Characterization of glycine betaine porter I from Listeria monocytogenes and its roles in salt and chill tolerance

Genomic and pathogenicity islands of Listeria monocytogenes-overview of selected aspects

Listeria monocytogenes causes listeriosis, a disease characterized by a high mortality rate (up to 30%). Since the pathogen is highly tolerant to changing conditions (high and low temperature, wide pH range, low availability of nutrients), it is widespread in the environment, e.g., water, soil, or food. L. monocytogenes possess a number of genes that determine its high virulence potential, i.e., genes involved in the intracellular cycle (e.g., prfA, hly, plcA, plcB, inlA, inlB), response to stress conditions (e.g., sigB, gadA, caspD, clpB, lmo1138), biofilm formation (e.g., agr, luxS), or resistance to disinfectants (e.g., emrELm, bcrABC, mdrL). Some genes are organized into genomic and pathogenicity islands. The islands LIPI-1 and LIPI-3 contain genes related to the infectious life cycle and survival in the food processing environment, while LGI-1 and LGI-2 potentially ensure survival and durability in the production environment. Researchers constantly have been searching for new genes determining the virulence of L. monocytogenes. Understanding the virulence potential of L. monocytogenes is an important element of public health protection, as highly pathogenic strains may be associated with outbreaks and the severity of listeriosis. This review summarizes the selected aspects of L. monocytogenes genomic and pathogenicity islands, and the importance of whole genome sequencing for epidemiological purposes.

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.

Adaptive Response of Listeria monocytogenes to the Stress Factors in the Food Processing Environment

Listeria monocytogenes are Gram-positive, facultatively anaerobic, non-spore-forming bacteria that easily adapt to changing environmental conditions. The ability to grow at a wide range of temperatures, pH, and salinity determines the presence of the pathogen in water, sewage, soil, decaying vegetation, and animal feed. L. monocytogenes is an etiological factor of listeriosis, especially dangerous for the elderly, pregnant women, and newborns. The major source of L. monocytogenes for humans is food, including fresh and smoked products. Its high prevalence in food is associated with bacterial adaptation to the food processing environment (FPE). Since the number of listeriosis cases has been progressively increasing an efficient eradication of the pathogen from the FPE is crucial. Understanding the mechanisms of bacterial adaptation to environmental stress will significantly contribute to developing novel, effective methods of controlling L. monocytogenes in the food industry.

Enhancement of pollutants removal from saline wastewater through simultaneous anammox and denitrification (SAD) process with glycine betaine addition

Enhanced pollutants removal from saline wastewater was investigated in simultaneous anammox and denitrification (SAD) process with glycine betaine (GB) addition. Long-term operation indicated the optimal GB dose was around 0.4 mM, which enhanced both anammox and denitrifying activity by 30% and 45%, respectively. The total nitrogen and organic removal rates were 0.38 ± 0.2 kgN/m³/d and 0.34 ± 0.3 kgCOD/m³/d, respectively, which increased by 34.5% and 20.5%. Independent of GB dose, denitrifying activity was promoted, but anammox activity was drastically deteriorated after excessive GB addition. The optimal GB dose predicated by both Gaussian and Modified-Boltzmann models were 0.42-0.45 mM. Besides, the bacterial activity recovery after excessive GB addition could be analyzed by the Modified-Boltzmann model. With 1.5 mM GB, granular floatation occurred since numerous gas bubbles were inside the granules. In general, exogenous GB addition can mitigate salinity inhibition and promote pollutants removal from saline wastewater.

Virulence genes expression in viable but non-culturable state of Listeria monocytogenes in fish meat

This study aimed to evaluate the fate of Listeria monocytogenes in water microcosm and rainbow trout fillet under salinity stress of 0% and 30% NaCl at refrigerator temperature (4 ± 2 ℃). Bacterial culturability was studied by standard culture and colony count method. Reverse transcription-PCR (RT-PCR) of 16 S rRNA gene was used to detect viability of non-culturable bacteria. Also, the qualitative expression of pathogenic genes (hly and inlA) was studied using RT-PCR. The results showed that bacteria in water microcosm lost their culturability at 13 days under 0% salinity (starvation or distilled water) and at 27 days under 30% salinity; however, bacteria in rainbow trout fillet remained culturable under 0% and 30% NaCl. RT-PCR of 16 S rRNA gene was positive for all treatments during the period of this study, indicating the entering of L. monocytogenes into the viable but non-culturable state in water microcosm under 0% and 30% NaCl. Also, viable but non-culturable L. monocytogenes retained the expression of hly and inlA genes. So, it could be concluded that L. monocytogenes in viable but non-culturable state can cause serious health problems and further investigation is necessary to elucidate the effects of other processing and storage conditions (light, dark, smoking, etc.) on behavior of L. monocytogenes in smoked and salted fish.

