilization in poultry manure by inhibiting growth of uric acid-utilizing microorganisms.

Efficient utilization of ectoine by halophilic Brevibacterium species and Escherichia coli subjected to osmotic downshock

Halophilic and non-halophilic bacteria subjected to osmotic downshock, from 0.7 M NaCl to deionized water, were examined for their survival, with the uptake and utilization of the cyclic amino acid ectoine, one of the representative compatible solutes, being taken into account. The uptake of ectoine added externally and survival of the cells were monitored as a function of incubation time in the presence and absence of NaCl. The halophilic Brevibacterium sp. JCM 6894 and B. epidermidis JCM 2593 actively accumulated ectoine regardless of the presence of NaCl, which led to cell survival. Brevibacterium casei JCM 2594 belonging to the same Brevibacterium species, however, revealed Na+-dependence of its uptake activity of ectoine. Non-halophilic Escherichia coli K-12 did not accumulate ectoine, and thereby this strain failed to survive irrespective of whether NaCl was present. The physiological meanings of the downshock procedure are discussed in connection with the uptake and the subsequent utilization of ectoine.

Effect of acid shock with hydrochloric, citric, and lactic acids on the survival and growth of Salmonella typhi and Salmonella typhimurium in acidified media

The effect of acid shock with hydrochloric, citric, or lactic acid on the survival and growth of Salmonella Typhi and Salmonella Typhimurium in acidified broth was evaluated. Salmonella serovars were acid shocked (1 h at 35 degrees C) in Trypticase soy broth acidified with hydrochloric, citric, or lactic acid at pH 5.5. Unshocked cells were exposed to the same media that had been neutralized before use to pH 7.0. Shocked and unshocked cells were inoculated into broth acidified with hydrochloric acid (pH 3.0), citric acid (pH 3.0), or lactic acid (pH 3.8), and growth and survival ability were evaluated. The acid shock conferred protection to Salmonella against the lethal effects of low pH and organic acids. The adaptive response was not specific to the anion used for adaptation. The biggest difference in reduction of survival between shocked and unshocked strains (approximately 2 log CFU/ml) was observed when the microorganisms were shocked with lactic acid and then challenged with citric acid. Salmonella Typhi was more tolerant of citric acid than was Salmonella Typhimurium, but Salmonella Typhimurium had higher acid tolerance in response to acid shock than did Salmonella Typhi. The acid shock decreased the extension of the lag phase and enhanced the physiological state values of Salmonella Typhi and Salmonella Typhimurium when the pH of growth was 4.5. This increased ability to tolerate acidity may have an important impact on food safety, especially in the case of Salmonella Typhi, given the very low infectious dose of this pathogen.

Effect of butyric acid on the performance and carcass yield of broiler chickens

Short-chain fatty acids such as butyrate are considered potential alternatives to antibiotic growth promoters. The efficacy of butyric acid on performance and carcass characteristics of broiler chickens was tested in two studies. The effect of dietary butyrate on the ability to withstand coccidial oocyte challenge also was investigated. In experiment 1, male broiler chickens were fed diets supplemented with 0 or 11 ppm virginiamycin or 0.2 or 0.4% butyric acid (as mono-, di-, and triglyceride). In experiment 2, broilers were fed bacitracin methylene disalicylate or 0.1 or 0.2% butyric acid. In another trial, birds vaccinated against coccidiosis were challenged with oocytes at 21 d and examined 6 d later. In experiment 1, diet treatments had no effect on body weight gain. Feed intake of the birds fed 0.4% butyric acid was decreased (P < 0.01) compared with birds fed the nonmedicated diet during the starter period, whereas birds fed 0.2% butyric acid had similar feed intake to the control birds. In experiment 2, diet treatments did not affect the performance of broiler chicks while carcass weight and breast meat yield increased (P < 0.01) in birds fed 0.2% butyric acid. With oocyte challenge, birds that had received butyric acid before challenge showed higher growth rate following the challenge compared with birds that received nonmedicated feed. Bacitracin decreased (P < 0.05%) duodenal villi crypt depth, whereas villus length was similar in birds fed butyric acid or the nonmedicated control diet. These results show that 0.2% butyric acid can help to maintain the performance and carcass quality of broilers, especially in vaccinated birds challenged with coccidiosis.

