Identification and disruption of BetL, a secondary glycine betaine transport system linked to the salt tolerance of Listeria monocytogenes LO28

Recent Advances in the Understanding of Stress Resistance Mechanisms in Probiotics: Relevance for the Design of Functional Food Systems

In recent years, more and more scientific community, food producers, and food industry show increased interest in functional foods containing probiotics, which is a big challenge. The consumption of probiotics in the context of a balanced diet through the consumption of functional foods or through the intake of pharmaceutical preparations has proven to contribute to the improvement of human health, even contributing to the prevention of diseases. In order for probiotics to be considered suitable for consumption, they must contain a minimum concentration of viable cells, namely, at least 107 colony forming units of beneficial microbes per gram. Ensuring the viability of bacterial cells until the moment of consumption is the overriding priority of functional probiotic food manufacturers. Probiotic bacteria are subject to stress conditions not only during food manufacturing but also during gastrointestinal passage, which limit or even compromise their functionality. This paper first examines all the stressful conditions faced by probiotic cells in their production stages and related to the conditions present in the bioreactor fermentation and drying processes as well as factors related to the food matrix and storage. The stress situations faced by probiotic microorganisms during the gastrointestinal transit especially during stomach and intestinal residence are also analyzed. In order to understand the adaptation mechanisms of probiotic bacteria to gastrointestinal stress, intrinsic and adaptive mechanisms identified in probiotic strains in response to acid stress and to bile and bile acid stress are analyzed. In addition, improvement strategies for multiple stress tolerance of lactic acid bacteria through directions dealing with stress, accumulation of metabolites, use of protectants, and regulation of technological parameters are examined. Finally, the definition of postbiotics, inanimate microorganisms and/or their components conferring health benefits, is also introduced. Postbiotics include cell lysates, enzymes, and cell wall fragments derived from probiotic bacteria and may represent an alternative to the use of probiotics, when they do not tolerate stressful conditions.

Listeria monocytogenes Pathogenesis: The Role of Stress Adaptation

Adaptive stress tolerance responses are the driving force behind the survival ability of Listeria monocytogenes in different environmental niches, within foods, and ultimately, the ability to cause human infections. Although the bacterial stress adaptive responses are primarily a necessity for survival in foods and the environment, some aspects of the stress responses are linked to bacterial pathogenesis. Food stress-induced adaptive tolerance responses to acid and osmotic stresses can protect the pathogen against similar stresses in the gastrointestinal tract (GIT) and, thus, directly aid its virulence potential. Moreover, once in the GIT, the reprogramming of gene expression from the stress survival-related genes to virulence-related genes allows L. monocytogenes to switch from an avirulent to a virulent state. This transition is controlled by two overlapping and interlinked transcriptional networks for general stress response (regulated by Sigma factor B, (SigB)) and virulence (regulated by the positive regulatory factor A (PrfA)). This review explores the current knowledge on the molecular basis of the connection between stress tolerance responses and the pathogenesis of L. monocytogenes. The review gives a detailed background on the currently known mechanisms of pathogenesis and stress adaptation. Furthermore, the paper looks at the current literature and theories on the overlaps and connections between the regulatory networks for SigB and PrfA.

Pathogenicity and virulence of Listeria monocytogenes: A trip from environmental to medical microbiology

Listeria monocytogenes is a saprophytic gram-positive bacterium, and an opportunistic foodborne pathogen that can produce listeriosis in humans and animals. It has evolved an exceptional ability to adapt to stress conditions encountered in different environments, resulting in a ubiquitous distribution. Because some food preservation methods and disinfection protocols in food-processing environments cannot efficiently prevent contaminations, L. monocytogenes constitutes a threat to human health and a challenge to food safety. In the host, Listeria colonizes the gastrointestinal tract, crosses the intestinal barrier, and disseminates through the blood to target organs. In immunocompromised individuals, the elderly, and pregnant women, the pathogen can cross the blood-brain and placental barriers, leading to neurolisteriosis and materno-fetal listeriosis. Molecular and cell biology studies of infection have proven L. monocytogenes to be a versatile pathogen that deploys unique strategies to invade different cell types, survive and move inside the eukaryotic host cell, and spread from cell to cell. Here, we present the multifaceted Listeria life cycle from a comprehensive perspective. We discuss genetic features of pathogenic Listeria species, analyze factors involved in food contamination, and review bacterial strategies to tolerate stresses encountered both during food processing and along the host's gastrointestinal tract. Then we dissect host-pathogen interactions underlying listerial pathogenesis in mammals from a cell biology and systemic point of view. Finally, we summarize the epidemiology, pathophysiology, and clinical features of listeriosis in humans and animals. This work aims to gather information from different fields crucial for a comprehensive understanding of the pathogenesis of L. monocytogenes.

Systematic review of the Listeria monocytogenes σB regulon supports a role in stress response, virulence and metabolism

Aim: Among the alternative sigma factors of Listeria monocytogenes, σB controls the largest regulon. The aim of this study was to perform a comprehensive review of σB-regulated genes, and the functions they confer. Materials & methods: A systematic search of PubMed and Web of Knowledge was carried out to identify members of the σB regulon based on experimental evidence of σB-dependent transcription and presence of a consensus σB-dependent promoter. Results: The literature review identified σB-dependent transcription units encompassing 304 genes encoding different functions including stress response and virulence. Conclusion: Our review supports the well-known roles of σB in virulence and stress response and provides new insight into novel roles for σB in metabolism and overall resilience of L. monocytogenes.

