Effect of sodium chloride on the intracellular solute pools of Listeria monocytogenes

A novel RofA-family transcriptional regulator, GadR, controls the development of acid resistance in Listeria monocytogenes

IMPORTANCE

The ability to survive the acidic conditions found in the stomach is crucial for the food-borne pathogen Listeria monocytogenes to gain access to the mammalian gastrointestinal tract. Little is currently known about how acid resistance is regulated in this pathogen and why this trait is highly variable between strains. Here, we used comparative genomics to identify a novel RofA-family transcriptional regulator, GadR, that controls the development of acid resistance. The RofA family of regulators was previously found only in a small group of bacterial pathogens, including streptococci, where they regulate virulence properties. We show that gadR encodes the dominant regulator of acid resistance in L. monocytogenes and that its sequence variability accounts for previously observed differences between strains in this trait. Together, these findings significantly advance our understanding of how this important pathogen copes with acid stress and suggest a potential molecular target to aid its control in the food chain.

Characterisation of the growth behaviour of Listeria monocytogenes in Listeria synthetic media

The foodborne pathogen Listeria monocytogenes can grow in a wide range of environmental conditions. For the study of the physiology of this organism, several chemically defined media have been developed over the past decades. Here, we examined the ability of L. monocytogenes wildtype strains EGD-e and 10403S to grow under salt and pH stress in Listeria synthetic medium (LSM). Furthermore, we determined that a wide range of carbon sources could support the growth of both wildtype strains in LSM. However, for hexose phosphate sugars such as glucose-1-phosphate, both L. monocytogenes strains need to be pre-grown under conditions, where the major virulence regulator PrfA is active. In addition, growth of both L. monocytogenes strains was observed when LSM was supplemented with the amino acid sugar N-acetylmannosamine (ManNAc). We were able to show that some of the proteins encoded in the operon lmo2795-nanE, such as the ManNAc-6-phosphate epimerase NanE, are required for growth in the presence of ManNAc. The first gene of the operon, lmo2795, encodes a transcriptional regulator of the RpiR family. Using electrophoretic mobility shift assays and quantitative real-time PCR analysis, we were able to show that Lmo2795 binds to the promoter region of the operon lmo2795-nanE and activates its expression.

Trehalose transport occurs via TreB in Listeria monocytogenes and it influences biofilm development and acid resistance

Listeria monocytogenes is a pathogenic bacterium that can inhabit a diverse range of environmental niches. This is largely attributed to the high proportion of carbohydrate-specific phosphotransferase system (PTS) genes in its genome. Carbohydrates can be assimilated as sources of energy but additionally they can serve as niche-specific cues for L. monocytogenes to shape its global gene expression, in order to cope with anticipated stresses. To examine carbon source utilization among wild L. monocytogenes isolates and to understand underlying molecular mechanisms, a diverse collection of L. monocytogenes strains (n = 168) with whole genome sequence (WGS) data available was screened for the ability to grow in chemically defined media with different carbon sources. The majority of the strains grew in glucose, mannose, fructose, cellobiose, glycerol, trehalose, and sucrose. Maltose, lactose, and rhamnose supported slower growth while ribose did not support any growth. In contrast to other strains, strain1386, which belonged to clonal complex 5 (CC5), was unable to grow on trehalose as a sole carbon source. WGS data revealed that it carried a substitution (N352K) in a putative PTS EIIBC trehalose transporter, TreB, while this asparagine residue is conserved in other strains in this collection. Spontaneous mutants of strain 1386 that could grow in trehalose were found to harbour a reversion of the substitution in TreB. These results provide genetic evidence that TreB is responsible for trehalose uptake and that the N352 residue is essential for TreB activity. Moreover, reversion mutants also restored other unusual phenotypes that strain 1386 displayed, i.e. altered colony morphology, impaired biofilm development, and reduced acid resistance. Transcriptional analysis at stationary phase with buffered BHI media revealed that trehalose metabolism positively influences the transcription of genes encoding amino acid-based acid resistance mechanisms. In summary, our results demonstrated that N352 is key to the function of the sole trehalose transporter TreB in L. monocytogenes and suggest that trehalose metabolism alters physiology to favour biofilm development and acid stress resistance. Moreover, since strain 1386 is among the strains recommended by the European Union Reference Laboratory for conducting food challenge studies in order to determine whether or not L. monocytogenes can grow in food, these findings have important implications for food safety.

Assessment of the influence of selected stress factors on the growth and survival of Listeria monocytogenes

BACKGROUND

Listeria monocytogenes are Gram-positive rods, which are the etiological factor of listeriosis. L. monocytogenes quickly adapts to changing environmental conditions. Since the main source of rods is food, its elimination from the production line is a priority. The study aimed to evaluate the influence of selected stress factors on the growth and survival of L. monocytogenes strains isolated from food products and clinical material.

RESULTS

We distinguished fifty genetically different strains of L. monocytogenes (PFGE method). Sixty-two percent of the tested strains represented 1/2a-3a serogroup. Sixty percent of the rods possessed ten examined virulence genes (fbpA, plcA, hlyA, plcB, inlB, actA, iap, inlA, mpl, prfA). Listeria Pathogenicity Island 1 (LIPI-1) was demonstrated among 38 (76.0%) strains. Majority (92.0%) of strains (46) were sensitive to all examined antibiotics. The most effective concentration of bacteriophage (inhibiting the growth of 22 strains; 44.0%) was 5 × 108 PFU. In turn, the concentration of 8% of NaCl was enough to inhibit the growth of 31 strains (62.0%). The clinical strain tolerated the broadest pH range (3 to 10). Five strains survived the 60-min exposure to 70˚C, whereas all were alive at each time stage of the cold stress experiment. During the stress of cyclic freezing-defrosting, an increase in the number of bacteria was shown after the first cycle, and a decrease was only observed after cycle 3. The least sensitive to low nutrients content were strains isolated from frozen food. The high BHI concentration promoted the growth of all groups.