Polyphosphate kinase protects Salmonella enterica from weak organic acid stress

Mutants of Salmonella enterica lacking polyphosphate kinase (ppk) grow poorly in the presence of the weak organic acids acetate, propionate, and benzoate. This sensitivity is corrected by methionine and seems to result from destabilization of MetA (homoserine transsuccinylase), the first enzyme in methionine biosynthesis. The MetA protein is known to be sensitive to thermal inactivation, and ppk mutants are more sensitive to heat-induced methionine auxotrophy. Peroxide increases the sensitivity of ppk mutants to both heat and acid and may oxidatively damage (carbonylate) destabilized MetA. While acid appears to impair methionine biosynthesis, it leads to derepression of MetA and may inhibit growth by causing toxic accumulation of denatured protein. This is supported by the observation that the overexpression of MetA in ppk mutants causes acid sensitivity that is not corrected by methionine. We propose that polyphosphate acts as a chemical chaperone that helps refold MetA and/or may stimulate proteolysis of toxic denatured protein. The instability of MetA protein may provide a metabolic fuse that blocks growth under conditions that denature proteins; the sensitivity of this fuse is modulated by polyphosphate.

Use of in vivo-induced antigen technology for identification of Escherichia coli O157:H7 proteins expressed during human infection

Using in vivo-induced antigen technology (IVIAT), a modified immunoscreening technique that circumvents the need for animal models, we directly identified immunogenic Escherichia coli O157:H7 (O157) proteins expressed either specifically during human infection but not during growth under standard laboratory conditions or at significantly higher levels in vivo than in vitro. IVIAT identified 223 O157 proteins expressed during human infection, several of which were unique to this study. These in vivo-induced (ivi) proteins, encoded by ivi genes, mapped to the backbone, O islands (OIs), and pO157. Lack of in vitro expression of O157-specific ivi proteins was confirmed by proteomic analysis of a mid-exponential-phase culture of E. coli O157 grown in LB broth. Because ivi proteins are expressed in response to specific cues during infection and might help pathogens adapt to and counter hostile in vivo environments, those identified in this study are potential targets for drug and vaccine development. Also, such proteins may be exploited as markers of O157 infection in stool specimens.

Alternatives to antibiotics: chemical and physical antimicrobial interventions and foodborne pathogen response

Successful control of foodborne pathogens requires placement of chemical and physical hurdles in the preharvest and postharvest food production sectors. Pathogens may also encounter indigenous antimicrobials in foods including certain botanical compounds that have historically been used for flavor enhancement as well as preservation. Chemical additives have traditionally included organic acids to control microbial contamination in foods and feeds. However, there is some concern that continuous application of certain chemical antimicrobials can lead to a buildup of microbial resistance. This creates problems if foodborne pathogens survive and develop resistance to a variety of environmental stressors encountered in pre- and postharvest animal production. To expand the diversity of potential antimicrobials that have practical application to food animal production requires exploring the interaction between the food matrix and foodborne pathogens. There is potential for isolating antimicrobial compounds that exhibit mechanisms unrelated to conventional antimicrobial compounds. However, understanding the potential for novel antimicrobial compounds in foods and feeds will require the physiological examination of foodborne pathogen response under experimental conditions comparable to the environment where the pathogen is most likely to occur. Research on foodborne Salmonella pathogenesis is extensive and should provide a model for detailed examination of the factors that influence antimicrobial effectiveness. Analysis of pathogen response to antimicrobials could yield clues for optimizing hurdle technologies to more effectively exploit vulnerabilities of Salmonella and other foodborne pathogens when administering antimicrobials during food and feed production.

The acetate switch

To succeed, many cells must alternate between life-styles that permit rapid growth in the presence of abundant nutrients and ones that enhance survival in the absence of those nutrients. One such change in life-style, the "acetate switch," occurs as cells deplete their environment of acetate-producing carbon sources and begin to rely on their ability to scavenge for acetate. This review explains why, when, and how cells excrete or dissimilate acetate. The central components of the "switch" (phosphotransacetylase [PTA], acetate kinase [ACK], and AMP-forming acetyl coenzyme A synthetase [AMP-ACS]) and the behavior of cells that lack these components are introduced. Acetyl phosphate (acetyl approximately P), the high-energy intermediate of acetate dissimilation, is discussed, and conditions that influence its intracellular concentration are described. Evidence is provided that acetyl approximately P influences cellular processes from organelle biogenesis to cell cycle regulation and from biofilm development to pathogenesis. The merits of each mechanism proposed to explain the interaction of acetyl approximately P with two-component signal transduction pathways are addressed. A short list of enzymes that generate acetyl approximately P by PTA-ACKA-independent mechanisms is introduced and discussed briefly. Attention is then directed to the mechanisms used by cells to "flip the switch," the induction and activation of the acetate-scavenging AMP-ACS. First, evidence is presented that nucleoid proteins orchestrate a progression of distinct nucleoprotein complexes to ensure proper transcription of its gene. Next, the way in which cells regulate AMP-ACS activity through reversible acetylation is described. Finally, the "acetate switch" as it exists in selected eubacteria, archaea, and eukaryotes, including humans, is described.