Role and regulation of the stress activated sigma factor sigma B (σB) in the saprophytic and host-associated life stages of Listeria monocytogenes

The stress activated sigma factor sigma B (σ^B) plays a pivotal role in allowing the food-borne bacterial pathogen Listeria monocytogenes to modulate its transcriptional landscape in order to survive in a variety of harsh environments both outside and within the host. While we have a comparatively good understanding of the systems under the control of this sigma factor much less is known about how the activity of σ^B is controlled. In this review, we present a current model describing how this sigma factor is thought to be controlled including an overview of what is known about stress sensing and the early signal transduction events that trigger its activation. We discuss the known regulatory overlaps between σ^B and other protein and RNA regulators in the cell. Finally, we describe the role of σ^B in surviving both saprophytic and host-associated stresses. The complexity of the regulation of this sigma factor reflects the significant role that it plays in the persistence of this important pathogen in the natural environment, the food chain as well as within the host during the early stages of an infection. Understanding its regulation will be a critical step in helping to develop rational strategies to prevent its growth and survival in the food destined for human consumption and in the prevention of listeriosis.

Resistance of Listeria monocytogenes to Stress Conditions Encountered in Food and Food Processing Environments

Listeria monocytogenes is a human food-borne facultative intracellular pathogen that is resistant to a wide range of stress conditions. As a consequence, L. monocytogenes is extremely difficult to control along the entire food chain from production to storage and consumption. Frequent and recent outbreaks of L. monocytogenes infections illustrate that current measures of decontamination and preservation are suboptimal to control L. monocytogenes in food. In order to develop efficient measures to prevent contamination during processing and control growth during storage of food it is crucial to understand the mechanisms utilized by L. monocytogenes to tolerate the stress conditions in food matrices and food processing environments. Food-related stress conditions encountered by L. monocytogenes along the food chain are acidity, oxidative and osmotic stress, low or high temperatures, presence of bacteriocins and other preserving additives, and stresses as a consequence of applying alternative decontamination and preservation technologies such high hydrostatic pressure, pulsed and continuous UV light, pulsed electric fields (PEF). This review is aimed at providing a summary of the current knowledge on the response of L. monocytogenes toward these stresses and the mechanisms of stress resistance employed by this important food-borne bacterium. Circumstances when L. monocytogenes cells become more sensitive or more resistant are mentioned and existence of a cross-resistance when multiple stresses are present is pointed out.

Cyclic di-adenosine monophosphate (c-di-AMP) is required for osmotic regulation in Staphylococcus aureus but dispensable for viability in anaerobic conditions

Cyclic di-adenosine monophosphate (c-di-AMP) is a recently discovered signaling molecule important for the survival of Firmicutes, a large bacterial group that includes notable pathogens such as Staphylococcus aureus However, the exact role of this molecule has not been identified. dacA, the S. aureus gene encoding the diadenylate cyclase enzyme required for c-di-AMP production, cannot be deleted when bacterial cells are grown in rich medium, indicating that c-di-AMP is required for growth in this condition. Here, we report that an S. aureus dacA mutant can be generated in chemically defined medium. Consistent with previous findings, this mutant had a severe growth defect when cultured in rich medium. Using this growth defect in rich medium, we selected for suppressor strains with improved growth to identify c-di-AMP-requiring pathways. Mutations bypassing the essentiality of dacA were identified in alsT and opuD, encoding a predicted amino acid and osmolyte transporter, the latter of which we show here to be the main glycine betaine-uptake system in S. aureus. Inactivation of these transporters likely prevents the excessive osmolyte and amino acid accumulation in the cell, providing further evidence for a key role of c-di-AMP in osmotic regulation. Suppressor mutations were also obtained in hepS, hemB, ctaA, and qoxB, coding proteins required for respiration. Furthermore, we show that dacA is dispensable for growth in anaerobic conditions. Together, these findings reveal an essential role for the c-di-AMP signaling network in aerobic, but not anaerobic, respiration in S. aureus.

Probiotic engineering: towards development of robust probiotic strains with enhanced functional properties and for targeted control of enteric pathogens

There is a growing concern about the increase in human morbidity and mortality caused by foodborne pathogens. Antibiotics were and still are used as the first line of defense against these pathogens, but an increase in the development of bacterial antibiotic resistance has led to a need for alternative effective interventions. Probiotics are used as dietary supplements to promote gut health and for prevention or alleviation of enteric infections. They are currently used as generics, thus making them non-specific for different pathogens. A good understanding of the infection cycle of the foodborne pathogens as well as the virulence factors involved in causing an infection can offer an alternative treatment with specificity. This specificity is attained through the bioengineering of probiotics, a process by which the specific gene of a pathogen is incorporated into the probiotic. Such a process will subsequently result in the inhibition of the pathogen and hence its infection. Recombinant probiotics offer an alternative novel therapeutic approach in the treatment of foodborne infections. This review article focuses on various strategies of bioengineered probiotics, their successes, failures and potential future prospects for their applications.

Shedding light on betL*: pPL2-lux mediated real-time analysis of betL* expression in Listeria monocytogenes

We propose a mechanism of action for the betL* mutation which is based on DNA topology. Removing a single thymine residue from the betL σ(A) promoter's -10 and -35 spacer results in a 'twist'-mediated activation of transcription which accounts for the osmotolerance phenotype observed for strains expressing betL*.

The Role of Stress and Stress Adaptations in Determining the Fate of the Bacterial Pathogen Listeria monocytogenes in the Food Chain

The foodborne pathogen Listeria monocytogenes is a highly adaptable organism that can persist in a wide range of environmental and food-related niches. The consumption of contaminated ready-to-eat foods can cause infections, termed listeriosis, in vulnerable humans, particularly those with weakened immune systems. Although these infections are comparatively rare they are associated with high mortality rates and therefore this pathogen has a significant impact on food safety. L. monocytogenes can adapt to and survive a wide range of stress conditions including low pH, low water activity, and low temperature, which makes it problematic for food producers who rely on these stresses for preservation. Stress tolerance in L. monocytogenes can be explained partially by the presence of the general stress response (GSR), a transcriptional response under the control of the alternative sigma factor sigma B (σB) that reconfigures gene transcription to provide homeostatic and protective functions to cope with the stress. Within the host σB also plays a key role in surviving the harsh conditions found in the gastrointestinal tract. As the infection progresses beyond the GI tract L. monocytogenes uses an intracellular infectious cycle to propagate, spread and remain protected from the host's humoral immunity. Many of the virulence genes that facilitate this infectious cycle are under the control of a master transcriptional regulator called PrfA. In this review we consider the environmental reservoirs that enable L. monocytogenes to gain access to the food chain and discuss the stresses that the pathogen must overcome to survive and grow in these environments. The overlap that exists between stress tolerance and virulence is described. We review the principal measures that are used to control the pathogen and point to exciting new approaches that might provide improved means of control in the future.