Assembly and Characterization of a Pathogen Strain Collection for Produce Safety Applications: Pre-growth Conditions Have a Larger Effect on Peroxyacetic Acid Tolerance Than Strain Diversity

Effective control of foodborne pathogens on produce requires science-based validation of interventions and control strategies, which typically involves challenge studies with a set of bacterial strains representing the target pathogens or appropriate surrogates. In order to facilitate these types of studies, a produce-relevant strain collection was assembled to represent strains from produce outbreaks or pre-harvest environments, including Listeria monocytogenes (n = 11), Salmonella enterica (n = 23), shiga-toxin producing Escherichia coli (STEC) (n = 13), and possible surrogate organisms (n = 8); all strains were characterized by whole genome sequencing (WGS). Strain diversity was assured by including the 10 most common S. enterica serotypes, L. monocytogenes lineages I-IV, and E. coli O157 as well as selected "non-O157" STEC serotypes. As it has previously been shown that strains and genetic lineages of a pathogen may differ in their ability to survive different stress conditions, a subset of representative strains for each "pathogen group" (e.g., Salmonella, STEC) was selected and assessed for survival of exposure to peroxyacetic acid (PAA) using strains pre-grown under different conditions including (i) low pH, (ii) high salt, (iii) reduced water activity, (iv) different growth phases, (v) minimal medium, and (vi) different temperatures (21°C, 37°C). The results showed that across the three pathogen groups pre-growth conditions had a larger effect on bacterial reduction after PAA exposure as compared to strain diversity. Interestingly, bacteria exposed to salt stress (4.5% NaCl) consistently showed the least reduction after exposure to PAA; however, for STEC, strains pre-grown at 21°C were as tolerant to PAA exposure as strains pre-grown under salt stress. Overall, our data suggests that challenge studies conducted with multi-strain cocktails (pre-grown under a single specific condition) may not necessarily reflect the relevant phenotypic range needed to appropriately assess different intervention strategies.

Expression profiling in a mammalian host reveals the strong induction of genes encoding LysM domain-containing proteins in Enterococcus faecium

Enterococcus faecium is an important health care-associated pathogen that is difficult to treat due to the high level of antibiotic resistance of clinical isolates. The identification of new potential therapeutic targets or vaccination strategies is therefore urgently needed. In this regard, we carried out a transcriptomic analysis of the E. faecium vancomycin-resistant strain AUS0004, comparing the gene expression of bacteria grown under laboratory conditions and bacteria isolated from an infection site. This analysis highlighted more than 360 genes potentially induced under infection conditions. Owing to their expression profiles, four LysM domain-containing proteins were characterized in more detail. The EFAU004_01059, 1150 and 494 proteins are highly homologous, whereas EFAU004_01209 has a unique domain-architecture and sequence. The analysis of corresponding mutants showed that all LysM proteins played relevant roles in the infection process of E. faecium in mice. The EFAU004_01209 mutant also displayed profound morphological modifications, suggesting it has a role in cell wall synthesis or cell division. Furthermore, the adhesion to kidney cells and growth of the mutant was affected in human urine. All these phenotypes and the surface exposure of EFAU004_01209 identify this protein as an interesting new drug target in E. faecium.

Ecology of cold environments: new insights of bacterial metabolic adaptation through an integrated genomic-phenomic approach

Cold environments dominate Earth's biosphere, hosting complex microbial communities with the ability to thrive at low temperatures. However, the underlying molecular mechanisms and the metabolic pathways involved in bacterial cold-adaptation mechanisms are still not fully understood. Herein, we assessed the metabolic features of the Antarctic bacterium Pseudoalteromonas haloplanktis TAC125 (PhTAC125), a model organism for cold-adaptation, at both 4 °C and 15 °C, by integrating genomic and phenomic (high-throughput phenotyping) data and comparing the obtained results to the taxonomically related Antarctic bacterium Pseudoalteromonas sp. TB41 (PspTB41). Although the genome size of PspTB41 is considerably larger than PhTAC125, the higher number of genes did not reflect any higher metabolic versatility at 4 °C as compared to PhTAC125. Remarkably, protein S-thiolation regulated by glutathione and glutathionylspermidine appeared to be a new possible mechanism for cold adaptation in PhTAC125. More in general, this study represents an example of how 'multi-omic' information might potentially contribute in filling the gap between genotypic and phenotypic features related to cold-adaptation mechanisms in bacteria.