CONCLUSIONS

Data on survival in stress conditions can form the basis for one of the hypotheses explaining the formation of persistent strains. Such studies are also helpful for planning appropriate hygiene strategies within the food industry.

Plasmidome of Listeria spp.-The repA-Family Business

Bacteria of the genus Listeria (phylum Firmicutes) include both human and animal pathogens, as well as saprophytic strains. A common component of Listeria spp. genomes are plasmids, i.e., extrachromosomal replicons that contribute to gene flux in bacteria. This study provides an in-depth insight into the structure, diversity and evolution of plasmids occurring in Listeria strains inhabiting various environments under different anthropogenic pressures. Apart from the components of the conserved plasmid backbone (providing replication, stable maintenance and conjugational transfer functions), these replicons contain numerous adaptive genes possibly involved in: (i) resistance to antibiotics, heavy metals, metalloids and sanitizers, and (ii) responses to heat, oxidative, acid and high salinity stressors. Their genomes are also enriched by numerous transposable elements, which have influenced the plasmid architecture. The plasmidome of Listeria is dominated by a group of related replicons encoding the RepA replication initiation protein. Detailed comparative analyses provide valuable data on the level of conservation of these replicons and their role in shaping the structure of the Listeria pangenome, as well as their relationship to plasmids of other genera of Firmicutes, which demonstrates the range and direction of flow of genetic information in this important group of bacteria.

Measurement of Protein Mobility in Listeria monocytogenes Reveals a Unique Tolerance to Osmotic Stress and Temperature Dependence of Diffusion

Protein mobility in the cytoplasm is essential for cellular functions, and slow diffusion may limit the rates of biochemical reactions in the living cell. Here, we determined the apparent lateral diffusion coefficient (D_L) of GFP in Listeria monocytogenes as a function of osmotic stress, temperature, and media composition. We find that D_L is much less affected by hyperosmotic stress in L. monocytogenes than under similar conditions in Lactococcus lactis and Escherichia coli. We find a temperature optimum for protein diffusion in L. monocytogenes at 30°C, which deviates from predicted trends from the generalized Stokes-Einstein equation under dilute conditions and suggests that the structure of the cytoplasm and macromolecular crowding vary as a function of temperature. The turgor pressure of L. monocytogenes is comparable to other Gram-positive bacteria like Bacillus subtilis and L. lactis but higher in a knockout strain lacking the stress-inducible sigma factor SigB. We discuss these findings in the context of how L. monocytogenes survives during environmental transmission and interaction with the human host.

Listeria monocytogenes σA Is Sufficient to Survive Gallbladder Bile Exposure

Listeria monocytogenes is a foodborne Gram-positive bacterium causing listeriosis in both animals and humans. It can persist and grow in various environments including conditions countered during saprophytic or intra-host lifestyles. Sigma (σ) subunit of RNA polymerase is a transcriptional factor responsible for guiding the core RNA polymerase and initiating gene expression under normal growth or physiological changes. In L. monocytogenes, there is one housekeeping sigma factor, σA, and four alternative sigma factors σB, σC, σH, and σL. Generally, σA directs expression of genes required for normal growth while alternative σ factors alter gene expression in response to specific conditions (e.g., stress). In this study, we aimed to determine the exclusive role of σA in L. monocytogenes by comparing a wild type strain with its isogenic mutant lacking genes encoding all alternative sigma factors (i.e., sigB, sigC, sigH, and sigL). We further investigated their survival abilities in 6% porcine bile (pH 8.2) mimicking gallbladder bile and their transcriptomics profiles in rich medium (i.e., BHI) and 1% porcine bile. Surprisingly, the results showed that survival abilities of wild type and ΔsigBΔsigCΔsigHΔsigL (or ΔsigBCHL) quadruple mutant strains in 6% bile were similar suggesting a compensatory role for σA. RNA-seq results revealed that bile stimulon of L. monocytogenes wild type contained 66 genes (43 and 23 genes were up- and down-regulated, respectively); however, only 29 genes (five up- and 24 down-regulated by bile) were differentially expressed in ΔsigBCHL. We have shown that bile exposure mediates increased transcription levels of dlt and ilv operons and decreased transcription levels of prfA and heat shock genes in wild type. Furthermore, we identified σA-dependent bile inducible genes that are involved in phosphotransferase systems, chaperones, and transporter systems; these genes appear to contribute to L. monocytogenes cellular homeostasis. As a result, σA seemingly plays a compensatory role in the absence of alternative sigma factors under bile exposure. Our data support that the bile stimulon is prone to facilitate resistance to bile prior to initiated infection.