Under the microscope: From pathogens to probiotics and back

The review centers on the human gastrointestinal tract; focusing first on the bacterial stress responses needed to overcome the physiochemical defenses of the host, specifically how these stress survival strategies can be used as targets for alternative infection control strategies. The concluding section focuses on recent developments in molecular diagnostics; centring on the shifting paradigm from culture to molecular based diagnostics.

Cronobacter sakazakii: stress survival and virulence potential in an opportunistic foodborne pathogen

A characteristic feature of the opportunistic foodborne pathogen Cronobacter sakazakii is its ability to survive in extremely arid environments, such as powdered infant formula, making it a dangerous opportunistic pathogen of individuals of all age groups, especially infants and neonates. Herein, we provide a brief overview of the pathogen; clinical manifestations, environmental reservoirs and our current understanding of stress response mechanisms and virulence factors which allow it to cause disease.

The food safety impact of salt and sodium reduction initiatives

Excessive or high salt or sodium intake is known to cause hypertension and other diseases. Within the United Kingdom voluntary targets for salt reduction have been set and laid out in the Secretary of State responsibility deal. This review considers the options available to food manufacturers to enable them to reduce salt and the potential food safety risks associated with those options. Gaps in research and knowledge within the areas of information supplied to food manufacturers, alternative solutions for salt replacement and the food safety impact of salt reduction are discussed.

Analysis of the role of the Cronobacter sakazakii ProP homologues in osmotolerance

Bacteria respond to elevated osmolality by the accumulation of a range of low molecular weight molecules, known as compatible solutes (owing to their compatibility with the cells' normal physiology at high internal concentrations). The neonatal pathogen Cronobacter sakazakii is uniquely osmotolerant, surviving in powdered infant formula (PIF) which typically has a water activity (aw) of 0.2 - inhospitable to most micro-organisms. Mortality rates of up to 80% in infected infants have been recorded making C. sakazakii a serious cause for concern. In silico analysis of the C. sakazakii BAA-894 genome revealed seven copies of the osmolyte uptake system ProP. Herein, we test the physiological role of each of these homologues following heterologous expression against an osmosensitive Escherichia coli host.

Metagenomics and novel gene discovery: promise and potential for novel therapeutics

Metagenomics provides a means of assessing the total genetic pool of all the microbes in a particular environment, in a culture-independent manner. It has revealed unprecedented diversity in microbial community composition, which is further reflected in the encoded functional diversity of the genomes, a large proportion of which consists of novel genes. Herein, we review both sequence-based and functional metagenomic methods to uncover novel genes and outline some of the associated problems of each type of approach, as well as potential solutions. Furthermore, we discuss the potential for metagenomic biotherapeutic discovery, with a particular focus on the human gut microbiome and finally, we outline how the discovery of novel genes may be used to create bioengineered probiotics.

A single point mutation in the listerial betL σ(A)-dependent promoter leads to improved osmo- and chill-tolerance and a morphological shift at elevated osmolarity

Betaine uptake in Listeria monocytogenes is mediated by three independent transport systems, the simplest of which in genetic terms is the secondary transporter BetL. Using a random mutagenesis approach, based on the E. coli XL1 Red mutator strain, we identified a single point mutation in a putative promoter region upstream of the BetL coding region which leads to a significant increase in betL transcript levels under osmo- and chill-stress conditions and a concomitant increase in stress tolerance. Furthermore, the mutation appears to counter the heretofore unreported “twisted” cell morphology observed for L. monocytogenes grown at elevated osmolarities in tryptone soy broth.

Enhancing the stress responses of probiotics for a lifestyle from gut to product and back again

Before a probiotic bacterium can even begin to fulfill its biological role, it must survive a battery of environmental stresses imposed during food processing and passage through the gastrointestinal tract (GIT). Food processing stresses include extremes in temperature, as well as osmotic, oxidative and food matrix stresses. Passage through the GIT is a hazardous journey for any bacteria with deleterious lows in pH encountered in the stomach to the detergent-like properties of bile in the duodenum. However, bacteria are equipped with an array of defense mechanisms to counteract intracellular damage or to enhance the robustness of the cell to withstand lethal external environments. Understanding these mechanisms in probiotic bacteria and indeed other bacterial groups has resulted in the development of a molecular toolbox to augment the technological and gastrointestinal performance of probiotics. This has been greatly aided by studies which examine the global cellular responses to stress highlighting distinct regulatory networks and which also identify novel mechanisms used by cells to cope with hazardous environments. This review highlights the latest studies which have exploited the bacterial stress response with a view to producing next-generation probiotic cultures and highlights the significance of studies which view the global bacterial stress response from an integrative systems biology perspective.

Osmoadaptive strategies of the archaeon Halococcus hamelinensis isolated from a hypersaline stromatolite environment