Short chain fatty acids influence virulence properties of Salmonella enterica serovar Typhimurium

The present study investigated SCFA exposed Salmonella enterica serovar Typhimurium for its virulence characteristics such as mouse lethality adhesion, surface hydrophobicity, phagocytic uptake, intracellular survival within the murine peritoneal macrophages, induction of acid tolerance response (ATR), and the expression of outer membrane proteins (OMPs). Lethal dose (LD50) for mice was found to be more for SCFA exposed cells as compared to the normal cells. In vitro adherence to murine intestinal epithelial cells as well as surface hydrophobicity of SCFA exposed cells were found to be lower than the normal cells. Though the phagocytic uptake of normal cells and SCFA exposed cells was similar, the SCFA exposed cells exhibited increased intracellular survival as compared to the normal cells, which were completely killed after 4 h. Moreover, SCFA exposed cells also survived exposure to extremely low pH (3.0). Outer membrane proteins extracted from SCFA exposed cells revealed an enhanced expression of proteins (porins) at 4% SCFA concentration. Thus, SCFA exposure may contribute to enhanced virulence of Salmonella enterica serovar Typhimurium by increased intracellular survival, induction of acid tolerance response, and the enhanced expression of outer membrane matrix proteins (porins).

Influence of animal origin and lineage on survival of Escherichia coli O157:H7 strains in strong and weak acid challenges

Twenty-five strains of Escherichia coli O157:H7 isolated from humans, cattle, and pigs were maintained in HCl (pH 2.5) and in a volatile fatty acid (VFA) mixture (pH 4.0) for up to 6 h at 37 degrees C to assess their ability to survive in acidic conditions that simulate those of the stomach and ileum, respectively. In HCl, the average group survival of bovine strains was significantly higher than that of porcine and human strains, whereas in VFAs, porcine strains were significantly more resistant than bovine and human strains. Bovine strains exhibited significantly higher average survival in HCl than in VFAs. The average survival of strains classified as octamer-based genome scanning (OBGS) lineage II was significantly superior to that of strains classified as OBGS lineage I in HCl. The group of lineage I strains was more resistant in VFAs compared with lineage II, but only after 6 h of challenge. The possible involvement of urease in acid resistance of E. coli O157:H7 was also examined. Although the strains possessed the ureC gene, as shown by PCR, this gene did not appear to contribute to acid resistance under the conditions tested. The data indicate that there is a relationship between acid resistance and source or lineage of O157:H7 strains.

Autoinducer-2-like activity associated with foods and its interaction with food additives

The autoinducer-2 (AI-2) molecule produced by bacteria as part of quorum sensing is considered to be a universal inducer signal in bacteria because it reportedly influences gene expression in a variety of both gram-negative and gram-positive bacteria. The objective of this study was to determine whether selected fresh produce and processed foods have AI-2-like activity and whether specific food additives can act as AI-2 mimics and result in AI-2-like activity. The luminescence-based response of the reporter strain Vibrio harveyi BB170 was used as the basis for determining AI-2 activity in the selected foods and food ingredients. Maximum AI-2 activity was seen on the frozen fish sample (203-fold, compared with the negative control) followed by tomato, cantaloupe, carrots, tofu, and milk samples. Interestingly, some samples were capable of inhibiting AI-2 activity. Turkey patties showed the highest inhibition (99.8% compared with the positive control) followed by chicken breast (97.5%), homemade cheeses (93.7%), beef steak (90.6%), and beef patties (84.4%). AI-2 activity was almost totally inhibited by sodium propionate, whereas sodium benzoate caused 93.3% inhibition, compared with 75% inhibition by sodium acetate. Sodium nitrate did not have any appreciable effect, even at 200 ppm. Understanding the relationships that exist between AI-2 activity on foods and the ecology of pathogens and food spoilage bacteria on foods could yield clues about factors controlling food spoilage and pathogen virulence.

Neutrophil and small intestinal lymphocyte migration after Salmonella typhimurium infection: impact of fermentable fiber

OBJECTIVES

Formula-fed infants have more episodes of acute diarrhea and intestinal infection than do breast-fed infants. Nutrient additions to infant formula that could confer some of the immune benefits of breast milk to formula-fed infants are currently under investigation. This study examined the impact of enteral formulas supplemented with fermentable substrates on small intestinal lymphocyte and neutrophil migration in piglets infected with Salmonella typhimurium. Small intestinal proinflammatory cytokine messenger RNA abundance and in vitro lipopolysaccharide-stimulated interleukin-6 release in whole blood were assessed.