Molecular analysis of the role of osmolyte transporters opuCA and betL in Listeria monocytogenes after cold and freezing stress

Listeria monocytogenes is a food-borne pathogen of humans and other animals. The striking ability to survive several stresses usually used for food preservation makes L. monocytogenes one of the biggest concerns to the food industry. This ubiquity can be partly explained by the ability of the organism to grow and persist at very low temperatures, a consequence of its ability to accumulate cryoprotective compound called osmolytes. A quantitative RT-PCR assay was used to measure mRNA transcript accumulation for the stress response genes opuCA and betL (encoding carnitine and betaine transporters, respectively) and the housekeeping gene 16S rRNA. Assays were conducted on mid-exponential phase L. monocytogenes cells exposed to conditions reflecting cold and freezing stress, conditions usually used to preserve foods. We showed that expression of the two cold-adapted genes encoded the transporters of the cryoprotectants carnitine and betaine in ATCC 19115 and the food-isolated L. monocytogenes S1 is induced after cold and freezing stress exposure. Furthermore, transcriptional analysis of the genes encoding opuCA and betL revealed that each transporter is induced to different degrees upon cold shock of L. monocytogenes ATCC 19115 and S1. Our results confirm an increase in carnitine uptake at low temperatures more than in betaine after cold-shocked temperature compared to the non-stress control treatment. It was concluded the use of carnitine and betaine as cryoprotectants is essential for rapid induction of the tested stress response under conditions typically encountered during food preservation.

Structural and functional analysis of betaine aldehyde dehydrogenase from Staphylococcus aureus

When exposed to high osmolarity, methicillin-resistant Staphylococcus aureus (MRSA) restores its growth and establishes a new steady state by accumulating the osmoprotectant metabolite betaine. Effective osmoregulation has also been implicated in the acquirement of a profound antibiotic resistance by MRSA. Betaine can be obtained from the bacterial habitat or produced intracellularly from choline via the toxic betaine aldehyde (BA) employing the choline dehydrogenase and betaine aldehyde dehydrogenase (BADH) enzymes. Here, it is shown that the putative betaine aldehyde dehydrogenase SACOL2628 from the early MRSA isolate COL (SaBADH) utilizes betaine aldehyde as the primary substrate and nicotinamide adenine dinucleotide (NAD(+)) as the cofactor. Surface plasmon resonance experiments revealed that the affinity of NAD(+), NADH and BA for SaBADH is affected by temperature, pH and buffer composition. Five crystal structures of the wild type and three structures of the Gly234Ser mutant of SaBADH in the apo and holo forms provide details of the molecular mechanisms of activity and substrate specificity/inhibition of this enzyme.

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.

Characterisation of the transcriptomes of genetically diverse Listeria monocytogenes exposed to hyperosmotic and low temperature conditions reveal global stress-adaptation mechanisms

The ability of Listeria monocytogenes to adapt to various food and food- processing environments has been attributed to its robustness, persistence and prevalence in the food supply chain. To improve the present understanding of molecular mechanisms involved in hyperosmotic and low-temperature stress adaptation of L. monocytogenes, we undertook transcriptomics analysis on three strains adapted to sub-lethal levels of these stress stimuli and assessed functional gene response. Adaptation to hyperosmotic and cold-temperature stress has revealed many parallels in terms of gene expression profiles in strains possessing different levels of stress tolerance. Gene sets associated with ribosomes and translation, transcription, cell division as well as fatty acid biosynthesis and peptide transport showed activation in cells adapted to either cold or hyperosmotic stress. Repression of genes associated with carbohydrate metabolism and transport as well as flagella was evident in stressed cells, likely linked to activation of CodY regulon and consequential cellular energy conservation.