Biogenic stromatolites are sources of significant information on the evolution of microbial life. Despite their evolutionary significance, little is known about the mechanisms of osmoadaptation by microorganisms that comprise living stromatolites thriving in hypersaline environments. Osmoadaptive strategies for Halococcus hamelinensis, a novel halophilic archaeon recently isolated from living stromatolites in the hypersaline reaches of Shark Bay, were thus a particular interest in this study. To investigate the possibility of "salt-in-cytoplasm"-associated osmoadaptation for this archaeon, flame photometry studies were performed. From the results, it was evident that this halophilic archaeon did not accumulate intracellular K(+) ions when cells were exposed to either osmotic shock or conditions with gradual increments in salinity. These results were further supported by polymerase chain reaction (PCR) analyses where there was no evidence for the existence of homologous genes to an ATP-driven, high-affinity potassium uptake system in Halococcus hamelinensis. To identify an alternative salt adaptation mechanism associated with accumulation of compatible solutes for this archaeon, (1)H nuclear magnetic resonance (NMR) spectroscopy experiments were carried out. Results indicate that glycine betaine, trehalose, and glutamate are solutes likely to be involved in osmoregulation in this archeaon. Subsequent (1)H NMR analysis of cell extracts from this microorganism grown under various NaCl concentrations revealed that intracellular levels of glycine betaine increased with increasing concentrations of NaCl. This behavior of increasing glycine betaine concentration with increasing external NaCl is consistent with its identity as an osmolyte. In contrast, intracellular levels of trehalose were decreased in high concentrations of NaCl. This provides evidence that compatible solute accumulation appears to be the preferential salt regulation mechanism for this haloarchaeon, in contrast to the salt-in-cytoplasm strategy employed by many other halophilic archaea.

A role for proline synthesis and transport in Listeria monocytogenes barotolerance

AIMS

To assess the contribution of proline biosynthesis to listerial barotolerance.

METHODS AND RESULTS

Using a Listeria monocytogenes proBA deletion mutant, incapable of synthesizing proline, together with a proline-overproducing strain, the contribution of proline synthesis to listerial barotolerance was determined. The ΔproBA strain does not survive as well as the wild type when subjected to treatment of 500 MPa in rich media and 400 MPa in minimal media (c. 1 log lower survival in both conditions). Betaine and carnitine decrease the ability of the wild type to survive at low pressures (300 MPa), but confer normal or slightly increased levels of protection at higher pressures (350 and 400 MPa).

CONCLUSIONS

A functional proline synthesis system is required for optimal survival of Listeria following treatment at high-pressure (HP) levels (500 MPa in brain heart infusion and 400 MPa in defined medium), particularly where other compatible solutes are absent or limiting.

SIGNIFICANCE AND IMPACT OF THE STUDY

  Given the potential of HP processing as an effective food processing/safety strategy, understanding how pathogens such as Listeria have evolved to cope with such stresses is an important food safety consideration. In this context, the work presented here may help to develop safer and more effective processing regimens.

The BCCT family of carriers: from physiology to crystal structure

Increases in the environmental osmolarity are key determinants for the growth of microorganisms. To ensure a physiologically acceptable level of cellular hydration and turgor at high osmolarity, many bacteria accumulate compatible solutes. Osmotically controlled uptake systems allow the scavenging of these compounds from scarce environmental sources as effective osmoprotectants. A number of these systems belong to the BCCT family (betaine-choline-carnitine-transporter), sodium- or proton-coupled transporters (e.g. BetP and BetT respectively) that are ubiquitous in microorganisms. The BCCT family also contains CaiT, an L-carnitine/γ-butyrobetaine antiporter that is not involved in osmotic stress responses. The glycine betaine transporter BetP from Corynebacterium glutamicum is a representative for osmoregulated symporters of the BCCT family and functions both as an osmosensor and osmoregulator. The crystal structure of BetP in an occluded conformation in complex with its substrate glycine betaine and two crystal structures of CaiT in an inward-facing open conformation in complex with L-carnitine and γ-butyrobetaine were reported recently. These structures and the wealth of biochemical data on the activity control of BetP in response to osmotic stress enable a correlation between the sensing of osmotic stress by a transporter protein with the ensuing regulation of transport activity. Molecular determinants governing the high-affinity binding of the compatible solutes by BetP and CaiT, the coupling in symporters and antiporters, and the osmoregulatory properties are discussed in detail for BetP and various BCCT carriers.

Novel listerial genetic loci conferring enhanced barotolerance in Escherichia coli

AIMS

To identify Listeria monocytogenes genes with a role in high-pressure (HP) resistance.

METHODS AND RESULTS

A L. monocytogenes genomic library constructed in an Escherichia coli background was screened for loci conferring increased HP resistance. Pressure treatments at 400 megapascals for 5 min in Luria-Bertani (LB) agar were used to screen for increased resistance to pressure. Colonies arising on the treated agar plates were isolated, the plasmid extracted and the inserts sequenced to identify the genetic loci conferring HP resistance. Seven different genetic regions were identified, which encoded proteins similar to an inorganic polyphosphate/ATP-NAD kinase, the septation ring formation regulator EzrA, flagellar motor proteins MotA and MotB, proteins similar to the quorum sensing Agr system from Staphylococcus (AgrA, AgrC and AgrD), proteins similar to a transcription regulator (RpiR family) and a fructose phosphotransferase system, proteins of unknown function, and a Fur regulator. Of the seven loci confirmed, three (EzrA, MotA/B and the Agr system) maintained significantly reproducible HP tolerance when expressed in a different E. coli background.

CONCLUSIONS

Novel genetic loci from the L. monocytogenes genome confer increased HP resistance when heterologously expressed in an E. coli background.

SIGNIFICANCE AND IMPACT OF THE STUDY

Molecular and functional approaches to the screening of genetic elements linked to HP resistance provide greater insights into microbial inactivation and/or survival mechanisms when using HP as a means of controlling/eliminating bacterial growth. This information will ultimately have significant implications for the use of HP processing in the food industry, in terms of both food quality and safety.

Compatible solutes: the key to Listeria's success as a versatile gastrointestinal pathogen?

Recently we reported a role for compatible solute uptake in mediating bile tolerance and increased gastrointestinal persistence in the foodborne pathogen Listeria monocytogenes[1]. Herein, we review the evolution in our understanding of how these low molecular weight molecules contribute to growth and survival of the pathogen both inside and outside the body, and how this stress survival mechanism may ultimately be used to target and kill the pathogen.