METHODS

Piglets were randomized to receive sow milk replacer formula supplemented with methylcellulose (control), soy polysaccharides (SPS) or fructooligosaccharides (FOS). On day 7, half of the piglets were infected with S. typhimurium. Intestinal lymphocyte, neutrophil and whole blood samples were obtained on day 14.

RESULTS

After infection, there was decreased lymphocyte migration in the control group but not in the SPS and FOS groups. The SPS group had greater neutrophil migration compared with the control and FOS groups, regardless of infection. Small intestinal abundance of proinflammatory cytokine messenger RNA was not significantly changed by either infection or diet. Blood from the FOS group challenged with lipopolysaccharide for 2 hours exhibited decreased interleukin-6 production compared with blood from the control and SPS groups, regardless of infection.

CONCLUSIONS

Supplementation of enteral formulas with SPS maintains the migratory function of small intestinal lymphocytes while increasing that of neutrophils.

Dietary fructo-oligosaccharides and inulin decrease resistance of rats to salmonella: protective role of calcium

BACKGROUND

We have shown recently that rapid fermentable fructo-oligosaccharides (FOS) decreased resistance of rats towards salmonella. It is not known whether inulin (which is fermented more gradually) has similar effects or whether buffering nutrients can counteract the adverse effects of rapid fermentation.

AIMS

To compare the effects of dietary inulin and FOS on resistance of rats to Salmonella enterica serovar Enteritidis and to determine whether calcium phosphate counteracts the effects of fermentation.

METHODS

Male Wistar rats (n = 8 per group) were fed a human "Western style diet". Diets with 60 g/kg cellulose (control), FOS, or inulin had either a low (30 mmol/kg) or high (100 mmol/kg) calcium concentration. After an adaptation period of two weeks, animals were orally infected with 2 x 10(9) colony forming units of Salmonella enterica serovar Enteritidis. Colonisation of salmonella was determined by quantification of salmonella in caecal contents. Translocation of salmonella was quantified by analysis of urinary nitric oxide metabolites in time.

RESULTS

Inulin and FOS decreased intestinal pH and increased faecal lactobacilli and enterobacteria. Moreover, both prebiotics increased the cytotoxicity of faecal water and faecal mucin excretion. Both prebiotics increased colonisation of salmonella in caecal contents and enhanced translocation of salmonella. Dietary calcium phosphate counteracted most of the adverse effects of inulin and FOS.

CONCLUSIONS

Both inulin and FOS impair resistance to intestinal infections in rats. This impairment is partially prevented by dietary calcium phosphate. The results of the present study await verification in other controlled animal and human studies.

Comparative acid stress response of Listeria monocytogenes, Escherichia coli O157:H7 and Salmonella Typhimurium after habituation at different pH conditions

AIMS

The aim of the study was to evaluate the effect of habituation at different pH conditions on the acid resistance of Listeria monocytogenes, Escherichia coli O157:H7 and Salmonella enterica serotype Typhimurium, and to identify potential differences between the adaptive responses of the three pathogens.

METHODS

Stationary phase cells of L. monocytogenes, E. coli O157:H7 and S. Typhimurium, grown in glucose-free media, were exposed to pH 3.5 broth directly or after habituation for 90 min at various pH conditions from 4.0 to 6.0. Survivors at pH 3.5 were determined by plating on tryptic soy agar and incubating at 30 degrees C for 48 h. The kinetics (death rate) of the pathogens at pH 3.5 was calculated by fitting the data to an exponential model.

RESULTS

Habituation to acidic environments provided protection of the pathogens against lethal acid conditions. This acid protection, however, was found to be pH dependent. For example, for E. coli O157:H7 an increased acid resistance was observed after habituation at a pH range from 4.0 to 5.5, while the maximum acid tolerance was induced at pH 5.0. Furthermore, the effect of low pH habituation was different among pathogens. For L. monocytogenes, E. coli O157:H7 and S. Typhimurium, the pH range within which habituation resulted to increased acid resistance was 5.0-6.0, 4.0-5.5 and 4.0-5.0, respectively, while the maximum acid tolerance was induced after habituation at pH 5.5, 5.0 and 4.5, respectively.

SIGNIFICANCE

Acid stress conditions are common within current food processing technologies. The information on adaptive responses of L. monocytogenes, E. coli O157:H7 and S. Typhimurium after habituation to different pH environments provided in the present study, could lead to a more realistic evaluation of food safety concerns and to a better selection of processes in order to avoid adaptation phenomena and to minimize the potential for food safety risks.