Dimethyl adenosine transferase (KsgA) deficiency in Salmonella enterica Serovar Enteritidis confers susceptibility to high osmolarity and virulence attenuation in chickens

Dimethyl adenosine transferase (KsgA) performs diverse roles in bacteria, including ribosomal maturation and DNA mismatch repair, and synthesis of KsgA is responsive to antibiotics and cold temperature. We previously showed that a ksgA mutation in Salmonella enterica serovar Enteritidis results in impaired invasiveness in human and avian epithelial cells. In this study, we tested the virulence of a ksgA mutant (the ksgA::Tn5 mutant) of S. Enteritidis in orally challenged 1-day-old chickens. The ksgA::Tn5 mutant showed significantly reduced intestinal colonization and organ invasiveness in chickens compared to those of the wild-type (WT) parent. Phenotype microarray (PM) was employed to compare the ksgA::Tn5 mutant and its isogenic wild-type strain for 920 phenotypes at 28°C, 37°C, and 42°C. At chicken body temperature (42°C), the ksgA::Tn5 mutant showed significantly reduced respiratory activity with respect to a number of carbon, nitrogen, phosphate, sulfur, and peptide nitrogen nutrients. The greatest differences were observed in the osmolyte panel at concentrations of ≥6% NaCl at 37°C and 42°C. In contrast, no major differences were observed at 28°C. In independent growth assays, the ksgA::Tn5 mutant displayed a severe growth defect in high-osmolarity (6.5% NaCl) conditions in nutrient-rich (LB) and nutrient-limiting (M9 minimum salts) media at 42°C. Moreover, the ksgA::Tn5 mutant showed significantly reduced tolerance to oxidative stress, but its survival within macrophages was not impaired. Unlike Escherichia coli, the ksgA::Tn5 mutant did not display a cold-sensitivity phenotype; however, it showed resistance to kasugamycin and increased susceptibility to chloramphenicol. To the best of our knowledge, this is the first report showing the role of ksgA in S. Enteritidis virulence in chickens, tolerance to high osmolarity, and altered susceptibility to kasugamycin and chloramphenicol.

Two-Dimensional Polyacrylamide Gel Electrophoresis Analysis of the Acid Tolerance Response in Listeria monocytogenes LO28

Listeria monocytogenes is capable of withstanding low pH after initial exposure to sublethal acidic conditions, a phenomenon termed the acid tolerance response (B. O'Driscoll, C. G. M. Gahan, and C. Hill, Appl. Environ. Microbiol. 62:1693-1698, 1996). Treatment of L. monocytogenes LO28 with chloramphenicol during acid adaptation abrogated the protective effect, suggesting that de novo protein synthesis is required for the acid tolerance response. Analysis of protein expression during acid adaptation by two-dimensional gel electrophoresis revealed changes in the levels of 53 proteins. Significant protein differences were also evident between nonadapted L. monocytogenes LO28 and a constitutively acid-tolerant mutant, ATM56. In addition, the analysis[S_TABC] revealed differences in protein expression between cells induced with a weak acid (lactic acid) and those induced with a strong acid (HCl). Comparison of both acid-adapted LO28 and ATM56 revealed that both are capable of maintaining their internal pH (pH(infi)) at higher levels than nonadapted control cells during severe acid stress. Collectively, the data demonstrate the profound alterations in protein synthesis which take place during acid adaptation in L. monocytogenes and ultimately lead to an increased ability to survive severe stress conditions.

Intracellular accumulation of high levels of gamma-aminobutyrate by Listeria monocytogenes 10403S in response to low pH: uncoupling of gamma-aminobutyrate synthesis from efflux in a chemically defined medium

It is well established that the glutamate decarboxylase (GAD) system is central to the survival of Listeria monocytogenes at low pH, both in acidic foods and within the mammalian stomach. The accepted model proposes that under acidic conditions extracellular glutamate is transported into the cell in exchange for an intracellular gamma-aminobutyrate (GABA(i)). The glutamate is then decarboxylated to GABA(i), a reaction that consumes a proton, thereby helping to prevent acidification of the cytoplasm. In this study, we show that glutamate supplementation had no influence on either growth rate at pH 5.0 or survival at pH 2.5 when L. monocytogenes 10403S was grown in a chemically defined medium (DM). In response to acidification, cells grown in DM failed to efflux GABA, even when glutamate was added to the medium. In contrast, in brain heart infusion (BHI), the same strain produced significant extracellular GABA (GABA(e)) in response to acidification. In addition, high levels of GABA(i) (>80 mM) were found in the cytoplasm in response to low pH in both growth media. Medium-swap and medium-mixing experiments revealed that the GABA efflux apparatus was nonfunctional in DM, even when glutamate was present. It was also found that the GadT2D2 antiporter/decarboxylase system was transcribed poorly in DM-grown cultures while overexpression of gadD1T1 and gadD3 occurred in response to pH 3.5. Interestingly, BHI-grown cells did not respond with upregulation of any of the GAD system genes when challenged at pH 3.5. The accumulation of GABA(i) in cells grown in DM in the absence of extracellular glutamate indicates that intracellular glutamate is the source of the GABA(i). These results demonstrate that GABA production can be uncoupled from GABA efflux, a finding that alters the way we should view the operation of bacterial GAD systems.