Salt stress phenotypes in Listeria monocytogenes vary by genetic lineage and temperature

Listeria monocytogenes can survive and grow under wide-ranging environmental stress conditions encountered both in foods and in the host. The ability of certain L. monocytogenes subtypes to thrive under stress conditions present in specific niches was hypothesized to reflect genetic characteristics and phenotypic capabilities conserved among strains within a subtype. To quantify variations in salt stress phenotypes among 40 strains selected to represent the diversity of the three major L. monocytogenes genetic lineages and to determine if salt stress phenotypes were associated with genetic relatedness, we measured growth under salt stress at both 7°C and 37°C. At 7°C, in brain-heart infusion with 6% NaCl, average growth rates among the lineages were similar. A comparison of doubling times after exposure to salt stress at 7°C or 37°C indicated that growth at 7°C provided crossprotection to subsequent salt stress for strains in lineages I and II. At 37°C, in brain-heart infusion with 6% NaCl, lineage I and III strains grew significantly faster (p<0.0001) than lineage II strains. Under salt stress at 37°C, differences in growth parameters were significantly (p<0.005) associated with genetic relatedness of the strains. Compatible solute uptake is part of the L. monocytogenes salt stress response, but growth differences between the lineages were not related to differences in transcript levels of osmolyte transporter-encoding genes betL, gbuA, oppA, and opuCA. The combination of phylogenetic and phenotypic data suggests that L. monocytogenes lineage I and III strains, which are most commonly associated with human and animal disease, may be better adapted to osmotic stress at 37°C, conditions that are present in the host gastrointestinal tract.

Compatible solutes: A listerial passe-partout?

Recently we reported a role for compatible solute uptake in mediating bile tolerance and increased gastrointestinal persistence in the foodborne pathogen Listeria monocytogenes.1 Herein, we review the evolution in our understanding of how these low molecular weight molecules contribute to growth and survival of the pathogen both inside and outside the body, and how this stress survival mechanism may ultimately be used to target and kill the pathogen.

Probiotics and gastrointestinal disease: successes, problems and future prospects

Gastrointestinal disease is a major cause of morbidity and mortality worldwide each year. Treatment of chronic inflammatory gastrointestinal conditions such as ulcerative colitis and Crohn's disease is difficult due to the ambiguity surrounding their precise aetiology. Infectious gastrointestinal diseases, such as various types of diarrheal disease are also becoming increasingly difficult to treat due to the increasing dissemination of antibiotic resistance among microorganisms and the emergence of the so-called 'superbugs'. Taking into consideration these problems, the need for novel therapeutics is essential. Although described for over a century probiotics have only been extensively researched in recent years. Their use in the treatment and prevention of disease, particularly gastrointestinal disease, has yielded many successful results, some of which we outline in this review. Although promising, many probiotics are hindered by inherent physiological and technological weaknesses and often the most clinically promising strains are unusable. Consequently we discuss various strategies whereby probiotics may be engineered to create designer probiotics. Such innovative approaches include; a receptor mimicry strategy to create probiotics that target specific pathogens and toxins, a patho-biotechnology approach using pathogen-derived genes to create more robust probiotic stains with increased host and processing-associated stress tolerance profiles and meta-biotechnology, whereby, functional metagenomics may be used to identify novel genes from diverse and vastly unexplored environments, such as the human gut, for use in biotechnology and medicine.

Rational design of improved pharmabiotics

Herein we review the most recent advances in probiotic research and applications with particular emphasis on the novel concept of patho-biotechnology: the application of pathogen-derived (ex vivo and in vivo) stress survival strategies for the design of more technologically robust and effective probiotic cultures with improved biotechnological and clinical applications.

Specific osmolyte transporters mediate bile tolerance in Listeria monocytogenes

The food-borne pathogenic bacterium Listeria monocytogenes has the potential to adapt to an array of suboptimal growth environments encountered within the host. The pathogen is relatively bile tolerant and has the capacity to survive and grow within both the small intestine and the gallbladder in murine models of oral infection. We have previously demonstrated a role for the principal carnitine transport system of L. monocytogenes (OpuC) in gastrointestinal survival of the pathogen (R. Sleator, J. Wouters, C. G. M. Gahan, T. Abee, and C. Hill, Appl. Environ. Microbiol. 67:2692-2698, 2001). However, the mechanisms by which OpuC, or indeed carnitine, protects the pathogen in this environment are unclear. In the current study, systematic analysis of strains with mutations in osmolyte transporters revealed a role for OpuC in resisting the acute toxicity of bile, with a minor role also played by BetL, a secondary betaine uptake system which also exhibits a low affinity for carnitine. In addition, the toxic effects of bile on wild-type L. monocytogenes cells were ameliorated when carnitine (but not betaine) was added to the medium. lux-promoter fusions to the promoters of the genes encoding the principal osmolyte uptake systems Gbu, BetL, and OpuC and the known bile tolerance system BilE were constructed. Promoter activity for all systems was significantly induced in the presence of bile, with the opuC and bilE promoters exhibiting the highest levels of bile-dependent expression in vitro and the betL and bilE promoters showing the highest expression levels in the intestines of orally inoculated mice. A direct comparison of all osmolyte transporter mutants in a murine oral infection model confirmed a major role for OpuC in intestinal persistence and systemic invasion and a minor role for the BetL transporter in fecal carriage. This study therefore demonstrates a previously unrecognized function for osmolyte uptake systems in bile tolerance in L. monocytogenes.