Microencapsulated Short-Chain Fatty Acids in Feed Modify Colonization and Invasion Early After Infection with Salmonella Enteritidis in Young Chickens

Short-chain fatty acids (SCFA) are widely used as feed additives in poultry for the control of pathogenic bacteria, such as Salmonella enteritidis. Recently, a new range of products was developed in which SCFA are encapsulated in mineral carriers, resulting in a slow release during the transport of these carriers through the intestinal tract. To test the efficacy of this type of products against early colonization after Salmonella infection in poultry, a challenge experiment with S. enteritidis was performed. Five groups of 20 chickens were given feed with no supplement or feed supplemented with acetic acid (0.24%), formic acid (0.22%), or propionic acid (0.27%) as film-coated microbeads or butyric acid (0.15%) as spray-cooled microcapsules. The 5 groups were challenged with 5 x 10(3) cfu S. enteritidis at d 5 and 6 posthatch, and samples of ceca, liver, and spleen were taken at d 8 and analyzed for the number of colony-forming units of Salmonella per gram of tissue. Feed supplementation with acetic acid, and to a lesser extent formic acid, resulted in an increase of colonization of ceca and internal organs. Birds receiving propionic acid-coated microbeads as feed supplement were colonized with Salmonella to the same extent as controls. Butyric acid-impregnated microbeads in the feed, however, resulted in a significant decrease of colonization by S. enteritidis in the ceca but not in liver and spleen.

Adaptive responses of Salmonella enterica serovar Typhimurium DT104 and other S. Typhimurium strains and Escherichia coli O157 to low pH environments

AIMS

Cattle are a known main reservoir for acid-resistant Escherichia coli O157 and Salmonella enterica serovar Typhimurium DT104. We studied the response of S. Typhimurium DT104 to extreme low pH environments and compared their response to that of acid-resistant E. coli O157 and other S. Typhimurium phage types.

METHODS AND RESULTS

Bacteria were grown in nutrient-rich medium and subsequently acid challenged at pH 2.5. We found that stationary phase cultures of various S. Typhimurium strains were able to survive a challenge for 2 h at pH 2.5. As in E. coli, the ability of S. Typhimurium to survive at pH 2.5 was shown to be dependent on the presence of amino acids, specifically arginine. The amount of proton pumping H+/ATPase, both in E. coli O157 and S. Typhimurium strains, was lower when grown at pH values <6 than after growth at pH 7.5. Cyclo fatty acid content of membranes of bacteria grown at pH values <6 was higher than that of membranes of bacteria grown at pH 7.5.

CONCLUSIONS

Various S. Typhimurium strains, both DT104 and non-DT104, are able to survive for a prolonged period of time at pH 2.5. Their response to such low pH environment is seemingly similar to that of E. coli O157.

SIGNIFICANCE AND IMPACT OF THE STUDY

Food-borne pathogens like S. Typhimurium DT104 and E. coli O157 form a serious threat to public health since such strains are able to survive under extreme low pH conditions as present in the human stomach. The emergence these acid-resistant strains suggests the presence of a selection barrier. The intestinal tract of ruminants fed a carbohydrate-rich diet might be such a barrier.

Perspectives on the use of organic acids and short chain fatty acids as antimicrobials

Organic acids have a long history of being utilized as food additives and preservatives for preventing food deterioration and extending the shelf life of perishable food ingredients. Specific organic acids have also been used to control microbial contamination and dissemination of foodborne pathogens in preharvest and postharvest food production and processing. The antibacterial mechanism(s) for organic acids are not fully understood, and activity may vary depending on physiological status of the organism and the physicochemical characteristics of the external environment. An emerging potential problem is that organic acids have been observed to enhance survivability of acid sensitive pathogens exposed to low pH by induction of an acid tolerance response and that acid tolerance may be linked to increased virulence. Although this situation has implications regarding the use of organic acids, it may only apply to circumstances in which reduced acid levels have induced resistance and virulence mechanisms in exposed organisms. Evaluating effectiveness of organic acids for specific applications requires more understanding general and specific stress response capabilities of foodborne pathogens. Development and application of molecular tools to study pathogen behavior in preharvest and postharvest food production environments will enable dissection of specific bacterial genetic regulation involved in response to organic acids. This could lead to the development of more targeted strategies to control foodborne pathogens with organic acids.

Weak organic acids: a panoply of effects on bacteria

Weak organic acids have been used for centuries to preserve foods, but only recently has the possible mechanism for bacterial growth inhibition been investigated. Although the lowering of internal pH was favored as the cause of growth inhibition, the emphasis has shifted to the anion and its specificity. There are a number of applications of weak organic acids to foods and in the food industry be they pre- or postharvest, However, there is concern that the ability of foodborne pathogens to adapt to these acids may allow longer survival in these commodities and also to better survive transit through the gastric acid barrier of the stomach. Genomic and proteomic approaches have been applied to the identification of genes and proteins that may allow prokaryotes to cope with organic acid stress. These technologies in combination with genetic approaches may provide better identification of genes essential for survival to organic acids. These acids may have other roles: they can induce phenotypic antibiotic resistance, and the high concentrations of these acids in the colon may signal a relationship to diet, colonic microflora, and human health.