Computational investigation on microsolvation of the osmolyte glycine betaine [GB (H(2)O)(1-7)]

The preferential interactions of glycine betaine (GB) with solvent components and the effect of solvent on its stability have been examined. In particular, the microsolvation of organic osmolyte and widely important osmoprotectant in nature as glycine betaine has been reported by using M06 method. A number of configurations (b(X) (a-z)) of the clusters for one to seven water molecules (× = 1-7) have been considered for the microsolvation. Structures of stable conformers are obtained and denoted as b1a, b2a, b3a, b4a, b5a, b6a and b7a. It is observed from the interaction energy difference (∆E) that only seven water molecules can be accommodated in the first solvation shell to stabilize GB. It is also observed that the calculated relative energy using M06 is in close agreement with calculations at the MP2 level of theory.

Global gene expression of Listeria monocytogenes to salt stress

Outbreaks of listeriosis caused by the ingestion of Listeria-contaminated ready-to-eat foods have been reported worldwide. Many ready-to-eat foods, such as deli meat products, contain high amounts of salt, which can disrupt the maintenance of osmotic balance within bacterial cells. To understand how Listeria monocytogenes adapts to salt stress, we examined the growth and global gene expression profiles of L. monocytogenes strain F2365 under salt stress using oligonucleotide probe-based DNA array and quantitative real-time PCR (qRT-PCR) analyses. The growth of L. monocytogenes in brain heart infusion (BHI) medium with various concentrations of NaCl (2.5, 5, and 10%) was significantly inhibited (P < 0.01) when compared with growth in BHI with no NaCl supplementation. Microarray data indicated that growth in BHI medium with 1.2% NaCl upregulated 4 genes and down-regulated 24 genes in L. monocytogenes, which was confirmed by qRT-PCR. The transcript levels of genes involved in the uptake of glycine betaine/(L)-proline were increased, whereas genes associated with a putative phosphoenolpyruvate-dependent sugar phosphotransferase system (PTS), metabolic enzymes, and virulence factor were down-regulated. Specifically, the expression levels of PTS transport genes were shown to be dependent on NaCl concentration. To further examine whether the down-regulation of PTS genes is related to decreased cell growth, the transcript levels of genes encoding components of enzyme II, involved in the uptake of various sugars used as the primary carbon source in bacteria, were also measured using qRT-PCR. Our results suggest that the decreased transcript levels of PTS genes may be caused by salt stress or reduced cell growth through salt stress. Here, we report global transcriptional profiles of L. monocytogenes in response to salt stress, contributing to an improved understanding of osmotolerance in this bacterium.

Comprehensive solid-state NMR analysis reveals the effects of N-methylation on the molecular dynamics of glycine

Molecular dynamics of metabolites are important for their interactions and functions. To understand the structural dependence of molecular dynamics for N-methylated glycines, we comprehensively measured the (13)C and (1)H spin-lattice relaxation times for sarcosine, N,N-dimethylglycine, betaine, and betaine hydrochloride over a temperature range of 178-460 K. We found that the reorientations of methyl groups were observed for all these molecules, whereas reorientations of whole trimethylamine groups were detected in betaines. While similar rotational properties were observed for methyl groups in N,N-dimethylglycine and those in betaine, three methyl groups in betaine hydrochloride had different motional properties (E(a) ≈ 20.5 kJ/mol, τ(0) ≈ 1.85 × 10(-13) s; E(a) ≈ 13.9 kJ/mol, τ(0) ≈ 2.1 × 10(-12) s; E(a) ≈ 15.8 kJ/mol, τ(0) ≈ 1.1 × 10(-12) s). N,N-Dimethylglycine showed a phase transition at 348.5 K with changed relaxation behavior for methyl groups showing distinct E(a) and τ(0) values. The DIPSHIFT experiments showed that CH(3) and CH(2) moieties in these molecules had dipolar-dephasing curves similar to these moieties in alanine and glycine. The activation energies for CH(3) rotations positively correlated with the number of substituted methyl groups. These findings provided useful information for the structural dependence of molecular dynamics for N-methylated glycines and demonstrated solid-state NMR as a useful tool for probing the structure-dynamics relationships.

ATP in current biotechnology: regulation, applications and perspectives

Adenosine tri-phosphate (ATP), the most important energy source for metabolic reactions and pathways, plays a vital role in the growth of industrial strain and the production of target metabolites. In this review, current advances in manipulating ATP in industrial strains, including altering NADH availability, and regulating NADH oxidation pathway, oxygen supply, proton gradient, the electron transfer chain activity and the F(0)F(1)-ATPase activity, are summarized and discussed. By applying these strategies, optimal product concentrations, yields and productivity in industrial biotechnology have been achieved. Furthermore, the mechanisms by which ATP extends the substrate utilization spectra and enhances the ability to challenge harsh environmental stress have been elucidated. Finally, three critical issues related to ATP manipulation have been addressed.