Listeria as an enteroinvasive gastrointestinal pathogen

The bacterium Listeria monocytogenes is the causative agent of listeriosis, a highly fatal opportunistic foodborne infection. Listeria spp. are isolated from a diversity of environmental sources, including soil, water, effluents, a large variety of foods, and the feces of humans and animals. Recent outbreaks demonstrated that L. monocytogenes can cause gastroenteritis in otherwise healthy individuals and more severe invasive disease in immunocompromised patients. Common symptoms include fever, watery diarrhea, nausea, headache, and pains in joints and muscles. The intestinal tract is the major portal of entry for L. monocytogenes, whereby strains penetrate the mucosal tissue either directly, via invasion of enterocytes, or indirectly, via active penetration of the Peyer's patches. Studies have revealed the strategy taken by the bacteria to overcome changes in oxygen tension, osmolarity, acidity, and the sterilizing effects of bile or antimicrobial peptides to adapt to conditions in the gut. In addition, L. monocytogenes has evolved species-specific strategies for intestinal entry by exploiting the interaction between the internalin protein and its receptor E-cadherin, or inducing diarrhea and an inflammatory response via the activity of its hemolytic toxin, listeriolysin. The ability of these bacteria to survive in bile-rich environments, and to induce depletion of sentinel cells such as Paneth cells that monitor the luminal burden of commensal bacteria, suggest strategies that have evolved to promote intestinal survival. Preexisting gastrointestinal disease may be a risk factor for infection of the gastrointestinal tract with L. monocytogenes. Currently, there is enough evidence to warrant consideration of L. monocytogenes as a possible etiology in outbreaks of febrile gastroenteritis, and for further studies to examine the genetic structure of Listeria strains that have a propensity to cause gastrointestinal versus systemic infections.

Molecular analysis of the microbial food safety implications of food reformulations for improved health

Food reformulation is commonly used as a strategy to produce foods for improved health; for example, replacing sugar with aspartame, and salt (NaCl) with KCl may help to reduce the incidence of obesity and heart disease. However, such reformulations will also change the intrinsic physicochemical properties of the food, which may in turn support the growth of foodborne pathogens and ultimately increase the incidence of foodborne disease. Thus, we need a better understanding of the microbiological food safety issues associated with product reformulation. Herein we review the most recent advances in our understanding of how microbial pathogens adapt to changes in the food composition, and how this information may ultimately be used for the design of effective pathogen control measures.

Pseudomonas syringae BetT is a low-affinity choline transporter that is responsible for superior osmoprotection by choline over glycine betaine

The plant pathogen Pseudomonas syringae derives better osmoprotection from choline than from glycine betaine, unlike most bacteria that have been characterized. In this report, we identified a betaine/carnitine/choline family transporter (BCCT) in P. syringae pv. tomato strain DC3000 that mediates the transport of choline and acetylcholine. This transporter has a particularly low affinity (K(m) of 876 microM) and high capacity (V(max) of 80 nmol/min/mg of protein) for choline transport relative to other known BCCTs. Although BetT activity increased in response to hyperosmolarity, BetT mediated significant uptake under low-osmolarity conditions, suggesting a role in transport for both osmoprotection and catabolism. Growth studies with mutants deficient in BetT and other choline transporters demonstrated that BetT was responsible for the superior osmoprotection conferred to P. syringae by choline over glycine betaine when these compounds were provided at high concentrations (>100 microM). These results suggest that P. syringae has evolved to survive in relatively choline-rich habitats, a prediction that is supported by the common association of P. syringae with plants and the widespread production of choline, but genus- and species-specific production of glycine betaine, by plants. Among the three putative BCCT family transporters in Pseudomonas aeruginosa and six in Pseudomonas putida, different transporters were predicted to function based on similarity to Escherichia coli BetT than to P. syringae BetT. Functional P. putida and P. aeruginosa transporters were identified, and their possession of a long C-terminal tail suggested an osmoregulatory function for this tail; this function was confirmed for P. syringae BetT using deletion derivatives.

High-pressure processing--effects on microbial food safety and food quality

High-pressure processing (HPP) is a nonthermal process capable of inactivating and eliminating pathogenic and food spoilage microorganisms. This novel technology has enormous potential in the food industry, controlling food spoilage, improving food safety and extending product shelf life while retaining the characteristics of fresh, preservative-free, minimally processed foods. As with other food processing methods, such as thermal processing, HPP has somewhat limited applications as it cannot be universally applied to all food types, such as some dairy and animal products and shelf-stable low-acid foods. Herein, we discuss the effects of high-pressure processing on microbial food safety and, to a lesser degree, food quality.

Improving gastric transit, gastrointestinal persistence and therapeutic efficacy of the probiotic strain Bifidobacterium breve UCC2003

Given the increasing commercial and clinical relevance of probiotic cultures, improving their stress tolerance profile and ability to overcome the physiological defences of the host is an important biological goal. In order to reach the gastrointestinal tract in sufficient numbers to exert a therapeutic effect, probiotic bacteria must resist the deleterious actions of low pH, elevated osmolarity and bile salts. Cloning the listerial betaine uptake system, BetL, into the probiotic strain Bifidobacterium breve UCC2003 significantly improved probiotic tolerance to gastric juice and conditions of elevated osmolarity mimicking the gut environment. Furthermore, whilst stable colonization of the murine intestine was achieved by oral administration of B. breve UCC2003, strains harbouring BetL were recovered at significantly higher levels in the faeces, intestines and caecum of inoculated animals. Finally, in addition to improved gastric transit and intestinal persistence, this approach improved the clinical efficacy of the probiotic culture: mice fed B. breve UCC2003-BetL(+) exhibited significantly lower levels of systemic infection compared to the control strain following oral inoculation with Listeria monocytogenes.

New frontiers in probiotic research

The increasing incidence of antibiotic resistance, coupled with a growing prevalence of cancer and allergic conditions in an aging population, has forced clinical research to explore alternative therapeutic and prophylactic avenues. One such approach involves the use of probiotics: beneficial bacterial cultures, which, when administered as a part of the daily dietary intake, reduce the incidence and severity of acute and chronic infection, facilitate prevention and reduced recurrence of certain cancers and lower the incidence of several atopic conditions. Herein, we review the most recent advances in the emerging area of patho-biotechnology in the context of improving probiotic production, delivery and clinical efficacy, in addition to the emerging area of 'designer probiotics'- strains specifically tailored to target certain pathogens and/or toxins in vivo.

Glycine betaine improves Listeria monocytogenes tolerance to desiccation on parsley leaves independent of the osmolyte transporters BetL, Gbu and OpuC

AIMS

To investigate the effect of glycine betaine (GB) on the survival of Listeria monocytogenes on leaf surfaces under low relative humidity (RH).