Involvement of surface polysaccharides in the organic acid resistance of Shiga Toxin-producing Escherichia coli O157:H7

In general, wild Escherichia coli strains can grow effectively under moderately acidic organic acid-rich conditions. We found that the Shiga Toxin-producing E. coli (STEC) O157:H7 NGY9 grows more quickly than a K-12 strain in Luria-Bertani (LB)-2-morpholinoethanesulphonic acid (MES) broth supplemented with acetic acid (pH 5.4). Hypothesizing that the resistance of STEC O157:H7 to acetic acid is as a result of a mechanism(s) other than those known, we screened for STEC mutants sensitive to acetic acid. NGY9 was subjected to mini-Tn5 mutagenesis and, from 50,000 colonies, five mutants that showed a clear acetic acid-sensitive phenotype were isolated. The insertion of mini-Tn5 in three mutants occurred at the fcl, wecA (rfe) and wecB (rffE) genes and caused loss of surface O-polysaccharide, loss of both O-polysaccharide and enterobacterial common antigen (ECA) and loss of ECA respectively. The other two mutants showed inactivation of the waaG (rfaG) gene but at different positions that caused a deep rough mutant with loss of the outer core oligosaccharide of lipopolysaccharide (LPS) as well as phenotypic loss of O-polysaccharide and ECA. With the introduction of plasmids carrying the fcl, wecA, wecB and waaG genes, respectively, all mutants were complemented in their production of O-polysaccharide and ECA, and normal growth was restored in organic acid-rich culture conditions. We also found that the growth of Salmonella LPS mutants Ra, Rb1, Rc, Rd1, Rd2 and Re was suppressed in the presence of acetic acid compared with that of the parents. These results suggest that the full expression of LPS (including O-polysaccharide) and ECA is indispensable to the resistance against acetic acid and other short chain fatty acids in STEC O157:H7 and Salmonella. To the best of our knowledge, this is a newly identified physiological role for O-polysaccharide and ECA as well as an acid resistance mechanism.

Extracellular sensing components and extracellular induction component alarmones give early warning against stress in Escherichia coli

The work reported here follows from the proposal that, for efficient induction of numerous extracellular stress responses, cultures contain extracellular stress-sensing molecules, termed extracellular sensing components (ESCs). These are directly converted to extracellular induction components (EICs) by stresses, thus providing an early warning system against stress, with very rapid responses occurring on exposure to increasing levels of stress. Although some stress responses appear to involve activation of intracellular sensors, the proposed ESCs and EICs function for many stress tolerance and sensitization responses and for several cross-tolerance and cross-sensitization responses. Because EICs can induce responses in unstressed cells, and because they are small molecules that can diffuse away from the site of formation, they can be considered to be 'alarmones', both warning unstressed organisms of future stress and preparing both stressed and unstressed ones to resist it. Therefore, EICs produced by one group of organisms could affect another group i.e. there could be 'cross-talk' (cell-to-cell communication) with other organisms in an area, to which the EICs diffuse, that has not yet faced the stress. In particular, stimuli that switch on acid tolerance, alkali tolerance, pH sensitization responses and alkylhydroperoxide tolerance are detected by ESCs; these molecules can give rise to EICs in the presence of the stress without organisms needing to be present. Not only does the ESC-EIC interconversion allow rapid switching on of responses, but for some responses it also allows rapid switching off. For some ESCs, the sensor can be modified by the culture conditions, modification leading to altered responsiveness to stress; such sensor changes appear to have evolved to allow the most efficient responses to stress to occur, under defined sets of conditions. In addition, the receptors on the organisms that interact with EICs are modified by culture conditions, so that extracellular components that function as ESCs for some cultures can act as EICs for others. In view of their role in early warning of stress, EICs and ESCs are likely to have important functions in the natural environment, especially in natural waters, in foods and food preparation and production, in hospital, domestic and commercial locations, and in the animal and human body. Findings of major importance relate to the extreme stress tolerance of some EICs. For example, because the acid-tolerance EIC formed at pH 5.0 is a heat-resistant molecule, heat-killed suspensions of acid-tolerant cultures can confer acid tolerance on living E. coli; cultures killed by extreme acidity and alkalinity and by exposure to high levels of UV irradiation or novobiocin are also able to confer acid tolerance on living E. coli. Extracellular components that inhibit induction of stress responses also occur in enterobacteria, since it has been found that AMP and HCO3-, which inhibit acid-tolerance induction, do so by forming extracellular agents that block the functioning of EICs. Similar agents to the above EICs and ESCs may occur in other non-stress-related processes. Systems using these extracellular components are quite distinct in their properties from quorum-sensing systems in Gram-negative bacteria and from those systems that use small peptides in intercellular communication and which induce virulence-related enzyme synthesis in Staphylococcus aureus and competence in streptococci and bacilli. Additionally, probably because the ESCs have evolved to become modified by cultural conditions, the components in the stress-related systems, although relatively small proteins, are much larger than the extracellular components used in the quorum-sensing processes and related systems. It is possible that the extracellular 'protectants' of Nikolaev, which protect E. coli from stress, act similarly to the EICs described here, e.g. by inducing stress tolerance. The antimutagenic factor of Vorobjeva may act similarly, although there is no evidence, so far, to suggest that it acts by inducing tolerance to mutagens.