Role of a sodium-dependent symporter homologue in the thermosensitivity of beta-lactam antibiotic resistance and cell wall composition in Staphylococcus aureus

Expression of high-level beta-lactam resistance is known to be thermosensitive in many methicillin-resistant Staphylococcus aureus (MRSA) strains, including strain COL, in which the high methicillin MIC for cultures grown at 37 degrees C (800 microg/ml) was reduced to 12 microg/ml at 42 degrees C. COL grew faster at 42 degrees C than at 37 degrees C and at the higher temperature produced cell walls of abnormal composition: there was an over-representation of the monomeric muropeptide without the oligoglycine chain and an increase in the representation of multimers that contained this wall component as the donor molecule. Screening of a Tn551 insertional library for mutants, in which the high and homogenous beta-lactam antibiotic resistance of strain COL is retained at 42 degrees C, identified mutant C245, which expressed high-level methicillin resistance and produced a cell wall of normal composition independent of the temperature. The Tn551 inactivated gene was found, by homology search, to encode for a sodium-dependent symporter, homologues of which are ubiquitous in both prokaryotic and eukaryotic genomes. Inactivation of this putative symporter in several heteroresistant clinical MRSA isolates caused striking increases in the level of their beta-lactam resistance.

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.

Structural Basis for the Binding of Compatible Solutes by ProX from the Hyperthermophilic Archaeon Archaeoglobus fulgidus

Compatible solutes such as glycine betaine and proline betaine serve as protein stabilizers because of their preferential exclusion from protein surfaces. To use extracellular sources of this class of compounds as osmo-, cryo-, or thermoprotectants, Bacteria and Archaea have developed high affinity uptake systems of the ATP-binding cassette type. These transport systems require periplasmic- or extracellular-binding proteins that are able to bind the transported substance with high affinity. Therefore, binding proteins that bind compatible solutes have to avoid the exclusion of their ligands within the binding pocket. In the present study we addressed the question to how compatible solutes can be effectively bound by a protein at temperatures around 83 degrees C as this is done by the ligand-binding protein ProX from the hyperthermophilic archaeon Archaeoglobus fulgidus. We solved the structures of ProX without ligand and in complex with both of its natural ligands glycine betaine and proline betaine, as well as in complex with the artificial ligand trimethylammonium. Cation-pi interactions and non-classical hydrogen bonds between four tyrosine residues, a main chain carbonyl oxygen, and the ligand have been identified to be the key determinants in binding the quaternary amines of the three investigated ligands. The comparison of the ligand binding sites of ProX from A. fulgidus and the recently solved structure of ProX from Escherichia coli revealed a very similar solution for the problem of compatible solute binding, although both proteins share only a low degree of sequence identity. The residues involved in ligand binding are functionally equivalent but not conserved in the primary sequence.

Growth of Listeria monocytogenes as influenced by viscosity and water activity

The effects of osmotic stress on Listeria monocytogenes growth parameters was examined in relation to the viscosity of the growth media. In low-viscosity systems, growth of L. monocytogenes in glucose-supplemented media was comparable to growth in sucrose-supplemented media. The relative lag time (RLT: the lag time divided by the generation time) responses were found to increase in the more restrictive water activity conditions. In high-viscosity systems containing polyvinylpyrrolidone (PVP), growth rate was reduced, whereas lag time showed no discernible modification. Osmotic stress in medium- and high-viscosity media supplemented with glucose resulted in approximately exponential increasing of the RLT values. Thus, the biological effects of osmotic stress on L. monocytogenes could be affected by the physical properties of the system, such as viscosity and diffusivity.

Molecular and physiological analysis of the role of osmolyte transporters BetL, Gbu, and OpuC in growth of Listeria monocytogenes at low temperatures