METHODS AND RESULTS

The addition of GB (> or = 25 mmol l(-1)) improved the survival of L. monocytogenes under low RH on parsley leaves, thus suggesting that GB can improve the tolerance of L. monocytogenes to desiccation. Ten times less GB was needed to improve L. monocytogenes survival under low RH on nonbiological surfaces compared with parsley leaves, suggesting that, on the leaf surface, L. monocytogenes may have to compete for the available GB with autochthonous bacteria and/or the plant itself. Wild type and mutants carrying deletions in the three GB uptake systems, BetL, Gbu and OpuC, behaved similarly with and without added GB on parsley leaves (P > 0.05). In addition, preaccumulation of GB, triggered by osmotic stress prior to inoculation, failed to improve survival under low RH compared with osmotic stress without GB accumulation.

CONCLUSIONS

Exogenous GB had a protective effect on L. monocytogenes cells from desiccation during survival on parsley leaves. This effect was independent of intracellular GB accumulation by the known uptake systems.

SIGNIFICANCE AND IMPACT OF THE STUDY

Presence of GB could improve the survival of L. monocytogenes to desiccation on leaf surfaces and nonbiological surfaces.

Patho-biotechnology; using bad bugs to make good bugs better

Given the increasing commercial and clinical relevance of probiotic cultures, improving their stress tolerance profile and ability to overcome the physiochemical defences of the host is an important biological goal. Pathogenic bacteria have evolved sophisticated strategies to overcome host defences, interact with the immune system and interfere with essential host systems. We coin the term 'patho-biotechnology' to describe the exploitation of these valuable traits in biotechnology and biomedicine. This approach shows promise for the design of more technologically robust and effective probiotic cultures with improved biotechnological and clinical applications as well as the development of novel vaccine and drug delivery platforms.

Bacterial osmosensing transporters

Cells faced with dehydration because of increasing extracellular osmotic pressure accumulate solutes through synthesis or transport. Water follows, restoring cellular hydration and volume. Prokaryotes and eukaryotes possess arrays of osmoregulatory genes and enzymes that are responsible for solute accumulation under osmotic stress. In bacteria, osmosensing transporters can detect increasing extracellular osmotic pressure and respond by mediating the uptake of organic osmolytes compatible with cellular functions ("compatible solutes"). This chapter reviews concepts and methods critical to the identification and study of osmosensing transporters. Like some experimental media, cytoplasm is a "nonideal" solution so the estimation of key solution properties (osmotic pressure, osmolality, water activity, osmolarity, and macromolecular crowding) is essential for studies of osmosensing and osmoregulation. Because bacteria vary widely in osmotolerance, techniques for its characterization provide an essential context for the elucidation of osmosensory and osmoregulatory mechanisms. Powerful genetic, molecular biological, and biochemical tools are now available to aid in the identification and characterization of osmosensory transporters, the genes that encode them, and the osmoprotectants that are their substrates. Our current understanding of osmosensory mechanisms is based on measurements of osmosensory transporter activity performed with intact cells, bacterial membrane vesicles, and proteoliposomes reconstituted with purified transporters. In the quest to elucidate the structural mechanisms of osmosensing and osmoregulation, researchers are now applying the full range of available biophysical, biochemical, and molecular biological tools to osmosensory transporter prototypes.

The sigma factor RpoN (sigma54) is involved in osmotolerance in Listeria monocytogenes

Listeria monocytogenes is able to grow under conditions of high osmolarity. We constructed a deletion mutant of rpoN, encoding the alternative sigma factor RpoN, and analyzed its response to osmotic stress. In a minimal medium with 4% NaCl and 1 mM betaine, the mutant showed a similar growth to that of the parental strain, EGD. In the same medium with 4% NaCl and 1 M carnitine, the growth rate of the mutant was greatly reduced, when the optical density at 600 nm (OD600) at the starting point of growth, was 0.15. However, when growth of the culture was started at an OD600 of 0.025, the growth of the mutant was similar to that of EGD. The mutant's expression of two betaine transporter genes, betL and gbuB, and the carnitine transporter gene opuCA, was osmotically induced at a level similar to EGD, and its rate of carnitine uptake was similar to that of EGD. These results suggest that the growth defect from the rpoN mutant is caused not by the transcriptional regulation of opuCA or by a decrease in carnitine uptake, but possibly by larger amounts of carnitine being needed for growth of the mutant in minimal medium when NaCl is present.

Halotolerant cyanobacterium Aphanothece halophytica contains a betaine transporter active at alkaline pH and high salinity

Aphanothece halophytica is a halotolerant alkaliphilic cyanobacterium which can grow in media of up to 3.0 M NaCl and pH 11. This cyanobacterium can synthesize betaine from glycine by three-step methylation using S-adenosylmethionine as a methyl donor. To unveil the mechanism of betaine uptake and efflux in this alkaliphile, we isolated and characterized a betaine transporter. A gene encoding a protein (BetT(A. halophytica)) that belongs to the betaine-choline-carnitine transporter (BCCT) family was isolated. Although the predicted isoelectric pH of a typical BCCT family transporter, OpuD of Bacillus subtilis, is basic, 9.54, that of BetT(A. halophytica) is acidic, 4.58. BetT(A. halophytica) specifically catalyzed the transport of betaine. Choline, gamma-aminobutyric acid, betaine aldehyde, sarcosine, dimethylglycine, and amino acids such as proline did not compete for the uptake of betaine by BetT(A. halophytica). Sodium markedly enhanced betaine uptake rates, whereas potassium and other cations showed no effect, suggesting that BetT(A. halophytica) is a Na(+)-betaine symporter. Betaine uptake activities of BetT(A. halophytica) were high at alkaline pH values, with the optimum pH around 9.0. Freshwater Synechococcus cells overexpressing BetT(A. halophytica) showed NaCl-activated betaine uptake activities with enhanced salt tolerance, allowing growth in seawater supplemented with betaine. Kinetic properties of betaine uptake in Synechococcus cells overexpressing BetT(A. halophytica) were similar to those in A. halophytica cells. These findings indicate that A. halophytica contains a Na(+)-betaine symporter that contributes to the salt stress tolerance at alkaline pH. BetT(A. halophytica) is the first identified transporter for compatible solutes in cyanobacteria.