Fate of surface-inoculated Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella typhimurium on kippered beef during extended storage at refrigeration and abusive temperatures

The behavior of Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella Typhimurium on kippered beef was evaluated. Individual pieces of the product were separately inoculated on the top and bottom surfaces with each three- to six-strain pathogen cocktail at ca. 6.0 log CFU per piece and stored at 4, 10, 21, or 30 degrees C for up to 28 days in each of two trials. When kippered beef was inoculated with E. coli O157:H7, Salmonella Typhimurium, or L. monocytogenes and stored at 4, 10, 21, or 30 degrees C for up to 28 days, pathogen numbers decreased ca. 0.4 to 0.9, 1.0 to 1.8, 3.0 to > or = 5.25, and > or = 5.0 to 5.25 log CFU per piece, respectively. Average D-values for E. coli O157:H7, Salmonella Typhimurium, and L. monocytogenes stored at 4 to 30 degrees C for 28 days were ca. 41 to 4.6, 40.8 to 5.3, and 29.5 to 4.3 days, respectively. As expected, the higher the storage temperature, the greater the level and rate of inactivation for all three pathogens. These data establish that kippered beef does not provide an environment conducive to proliferation of these pathogens.

Organic vs inorganic: What makes the major contribution to osmotic adjustment in bacteria?

In order to survive hyperosmotic stress bacteria should adjust their cell turgor to altered conditions by increasing the intracellular osmolality. The classical view is that bacterial osmotic adjustment is achieved via accumulation of so-called "compatible solutes"-some organic osmolytes that can be accumulated in the cytosol at high concentrations without interfering with cell metabolism. In our recently published paper,11 we have shown that in the absence of osmolytes in the environment uptake of inorganic ions (and, specifically, K(+)) is central to osmotic adjustment in E. coli under hyperosmotic stress conditions. Here we show that optimal E. coli growth, observed at 2% NaCl, corresponds to an osmotic balance between external and internal osmolality within bacterial cells. This is achieved by the regulation of net K(+) fluxes across the bacterial membrane. We suggest that the role of compatible solutes in osmotic adjustment in bacteria is indirect and confined to the fine tuning of a number of ion channels and transporters in order to achieve osmotic balance.

Relative importance of amino acids, glycine-betaine and ectoine synthesis in the biocontrol agent Pantoea agglomerans CPA-2 in response to osmotic, acidic and heat stress

AIMS

The objective of this work was to determine the role of different compatible solutes in adaptation of Pantoea agglomerans CPA-2 at different stages of growth to solute (0.98, 0.97, 0.96 aw), heat (35 and 40 degrees C) and acidic (pH 4.0, 5.0, 6.0) stress.

METHODS AND RESULTS

Solute stress was imposed by using NaCl, glucose or glycerol, and pH was imposed with malic and citric acids. The accumulation of glycine-betaine, ectoine and amino acids in bacterial cells was quantified using high performance liquid chromathography (HPLC). There was a significant (P<0.05) accumulation of glycine-betaine (NaCl modified, 100-150 micromol g(-1) dry weight of cells) and ectoine (glucose modified media, >340 micromol g(-1) dry weight of cells) in the cells over a 48 h incubation period when compared with controls (<10 micromol g(-1) dry weight of cells). Chromatographic profile of amino acids was different with respect to control when NaCl or glucose was used as osmolyte.

CONCLUSIONS

Pantoea agglomerans CPA-2 cells synthesised significant amounts of glycine-betaine and ectoine in response to imposed solute stress. However, these compounds and tested amino acids were not involved in cellular adaptation to either heat or pH stress.

SIGNIFICANCE AND IMPACT OF THE STUDY

This type of information can be effectively applied to improve ecophysiological quality of cells of bacterial biocontrol agents for better survival and biocontrol efficacy in the phyllosphere of plants.

A predictive model for the effect of temperature and predrying treatments in reducing Listeria monocytogenes populations during drying of beef jerky

The objective of this study was to model the effect of drying temperatures (52, 57, and 63 degrees C) and predrying treatments on the inactivation of Listeria monocytogenes on beef jerky. Before drying, beef slices were inoculated with a 10-strain composite of L. monocytogenes and then treated with the following: (i) nothing (C), (ii) traditional marinade (M), or (iii) dipping in 5% acetic acid solution for 10 min, followed by M (AM). In addition, sequential stresses (exposure to 10% NaCl, followed by an adjustment of the pH to 5.0 and, subsequently, a water bath at 45 degrees C) were applied to the inocula before beef contamination and drying at 63 degrees C. Surviving L. monocytogenes were determined on tryptic soy agar plus 0.6% yeast extract (TSAYE) and on PALCAM agar at 0, 2, 4, 6, 8, and 10 h during drying. Data were modeled by a linear regression (treatment AM) and a logistic-based equation capable of fitting biphasic inactivation curves without initial shoulder (treatments C and M). The total log reductions expressed as the CFU per square centimeter of L. monocytogenes (3.9 to 5.1) for the samples treated with M (3.5 to 5.4) when compared with C were similar, whereas AM-treated samples had higher (6.1 to 6.8) reductions. All survival curves were characterized by an initial rapid decrease in populations within the first 2 h, which was followed by a secondary death phase at a lower rate. No significant (P > or = 0.05) differences in inactivation were observed due to drying temperatures in the range (52 to 63 degrees C) tested. Inactivation differences between recovered counts of stressed and unstressed cells were significant (P < 0.05) in PALCAM but not in TSAYE. The acidified predrying treatment (AM) had higher pathogen inactivation during drying than other treatments, regardless of drying temperature. The models developed may be useful in designing effective drying processes for beef jerky.