Listeria monocytogenes is a ubiquitous food-borne pathogen found widely distributed in nature as well as an undesirable contaminant in a variety of fresh and processed foods. This ubiquity can be at least partly explained by the ability of the organism to grow at high osmolarity and reduced temperatures, a consequence of its ability to accumulate osmo- and cryoprotective compounds termed osmolytes. Single and multiple deletions of the known osmolyte transporters BetL, Gbu, and OpuC significantly reduce growth at low temperatures. During growth in brain heart infusion broth at 7 degrees C, Gbu and OpuC had a more pronounced role in cryoprotection than did BetL. However, upon the addition of betaine to defined medium, the hierarchy of transporter importance shifted to Gbu > BetL > OpuC. Upon the addition of carnitine, only OpuC appeared to play a role in cryoprotection. Measurements of the accumulated osmolytes showed that betaine is preferred over carnitine, while in the absence of a functional Gbu, carnitine was accumulated to higher levels than betaine was at 7 degrees C. Transcriptional analysis of the genes encoding BetL, Gbu, and OpuC revealed that each transporter is induced to different degrees upon cold shock of L. monocytogenes LO28. Additionally, despite being transcriptionally up-regulated upon cold shock, a putative fourth osmolyte transporter, OpuB (identified by bioinformatic analysis and encoded by lmo1421 and lmo1422), showed no significant contribution to listerial chill tolerance. Growth of the quadruple mutant LO28deltaBCGB (deltabetL deltaopuC deltagbu deltaopuB) was comparable to the that of the triple mutant LO28deltaBCGsoe (deltabetL deltaopuC deltagbu) at low temperatures. Here, we conclude that betaine and carnitine transport upon low-temperature exposure is mediated via three osmolyte transporters, BetL, Gbu, and OpuC.

Influence of the sigB gene on the cold stress survival and subsequent recovery of two Listeria monocytogenes serotypes

The influence of serotype and the role of the sigB gene of Listeria monocytogenes during the survival and recovery on different substrates were determined. Wild-type and sigB mutants of two serotypes of L. monocytogenes were inoculated into buffer and onto beef steaks, and incubated at 4 degrees C for 8 weeks during which samples were removed and Listeria numbers determined. Growth kinetics of stationary phase wild-type and sigB mutant cells were compared, without prechilling and after prechilling at 4 degrees C. The two serotypes had similar survival capabilities under the conditions examined, and the sigB gene was influential in survival of chill stress, but was dependent upon additional nutritional factors. Prechilling cells prior to growth extended the lag phase of both strains, and this lag phase extension was compounded by the absence of a functional sigB gene. In conclusion, the sigB gene is involved in the survival and recovery from chill stress by the two serotypes tested. Additional factors such as previous growth conditions, nutritional requirements and serotype susceptibility are also contributory. This study adds relevant information regarding the influence of the sigB gene, in conjunction with the historical growth conditions and serotype differences. Understanding the significance of these factors may be useful in creating improved recovery systems for the detection of L. monocytogenes from at-risk foods.

Role of ectoine in Vibrio cholerae osmoadaptation

Vibrio cholerae is both an intestinal pathogen and a microbe in the estuarine community. To persist in the estuarine environment, V. cholerae must adjust to changes in ionic composition and osmolarity. These changes in the aquatic environment have been correlated with cholera epidemics. In this work, we study the response of V. cholerae to increases in environmental osmolarity. Optimal growth of V. cholerae in minimal medium requires supplementation with 200 mM NaCl and KCl. However, when the NaCl concentration is increased beyond 200 mM, a proportionate delay in growth is observed. During this delay in growth, osmotic equilibrium is reached by cytoplasmic accumulation of small, uncharged solutes that are compatible with growth. We show that synthesis of the compatible solute ectoine and transport of the compatible solute glycine betaine impact the length of the osmoadaptive growth delay. We also demonstrate that high-osmolarity-adapted V. cholerae displays a growth advantage when competed against unadapted cells in high-osmolarity medium. In contrast, low-osmolarity-adapted V. cholerae displays no growth advantage when competed against high-osmolarity-adapted cells in low-osmolarity medium. These results may have implications for V. cholerae population dynamics when seawater and freshwater and their attendant microbes mix.

Transcriptional regulation and posttranslational activity of the betaine transporter BetL in Listeria monocytogenes are controlled by environmental salinity

While the genetic elements contributing to the salinity tolerance of Listeria monocytogenes have been well characterized, the regulatory signals and responses (genetic and/or biochemical) that govern these mechanisms have yet to be elucidated. Encoded by betL, the first genetic element to be linked to listerial osmotolerance, the secondary betaine uptake system BetL is a member of the betaine-carnitine-choline transporter family. Preceded by consensus sigma(A)- and sigma(B)-dependent promoter sites, betL is constitutively expressed and transcriptionally up-regulated in response to salt stress. The nisin-controlled expression system was used to achieve salinity-independent, controlled betL expression in Listeria. In the absence of NaCl-activated transcriptional control, BetL activity was found to be a function of environmental salinity, showing optimal activity in buffer supplemented with 1 to 2% NaCl (osmolality, 417 to 719 mosmol/kg). In addition, BetL was activated rapidly (half-life, 2 min) in response to an osmotic upshift imposed by adding 2% NaCl to 50 mM potassium phosphate buffer.