Cold stress tolerance of Listeria monocytogenes: A review of molecular adaptive mechanisms and food safety implications

The foodborne pathogen Listeria monocytogenes has many physiological adaptations that enable survival under a wide range of environmental conditions. The microbes overcome various types of stress, including the cold stress associated with low temperatures in food-production and storage environments. Cold stress adaptation mechanisms are therefore an important attribute of L. monocytogenes, enabling these food pathogens to survive and proliferate to reach minimal infectious levels on refrigerated foods. This phenomenon is a function of many molecular adaptation mechanisms. Therefore, an improved understanding of how cold stress is sensed and adaptation measures implemented by L. monocytogenes may facilitate the development of better ways of controlling these pathogens in food and related environments. Research over the past few years has highlighted some of the molecular aspects of cellular mechanisms behind cold stress adaptation in L. monocytogenes. This review provides an overview of the molecular and physiological constraints of cold stress and discusses the various cellular cold stress response mechanisms in L. monocytogenes, as well as their implications for food safety.

Heterologous expression of BetL, a betaine uptake system, enhances the stress tolerance of Lactobacillus salivarius UCC118

Given the increasing commercial and clinical relevance of probiotic cultures, improving the technological robustness of what are often process-sensitive cultures is an important biological goal. The nisin-controlled expression system was used to direct the heterologous expression of the listerial betaine uptake system BetL in the probiotic strain Lactobacillus salivarius UCC118. Following nisin induction, strains expressing betL exhibited a significant increase in resistance to several stresses, including elevated osmo-, cryo-, baro-, and chill tolerance, as well as increased resistance to spray- and freeze-drying. The ability to confer additional stress tolerance on a probiotic culture may be an important step in delivering viable cultures for maximal efficacy.

Identification and characterization of Di- and tripeptide transporter DtpT of Listeria monocytogenes EGD-e

Listeria monocytogenes is a gram-positive intracellular pathogen responsible for opportunistic infections in humans and animals. Here we identified and characterized the dtpT gene (lmo0555) of L. monocytogenes EGD-e, encoding the di- and tripeptide transporter, and assessed its role in growth under various environmental conditions as well as in the virulence of L. monocytogenes. Uptake of the dipeptide Pro-[14C]Ala was mediated by the DtpT transporter and was abrogated in a DeltadtpT isogenic deletion mutant. The DtpT transporter was shown to be required for growth when the essential amino acids leucine and valine were supplied as peptides. The protective effect of glycine- and proline-containing peptides during growth in defined medium containing 3% NaCl was noted only in L. monocytogenes EGD-e, not in the DeltadtpT mutant strain, indicating that the DtpT transporter is involved in salt stress protection. Infection studies showed that DtpT contributes to pathogenesis in a mouse infection model but has no role in bacterial growth following infection of J774 macrophages. These studies reveal that DptT may contribute to the virulence of L. monocytogenes.

OusB, a broad-specificity ABC-type transporter from Erwinia chrysanthemi, mediates uptake of glycine betaine and choline with a high affinity

The ability of Erwinia chrysanthemi to cope with environments of elevated osmolality is due in part to the transport and accumulation of osmoprotectants. In this study we have identified a high-affinity glycine betaine and choline transport system in E. chrysanthemi. By using a pool of Tn5-B21 ousA mutants, we isolated a mutant that could grow in the presence of a toxic analogue of glycine betaine (benzyl-glycine betaine) at high osmolalities. This mutant was impaired in its ability to transport all effective osmoprotectants in E. chrysanthemi. The DNA sequence of the regions flanking the transposon insertion site revealed three chromosomal genes (ousVWX) that encode components of an ABC-type transporter (OusB): OusV (ATPase), OusW (permease), and OusX (periplasmic binding protein). The OusB components showed a significant degree of sequence identity to components of ProU from Salmonella enterica serovar Typhimurium and Escherichia coli. OusB was found to restore the uptake of glycine betaine and choline through functional complementation of an E. coli mutant defective in both ProU and ProP osmoprotectant uptake systems. Competition experiments demonstrated that choline, dimethylsulfoniacetate, dimethylsulfoniopropionate, and ectoine were effective competitors for OusB-mediated betaine transport but that carnitine, pipecolate, and proline were not effective. In addition, the analysis of single and double mutants showed that OusA and OusB were the only osmoprotectant transporters operating in E. chrysanthemi.

Identification and functional characterisation of genes and corresponding enzymes involved in carnitine metabolism of Proteus sp

Enzymes involved in carnitine metabolism of Proteus sp. are encoded by the cai genes organised as the caiTABCDEF operon. The complete operon could be sequenced from the genomic DNA of Proteus sp. Amino acid sequence similarities and/or enzymatic analysis confirmed the function assigned to each protein involved in carnitine metabolism. CaiT was suggested to be an integral membrane protein responsible for the transport of betaines. The caiA gene product was shown to be a crotonobetainyl-CoA reductase catalysing the irreversible reduction of crotonobetainyl-CoA to gamma-butyrobetainyl-CoA. CaiB and CaiD were identified to be the two components of the crotonobetaine hydrating system, already described. CaiB and caiD were cloned and expressed in Escherichia coli. After purification of both proteins, their individual enzymatic functions were solved. CaiB acts as betainyl-CoA transferase specific for carnitine, crotonobetaine, gamma-butyrobetaine and its CoA derivatives. Transferase reaction proceeds, following a sequential bisubstrate mechanism. CaiD was identified to be a crotonobetainyl-CoA hydratase belonging to the crotononase superfamily. Because of amino acid sequence similarities, CaiC was suggested to be a betainyl-CoA ligase. Taken together, these results show that the metabolism of carnitine and crotonobetaine in Proteus sp. proceeds at the CoA level.