Comparative assessment of acid, alkali and salt tolerance inListeria monocytogenesvirulent and avirulent strains

Listeria monocytogenes is an opportunistic bacterial pathogen of man and animals that has the capacity to survive under extreme environmental conditions. While our knowledge on L. monocytogenes and its ability to sustain within wide pH and temperature ranges and salt concentrations has been largely built on the virulent strains of this species, relatively little is known about avirulent strains in this regard. In this study, we extend our analysis on avirulent L. monocytogenes strains. By subjecting three virulent (EGD, 874 and ATCC 19196) and three avirulent (ATCC 19114, HCC23 and HCC25) strains to various pH and salt concentrations, it was found that L. monocytogenes recovered well after treatment with 100 mM Tris at pH 12.0, and to a lesser extent at pH 3.0. Interestingly, avirulent L. monocytogenes strains showed a somewhat higher tolerance to alkali than virulent strains. This unique feature of avirulent L. monocytogenes strains may potentially be exploited for the development of a rapid technique for differentiation between avirulent and virulent strains. Furthermore, all L. monocytogenes strains tested were resistant to saturated NaCl (about 7 M, or 40% w/v) for a long period of time (20 h and possibly longer). Together, these results highlight that acid, alkali, and/or salt treatments commonly used in food product processing may not be sufficient to eliminate L. monocytogenes, and therefore stringent quality control measures at the beginning and end of the food manufacturing process is essential to ensure that such food products are free of listerial contamination.

Role of the glycine betaine and carnitine transporters in adaptation of Listeria monocytogenes to chill stress in defined medium

The food-borne pathogen Listeria monocytogenes proliferates at refrigeration temperatures, rendering refrigeration ineffective in the preservation of Listeria-contaminated foods. The uptake and intracellular accumulation of the potent compatible solutes glycine betaine and carnitine has been shown to be a key mediator of the pathogen's cold-tolerant phenotype. To date, three compatible solute systems are known to operate in L. monocytogenes: glycine betaine porter I (BetL), glycine betaine porter II (Gbu), and the carnitine transporter OpuC. We investigated the specificity of each transporter towards each compatible solute at 4 degrees C by examining mutant derivatives of L. monocytogenes 10403S that possess each of the transporters in isolation. Kinetic and steady-state compatible solute accumulation data together with growth rate experiments demonstrated that under cold stress glycine betaine transport is primarily mediated by Gbu and that Gbu-mediated betaine uptake results in significant growth stimulation of chill-stressed cells. BetL and OpuC can serve as minor porters for the uptake of betaine, and their action is capable of providing a small degree of cryotolerance. Under cold stress, carnitine transport occurs primarily through OpuC and results in a high level of cryoprotection. Weak carnitine transport occurs via Gbu and BetL, conferring correspondingly weak cryoprotection. No other transporter in L. monocytogenes 10403S appears to be involved in transport of either compatible solute at 4 degrees C, 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 at that temperature.

The effect of abrupt osmotic shifts on the lag phase duration of foodborne bacteria

The effects of osmotic environment and inoculum history on lag times were examined. Abrupt osmotic shifts of cultures were found to induce lag phases in a variety of foodborne bacteria. Relative lag times (RLT; the ratio of lag time to generation time) were used to differentiate the effects of the shift from those of the outgrowth environment. In general, osmotic downshifts induced larger RLTs than equivalent upshifts. An observed reduction in RLT at very low a(w), however, was unexpected. For an osmotic downshift, differences were observed in the RLT response of the Gram-negative and -positive strains tested. RLTs were usually extended for Gram-negative organisms as conditions became less favourable for growth. In comparison, RLT remained relatively unaffected for Gram-positive organisms. The observations reported in this study demonstrate that lag time can be understood in terms of the amount of work to be done to adjust to new environmental conditions and the rate at which that work is done, and are consistent with known strategies for osmoregulation employed by the various organisms studied.

Multiple deletions of the osmolyte transporters BetL, Gbu, and OpuC of Listeria monocytogenes affect virulence and growth at high osmolarity

The success of Listeria monocytogenes as a food-borne pathogen owes much to its ability to survive a variety of stresses, both in the food environment and, after ingestion, within the animal host. Growth at high salt concentrations is attributed mainly to the accumulation of organic solutes such as glycine betaine and carnitine. We characterized L. monocytogenes LO28 strains with single, double, and triple deletions in the osmolyte transport systems BetL, Gbu, and OpuC. When single deletion mutants were tested, Gbu was found to have the most drastic effect on the rate of growth in brain heart infusion (BHI) broth with 6% added NaCl. The highest reduction in growth rate was found for the triple mutant LO28BCG (DeltabetL DeltaopuC Deltagbu), although the mutant was still capable of growth under these adverse conditions. In addition, we analyzed the growth and survival of this triple mutant in an animal (murine) model. LO28BCG showed a significant reduction in its ability to cause systemic infection following peroral coinoculation with the wild-type parent. Altering OpuC alone resulted in similar effects (R. D. Sleator, J. Wouters, C. G. M. Gahan, T. Abee, and C. Hill, Appl. Environ. Microbiol. 67:2692-2698, 2001), leading to the assumption that OpuC may play an important role in listerial pathogenesis. Analysis of the accumulation of osmolytes revealed that betaine is accumulated up to 300 micro mol/g (dry weight) when grown in BHI broth plus 6% NaCl whereas no carnitine accumulation could be detected. Radiolabeled-betaine uptake studies revealed an inability of BGSOE (DeltabetL Deltagbu) and LO28BCG to transport betaine. Indeed, for LO28BCG, no accumulated betaine was found, but carnitine was accumulated in this strain up to 600 micro mol/g (dry weight) of cells, indicating the presence of a possible fourth osmolyte transporter.