Role of the glutamate decarboxylase acid resistance system in the survival of Listeria monocytogenes LO28 in low pH foods

The glutamate decarboxylase (GAD) acid resistance system of the foodborne pathogen Listeria monocytogenes plays a major role in its survival at low pH. It was found that survival of the wild-type strain. LO28, in acidified reconstituted skim milk, diluted to reduce free glutamate levels. improves in response to supplementation with monosodium glutamate. A mutant, in which the two listerial GAD homologs have been deleted (and in which there is no discernible GAD activity), did not respond to glutamate supplementation and displayed greatly enhanced sensitivity in a number of low pH foods, even when levels of free glutamate were as low as 0.22 mM. We thus show that the GAD system plays a major role in the survival of L. monocytogenes in acidic foods even when levels of free glutamate are low.

Impedance measurements to study the antimicrobial activity of essential oils from Lamiaceae and Compositae

A wide range of essential oils from sage, mint, hyssop, camomile and oregano were tested for their inhibitory effects against nine strains of gram-negative bacteria and six strains of gram-positive bacteria. Three principles were used in describing the antimicrobial effects of the essential oils: the overall antimicrobial activity determined by use of an impedometric method, the bactericidal effect determined as colony forming units after exposure to the essential oils, and the number of apparent dead cells determined after further enrichment. The data obtained indicate that while the essential oils of sage, mint, hyssop and camomile had generally a bacteriostatic activity, the essential oil from oregano appeared to be bactericidal at concentrations above 400 ppm, probably because of high contents in phenolic compounds. For the other essential oils, the chemical analysis was unable to explain the antimicrobial effect. The bacteriostatic activity was more marked against gram-positive bacteria; in contrast, the bactericidal activity was greatest against gram-negative bacteria. The most sensitive strain was Escherichia coli O157:H7 and, of the gram-positive species even at the lowest oil concentrations, Listeria innocua was the most sensitive. The data obtained from the study of the bactericidal effect of oregano essential oil indicated that the major part of the species was irreversibly inactivated, i.e. they could not be revived by enrichment.

Breaking through the acid barrier: an orchestrated response to proton stress by enteric bacteria

The ability of enteropathogens such as Salmonella and Escherichia coli to adapt and survive acid stress is fundamental to their pathogenesis. Once inside the host, these organisms encounter life-threatening levels of inorganic acid (H+) in the stomach and a combination of inorganic and organic acids (volatile fatty acids) in the small intestine. To combat these stresses, enteric bacteria have evolved elegant, overlapping strategies that involve both constitutive and inducible defense systems. This article reviews the recent progress made in understanding the pH 3 acid tolerance systems of Salmonella and the even more effective pH 2 acid resistance systems of E. coli. Focus is placed on how Salmonella orchestrates acid tolerance by modulating the activities or levels of diverse regulatory proteins in response to pH stress. The result is induction of overlapping arrays of acid shock proteins that protect the cell against acid and other environmental stresses. Most notable among these pH-response regulators are RpoS, Fur, PhoP and OmpR. In addition, we will review three dedicated acid resistance systems of E. coli, not present in Salmonella, that allow this organism to survive extreme (pH 2) acid challenge.

Global analysis of Escherichia coli gene expression during the acetate-induced acid tolerance response

The ability of Escherichia coli to survive at low pH is strongly affected by environmental factors, such as composition of the growth medium and growth phase. Exposure to short-chain fatty acids, such as acetate, proprionate, and butyrate, at neutral or nearly neutral pH has also been shown to increase acid survival of E. coli and Salmonella enterica serovar Typhimurium. To investigate the basis for acetate-induced acid tolerance in E. coli O157:H7, genes whose expression was altered by exposure to acetate were identified using gene arrays. The expression of 60 genes was reduced by at least twofold; of these, 48 encode components of the transcription-translation machinery. Expression of 26 genes increased twofold or greater following treatment with acetate. This included six genes whose products are known to be important for survival at low pH. Five of these genes, as well as six other acetate-induced genes, are members of the E. coli RpoS regulon. RpoS, the stress sigma factor, is known to be required for acid tolerance induced by growth at nonlethal low pH or by entry into stationary phase. Disruption of the rpoS gene by a transposon insertion mutation also prevented acetate-induced acid tolerance. However, induction of RpoS expression did not appear to be sufficient to activate the acid tolerance response. Treatment with either NaCl or sodium acetate (pH 7.0) increased expression of an rpoS::lacZ fusion protein, but only treatment with acetate increased acid survival.