Betaine and carnitine uptake systems in Listeria monocytogenes affect growth and survival in foods and during infection

AIMS

To establish the relative importance of the osmo- and cryoprotective compounds glycine betaine and carnitine, and their transporters, for listerial growth and survival, in foods and during infection.

METHODS AND RESULTS

A set of Listeria monocytogenes mutants with single, double and triple mutations in the genes encoding the principal betaine and carnitine uptake systems (gbu, betL and opuC, respectively) was used to determine the specific contribution of each transporter to listerial growth and survival. Food models were chosen to represent high-risk foods of plant and animal origin i.e. coleslaw and frankfurters, which have previously been linked to major human outbreaks of listeriosis. BALB/c mice were used as an in vivo model of infection. Interestingly, while betaine appeared to confer most protection in foods, the hierarchy of transporter importance differs depending on the food type: Gbu>BetL>OpuC for coleslaw, as opposed to Gbu>OpuC>BetL in frankfurters. By contrast in the animal model, OpuC and thus carnitine, appears to play the dominant role, with the remaining systems contributing little to the infection process.

CONCLUSIONS

This study demonstrates that the individual contribution of each system appears dependent on the immediate environment. In foods Gbu appears to play the dominant role, while during infection OpuC is most important.

SIGNIFICANCE AND IMPACT OF THE STUDY

It is envisaged that this information may ultimately facilitate the design of effective control measures specifically targeting this pathogen in foods and during infection.

Cation-pi interactions as determinants for binding of the compatible solutes glycine betaine and proline betaine by the periplasmic ligand-binding protein ProX from Escherichia coli

Compatible solutes such as glycine betaine and proline betaine are accumulated to exceedingly high intracellular levels by many organisms in response to high osmolarity to offset the loss of cell water. They are excluded from the immediate hydration shell of proteins and thereby stabilize their native structure. Despite their exclusion from protein surfaces, the periplasmic ligand-binding protein ProX from the Escherichia coli ATP-binding cassette transport system ProU binds the compatible solutes glycine betaine and proline betaine with high affinity and specificity. To understand the mechanism of compatible solute binding, we determined the high resolution structure of ProX in complex with its ligands glycine betaine and proline betaine. This crystallographic study revealed that cation-pi interactions between the positive charge of the quaternary amine of the ligands and three tryptophan residues forming a rectangular aromatic box are the key determinants of the high affinity binding of compatible solutes by ProX. The structural analysis was combined with site-directed mutagenesis of the ligand binding pocket to estimate the contributions of the tryptophan residues involved in binding.

Cloning, expression, and purification of choline dehydrogenase from the moderate halophile Halomonas elongata

Choline dehydrogenase (EC 1.1.99.1) catalyzes the four-electron oxidation of choline to glycine-betaine via a betaine-aldehyde intermediate. Such a reaction is of considerable interest for biotechnological applications in that transgenic plants engineered with bacterial glycine-betaine-synthesizing enzymes have been shown to have enhanced tolerance towards various environmental stresses, such as hypersalinity, freezing, and high temperatures. To date, choline dehydrogenase has been poorly characterized in its biochemical and kinetic properties, mainly because its purification has been hampered by instability of the enzyme in vitro. In the present report, we cloned and expressed in Escherichia coli the betA gene from the moderate halophile Halomonas elongata which codes for a hypothetical choline dehydrogenase. The recombinant enzyme was purified to more than 70% homogeneity as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and by treatment with 30 to 50% saturation of ammonium sulfate followed by column chromatography using DEAE-Sepharose. The purified enzyme showed similar substrate specificities with either choline or betaine-aldehyde as the substrate, as indicated by the apparent V/K values (where V is the maximal velocity and K is the Michaelis constant) of 0.9 and 0.6 micro mol of O(2) min(-1) mg(-1) mM(-1) at pH 7 and 25 degrees C, respectively. With 1 mM phenazine methosulfate as the primary electron acceptor, the apparent V(max) values for choline and betaine-aldehyde were 10.9 and 5.7 micro mol of O(2) min(-1) mg(-1), respectively. These V(max) values decreased four- to sevenfold when molecular oxygen was used as the electron acceptor. Altogether, the kinetic data are consistent with the conclusion that H. elongata betA codes for a choline dehydrogenase that can also act as an oxidase when electron acceptors other than molecular oxygen are not available.