Acid stress in the food pathogen Bacillus cereus

AIMS

The pathogen Bacillus cereus, which is associated with a number of foods including dairy products, was studied for its response to acid stress during the exponential phase.

METHODS AND RESULTS

Bacillus cereus was found to adapt to acid stress (pH 4.6) when pre-exposed to a non-lethal, inducing pH of 6.3 or to inducing concentrations of heat, ethanol, salt or hydrogen peroxide. Cells were found to maintain their internal pH at a higher level than the external acid pH and adapted cells had a higher internal pH than unadapted cells. A constitutive acid-sensitive mutant that was also heat- and ethanol-sensitive was found to be capable of high levels of adaptation despite its lack of induction of proteins induced in the wild type by exposure to moderate pH (6.3) values.

CONCLUSIONS

A number of proteins were found to be underexpressed in the mutant compared with the wild type at pH 6.3, including some with homology to ribosomal proteins and to the sporulation regulator RapK, while one differentially expressed band contained two proteins, one of which was homologous to the competence regulator CodY.

SIGNIFICANCE AND IMPACT OF THE STUDY

The work has implications for the processing of B. cereus-associated foods by acidification. The linked developmental processes of stationary phase, sporulation and possibly competence appear to be involved in the response to acid stress.

Identification of opuC as a chill-activated and osmotically activated carnitine transporter in Listeria monocytogenes

The food-borne pathogen Listeria monocytogenes is notable for its ability to grow under osmotic stress and at low temperatures. It is known to accumulate the compatible solutes glycine betaine and carnitine from the medium in response to osmotic or chill stress, and this accumulation confers tolerance to these stresses. Two permeases that transport glycine betaine have been identified, both of which are activated by hyperosmotic stress and one of which is activated by low temperature. An osmotically activated transporter for carnitine, OpuC, has also been identified. We have isolated a Tn917-LTV3 insertional mutant that could not be rescued from hyperosmotic stress by exogenous carnitine. The mutant, LTS4a, grew indistinguishably from a control strain (DP-L1044) in the absence of stress or in the absence of carnitine, but DP-L1044 grew substantially faster under osmotic or chill stress in the presence of carnitine. LTS4a was found to be strongly impaired in KCl-activated as well as chill-activated carnitine transport. 13C nuclear magnetic resonance spectroscopy of perchloric acid extracts showed that accumulation of carnitine by LTS4a was negligible under all conditions tested. Direct sequencing of LTS4a genomic DNA with a primer based on Tn917-LTV3 yielded a 487-bp sequence, which allowed us to determine that the opuC operon had been interrupted by the transposon. It can be concluded that opuC encodes a carnitine transporter that can be activated by either hyperosmotic stress or chill and that the transport system plays a significant role in the tolerance of L. monocytogenes to both forms of environmental stress.

Identification and disruption of the proBA locus in Listeria monocytogenes: role of proline biosynthesis in salt tolerance and murine infection

Intracellular accumulation of the amino acid proline has previously been linked to the salt tolerance and virulence potential of a number of bacteria. Taking advantage of the proBA mutant Escherichia coli CSH26, we identified a listerial proBA operon coding for enzymes functionally similar to the glutamyl kinase (GK) and glutamylphosphate reductase (GPR) enzyme complex which catalyzes the first and second steps of proline biosynthesis in E. coli. The first gene of the operon, proB, is predicted to encode GK, a 276-residue protein with a calculated molecular mass of 30.03 kDa and pl of 5.2. Distal to the promoter and overlapping the 3' end of proB by 17 bp is proA, which encodes GPR, a 415-residue protein with a calculated molecular mass of 45.50 kDa (pl 5.3). Using this information, we created a chromosomal deletion mutant by allelic exchange which is auxotrophic for proline. This mutant was used to assess the contribution of proline anabolism to osmotolerance and virulence. While inactivation of proBA had no significant effect on virulence in mouse assays (either perorally or intraperitoneally), growth at low (2 to 4% NaCl) and high (>6% NaCl) salt concentrations in complex media was significantly reduced in the absence of efficient proline synthesis. We conclude that while proline biosynthesis plays little, if any, role in the intracellular life cycle and infectious nature of Listeria monocytogenes, it can play an important role in survival in osmolyte-depleted environments of elevated osmolarity.

Analysis of the role of OpuC, an osmolyte transport system, in salt tolerance and virulence potential of Listeria monocytogenes

The success of Listeria monocytogenes as a food-borne pathogen owes much to its ability to survive a variety of stresses, both in the external environment prior to ingestion and subsequently within the animal host. Growth at high salt concentrations and low temperatures is attributed mainly to the accumulation of organic solutes such as glycine betaine and carnitine. We utilized a novel system for generating chromosomal mutations (based on a lactococcal pWVO1-derived Ori(+) RepA(-) vector, pORI19) to identify a listerial OpuC homologue. Mutating the operon in two strains of L. monocytogenes revealed significant strain variation in the observed activity of OpuC. Radiolabeled osmolyte uptake studies, together with growth experiments in defined media, linked OpuC to carnitine and glycine betaine uptake in Listeria. We also investigated the role of OpuC in contributing to the growth and survival of Listeria in an animal (murine) model of infection. Altering OpuC resulted in a significant reduction in the ability of Listeria to colonize the upper small intestine and cause subsequent systemic infection following peroral inoculation.