Short-chain volatile fatty acids modulate the expression of the hilA and invF genes of Salmonella typhimurium

The ability of Salmonella typhimurium to invade the intestinal mucosal cells is an important step in pathogenesis. This invasion process requires genes encoded on the Salmonella pathogenicity island 1 (SPI1). Two transcriptional activators, HilA and InvF, encoded in SPII regulate the expression of invasion genes in response to environmental stimuli such as osmolarity, oxygen tension, and pH. During its pathogenic life cycle, Salmonella typhimurium is also exposed to short-chain fatty acids (SCFA), especially acetate, propionate, and butyrate, in the intestinal lumen, as well as the SCFA used as food preservatives. The effects of SCFA on the expression of hilA and invF-lacZY transcriptional fusions were examined to determine the potential role of SCFA in the pathogenesis of Salmonella typhimurium. Growth rates were reduced by increasing SCFA concentrations at pH 6 but not at pH 7. At pH 7, hilA and invF expression was induced by acetate but not by propionate or butyrate, while at pH 6, all SCFA induced hilA and invF expression at 1 h. In general, hilA and invF expression levels when compared to respective control responses were higher at 1 h than at 4 and 8 h in the presence of most SCFA concentrations at pH 6. However, expression levels at 4 and 8 h were either similar or higher than the 1-h responses for the hilA-lacZY fusion strain in the presence of acetate while exposure to 20 mM propionate yielded similar levels of expression at 1, 4, and 8 h. The pH-dependent manner of induction suggests that entry of SCFA into the cell was necessary for induction. We speculate that SCFA may serve as an environmental signal that triggers the expression of invasion genes in the gastrointestinal tract.

Short-chain fatty acids affect cell-association and invasion of HEp-2 cells by Salmonella typhimurium

This study demonstrates that the growth of S. typhimurium in Luria Bertani broth supplemented with acetate, propionate, butyrate, or a mixture of the three SCFA, affected cell-association and the ability to invade cultured HEp-2 cells. Cell-association and invasion was determined after growth for 4 h of growth in the presence of the SCFA at pH 6 and 7. The results suggest that the growth rate of the culture may have affected cell-association and invasion since accompanying the significant decrease in growth rate in the presence of SCFA at pH 6 was a decrease in cell-association and invasion. However, the results also suggest that the individual SCFA may play a role in modulating cell-association and the invasion phenotype and the regulation of cell-association and invasion by the SCFA was dependent on the concentration and the pH of the medium. Although the growth rates were similar for S. typhimurium in the SCFA mixture, butyrate (100 mM) and propionate (50 mM) at pH 6, differences in cell-association and invasion were observed among these cultures. Also, at pH 7, differences were observed among the SCFA treatments even though the growth rates were similar.

Foodborne infections

The role of foodborne infections in the health of the population has become of major concern recently. Numerous agents are transmitted in food and water and typically result in acute gastroenteritis, although long-term complications such as reactive arthritis (due to Salmonella, Yersinia, and Shigella organisms), Guillain-Barré syndrome (due to Campylobacter organisms), and renal failure (due to Escherichia coli) are now well recognized. The development of FoodNet to follow the epidemiology of select foodborne infections in the United States has been a major advance in recent years and is now beginning to show interesting trends. Our understanding of the pathogenesis of some of the major foodborne pathogens, especially Salmonella, is advancing and the genome sequencing of these organisms will advance the field further. Of particular concern of late is the increasing number of antibiotic-resistant bacterial isolates, especially for Salmonella and Campylobacter. Irrespective of their cause, these changes in susceptibility patterns pose a major threat to the appropriate treatment of patients. Overall, our knowledge of foodborne infections is advancing rapidly, but new factors such as the emergence of antibiotic resistance means that vigilance must be maintained.

When protons attack: microbial strategies of acid adaptation

Inducible tolerance to acidic and alkaline environments is recognized as an important survival strategy for many prokaryotic and eukaryotic microorganisms. Recent developments in understanding this phenomenon include the identification of regulatory genes, specific tolerance mechanisms and genes associated with tolerance. In addition, there is significant evidence linking pH responses with virulence.