Influence of interactions between temperature, ferric ammonium citrate and glycine betaine on the growth of Listeria monocytogenes in a defined medium

AIMS

To investigate interactions, if any, between temperature, ferric ammonium citrate and glycine betaine on the growth of Listeria monocytogenes in modified Pine's medium (Pine et al. 1989).

METHODS AND RESULTS

Modified Pine's medium containing 0, 0.044, 0.088 or 0.176 g l(-1) ferric ammonium citrate, and 0 or 1 mM glycine betaine, was inoculated with each of two L. monocytogenes strains and incubated at 4, 25 or 37 degrees C. The optical density at 600 nm, and cell numbers, were determined at appropriate time intervals. At 4 degrees C, but not other temperatures, increasing ferric ammonium citrate resulted in improved growth in the absence, but not the presence, of glycine betaine. The presence of glycine betaine was inhibitory at 25 and 37 degrees C, but not at 4 degrees C.

CONCLUSIONS

Interactions affecting the growth kinetics of L. monocytogenes were apparent between the parameters investigated.

SIGNIFICANCE AND IMPACT OF THE STUDY

Limitations on the use of modified Pine's medium, and the significance of iron metabolism at lower temperatures, were revealed.

Three transporters mediate uptake of glycine betaine and carnitine by Listeria monocytogenes in response to hyperosmotic stress

The uptake and accumulation of the potent osmolytes glycine betaine and carnitine enable the food-borne pathogen Listeria monocytogenes to proliferate in environments of elevated osmotic stress, often rendering salt-based food preservation inadequate. To date, three osmolyte transport systems are known to operate in L. monocytogenes: glycine betaine porter I (BetL), glycine betaine porter II (Gbu), and a carnitine transporter OpuC. We investigated the specificity of each transporter towards each osmolyte by creating mutant derivatives of L. monocytogenes 10403S that possess each of the transporters in isolation. Kinetic and steady-state osmolyte accumulation data together with growth rate experiments demonstrated that osmotically activated glycine betaine transport is readily and effectively mediated by Gbu and BetL and to a lesser extent by OpuC. Osmotically stimulated carnitine transport was demonstrated for OpuC and Gbu regardless of the nature of stressing salt. BetL can mediate weak carnitine uptake in response to NaCl stress but not KCl stress. No other transporter in L. monocytogenes 10403S appears to be involved in osmotically stimulated transport of either osmolyte, since a triple mutant strain yielded neither transport nor accumulation of glycine betaine or carnitine and could not be rescued by either osmolyte when grown under elevated osmotic stress.

Gbu glycine betaine porter and carnitine uptake in osmotically stressed Listeria monocytogenes cells

The food-borne pathogen Listeria monocytogenes grows actively under high-salt conditions by accumulating compatible solutes such as glycine betaine and carnitine from the medium. We report here that the dominant transport system for glycine betaine uptake, the Gbu porter, may act as a secondary uptake system for carnitine, with a K(m) of 4 mM for carnitine uptake and measurable uptake at carnitine concentrations as low as 10 microM. This porter has a K(m) for glycine betaine uptake of about 6 micro M. The dedicated carnitine porter, OpuC, has a K(m) for carnitine uptake of 1 to 3 microM and a V(max) of approximately 15 nmol/min/mg of protein. Mutants lacking either opuC or gbu were used to study the effects of four carnitine analogs on growth and uptake of osmolytes. In strain DP-L1044, which had OpuC and the two glycine betaine porters Gbu and BetL, triethylglycine was most effective in inhibiting growth in the presence of glycine betaine, but trigonelline was best at inhibiting growth in the presence of carnitine. Carnitine uptake through OpuC was inhibited by gamma-butyrobetaine. Dimethylglycine inhibited both glycine betaine and carnitine uptake through the Gbu porter. Carnitine uptake through the Gbu porter was inhibited by triethylglycine. Glycine betaine uptake through the BetL porter was strongly inhibited by trigonelline and triethylglycine. These results suggest that it is possible to reduce the growth of L. monocytogenes under osmotically stressful conditions by inhibiting glycine betaine and carnitine uptake but that to do so, multiple uptake systems must be affected.