The role of L-carnitine and glycine betaine in the survival and sub-lethal injury of non-growing Listeria monocytogenes cells during chilled storage

AIMS

To determine the role played by previous growth in the presence of osmolytes on the subsequent survival and sub-lethal injury of L. monocytogenes during long-term chilled storage in a model buffer system.

METHODS AND RESULTS

Four Listeria monocytogenes strains were grown separately to stationary phase in Listeria minimal medium (DM) alone or in DM with 4% NaCl alone, or both these media supplemented with 1 mM L-carnitine and/or 1 mM glycine betaine. Cells were resuspended in phosphate buffered saline (pH 5.5) and stored for four weeks at 4 degrees C. Initially, and at weekly intervals, samples were plated on both Tryptic Soy Agar and Tryptic Soy Agar with 4% NaCl to determine total numbers and degree of sub-lethal injury in the populations. The numbers of cells within all strains after growth to stationary phase, except one which increased ( approximately 2 log cfu ml-1, P < 0.05) in the presence of NaCl, were not influenced significantly by previous growth conditions (P > 0.05). During subsequent chilled storage, however, numbers of all strains grown in the presence of NaCl remained constant while those grown in its absence decreased. The rate and magnitude of the decrease in cell numbers was strain dependent. The initial percentage of sub-lethal injury increased significantly in all strains when grown previously in the presence of L-carnitine (P < 0.05). During subsequent chilled storage sub-lethal injury increased for all strains in a manner that was strain dependent, but not related to the previous growth conditions.

CONCLUSION

Previous growth in the presence of osmolytes of NaCl, but not osmolytes alone, increases the subsequent survival, but not percentage sub-lethal injury, of L. monocytogenes during subsequent chilled storage in buffer.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study shows that risks associated with L. monocytogenes in chilled food may be influenced by the individual life histories of the cells.

Does enterohemorrhagic Escherichia coli O157:H7 enter the viable but nonculturable state in salted salmon roe?

An outbreak caused by salted salmon roe contaminated with enterohemorrhagic Escherichia coli O157 occurred in Japan in 1998. Since about 0.75 to 1.5 viable cells were estimated to cause infection, we presumed that O157 might enter the viable but nonculturable (VNC) state in salted salmon roe and consequently that viable cell numbers might be underestimated. Although patient-originating O157 cells could not grow on agar plates after 72 h of incubation in 13% NaCl, they were resuscitated in yeast extract broth, and more than 90% of the cells were shown to be viable by fluorescent staining, suggesting that almost all of them could enter the VNC state in NaCl water. Roe-originating O157 was resistant to NaCl because it could grow on agar after 72 h of incubation in NaCl water, but about 20% of cells appeared to enter the VNC state. Therefore, germfree mice were infected with O157 to examine the resuscitation of cells in the VNC state and the retention of pathogenicity. O157 that originated in roe, but not patients, killed mice and was isolated from the intestine. However, these isolates had become sensitive to NaCl. O157 cells of roe origin incubated in normal media also killed mice and were isolated from the intestine, but they also became transiently NaCl sensitive. We therefore propose that bacterial cells might enter the VNC state under conditions of stress, such as those encountered in vivo or in high salt concentrations, and then revive when those conditions have eased. If so, the VNC state in food is potentially dangerous from a public health viewpoint and may have to be considered at the time of food inspection. Finally, the establishment of a simple recovery system for VNC cells should be established.

Identification and characterization of an ATP binding cassette L-carnitine transporter in Listeria monocytogenes

We identified an operon in Listeria monocytogenes EGD with high levels of sequence similarity to the operons encoding the OpuC and OpuB compatible solute transporters from Bacillus subtilis, which are members of the ATP binding cassette (ABC) substrate binding protein-dependent transporter superfamily. The operon, designated opuC, consists of four genes which are predicted to encode an ATP binding protein (OpuCA), an extracellular substrate binding protein (OpuCC), and two membrane-associated proteins presumed to form the permease (OpuCB and OpuCD). The operon is preceded by a potential SigB-dependent promoter. An opuC-defective mutant was generated by the insertional inactivation of the opuCA gene. The mutant was impaired for growth at high osmolarity in brain heart infusion broth and failed to grow in a defined medium. Supplementation of the defined medium with peptone restored the growth of the mutant in this medium. The mutant was found to accumulate the compatible solutes glycine betaine and choline to same extent as the parent strain but was defective in the uptake of L-carnitine. We conclude that the opuC operon in L. monocytogenes encodes an ABC compatible solute transporter which is capable of transporting L-carnitine and which plays an important role in osmoregulation in this pathogen.

Osmotic and chill activation of glycine betaine porter II in Listeria monocytogenes membrane vesicles

Listeria monocytogenes is a foodborne pathogen known for its tolerance to conditions of osmotic and chill stress. Accumulation of glycine betaine has been found to be important in the organism's tolerance to both of these stresses. A procedure was developed for the purification of membranes from L. monocytogenes cells in which the putative ATP-driven glycine betaine permease glycine betaine porter II (Gbu) is functional. As is the case for the L. monocytogenes sodium-driven glycine betaine uptake system (glycine betaine porter I), uptake in this vesicle system was dependent on energization by ascorbate-phenazine methosulfate. Vesicles lacking the gbu gene product had no uptake activity. Transport by this porter did not require sodium ion and could be driven only weakly by artificial gradients. Uptake rates could be manipulated under conditions not affecting secondary transport but known to affect ATPase activity. The system was shown to be both osmotically activated and cryoactivated. Under conditions of osmotic activation, the system exhibited Arrhenius-type behavior although the uptake rates were profoundly affected by the physical state of the membrane, with breaks in Arrhenius curves at approximately 10 and 18 degrees C. In the absence of osmotic activation, the permease could be activated by decreasing temperature within the range of 15 to 4 degrees C. Kinetic analyses of the permease at 30 degrees C revealed K(m) values for glycine betaine of 1.2 and 2.9 microM with V(max) values of 2,200 and 3,700 pmol/min. mg of protein under conditions of optimal osmotic activation as mediated by KCl and sucrose, respectively.

Osmoprotectants and cryoprotectants for Listeria monocytogenes

Listeria monocytogenes is a foodborne pathogen that can grow in high osmotic strength environments and at refrigeration temperatures. Glycine betaine, proline betaine, acetylcarnitine, carnitine, gamma-butyrobetaine and 3-dimethylsulphoniopropionate all acted as osmoprotectants, as evidenced by an increase in growth rate of L. monocytogenes 10403S and Scott A when provided with these compounds, while being stressed in defined medium containing 0.7 M NaCl. These same compounds exhibited cryoprotective activity, as evidenced by increasing the growth rate of L. monocytogenes at 5 degrees C. Ectoine, hydroxy ectoine, pipecolic acid and proline were ineffective as osmoprotectants or cryoprotectants under these conditions. The presence of osmoprotectants and cryoprotectants in foods may provide compounds assisting L. monocytogenes to overcome the barriers of high osmotic strength and low temperature that otherwise control microbial growth.

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

The trimethylammonium compound glycine betaine (N,N, N-trimethylglycine) can be accumulated to high intracellular concentrations, conferring enhanced osmo- and cryotolerance upon Listeria monocytogenes. We report the identification of betL, a gene encoding a glycine betaine uptake system in L. monocytogenes, isolated by functional complementation of the betaine uptake mutant Escherichia coli MKH13. The betL gene is preceded by a consensus sigmaB-dependent promoter and is predicted to encode a 55-kDa protein (507 amino acid residues) with 12 transmembrane regions. BetL exhibits significant sequence homologies to other glycine betaine transporters, including OpuD from Bacillus subtilis (57% identity) and BetP from Corynebacterium glutamicum (41% identity). These high-affinity secondary transporters form a subset of the trimethylammonium transporter family specific for glycine betaine, whose substrates possess a fully methylated quaternary ammonium group. The observed Km value of 7.9 microM for glycine betaine uptake after heterologous expression of betL in E. coli MKH13 is consistent with values obtained for L. monocytogenes in other studies. In addition, a betL knockout mutant which is significantly affected in its ability to accumulate glycine betaine in the presence or absence of NaCl has been constructed in L. monocytogenes. This mutant is also unable to withstand concentrations of salt as high as can the BetL+ parent, signifying the role of the transporter in Listeria osmotolerance.

Osmosensing by bacteria: signals and membrane-based sensors

Bacteria can survive dramatic osmotic shifts. Osmoregulatory responses mitigate the passive adjustments in cell structure and the growth inhibition that may ensue. The levels of certain cytoplasmic solutes rise and fall in response to increases and decreases, respectively, in extracellular osmolality. Certain organic compounds are favored over ions as osmoregulatory solutes, although K+ fluxes are intrinsic to the osmoregulatory response for at least some organisms. Osmosensors must undergo transitions between "off" and "on" conformations in response to changes in extracellular water activity (direct osmosensing) or resulting changes in cell structure (indirect osmosensing). Those located in the cytoplasmic membranes and nucleoids of bacteria are positioned for indirect osmosensing. Cytoplasmic membrane-based osmosensors may detect changes in the periplasmic and/or cytoplasmic solvent by experiencing changes in preferential interactions with particular solvent constituents, cosolvent-induced hydration changes, and/or macromolecular crowding. Alternatively, the membrane may act as an antenna and osmosensors may detect changes in membrane structure. Cosolvents may modulate intrinsic biomembrane strain and/or topologically closed membrane systems may experience changes in mechanical strain in response to imposed osmotic shifts. The osmosensory mechanisms controlling membrane-based K+ transporters, transcriptional regulators, osmoprotectant transporters, and mechanosensitive channels intrinsic to the cytoplasmic membrane of Escherichia coli are under intensive investigation. The osmoprotectant transporter ProP and channel MscL act as osmosensors after purification and reconstitution in proteoliposomes. Evidence that sensor kinase KdpD receives multiple sensory inputs is consistent with the effects of K+ fluxes on nucleoid structure, cellular energetics, cytoplasmic ionic strength, and ion composition as well as on cytoplasmic osmolality. Thus, osmoregulatory responses accommodate and exploit the effects of individual cosolvents on cell structure and function as well as the collective contribution of cosolvents to intracellular osmolality.

Effect of osmotic, alkaline, acid or thermal stresses on the growth and inhibition of Listeria monocytogenes

Five strains of Listeria monocytogenes (a, b, c, d and e) isolated from industrial plants have been subjected to different osmotic, alkaline, acid or thermal stresses. The effects of these treatments on lag-phase (L) and growth rate (mu) of cells in mid-log phase have been followed using an automated optical density monitoring system. Increasing the osmotic pressure by the addition of different amounts of NaCl increased the lag phase and decreased the growth rate. The same phenomena were observed after decreasing the pH of the medium to 5.8, 5.6 or 5.4 by addition of acetic, lactic or hydrochloric acids. The inhibitory effect was: acetic acid > lactic acid > hydrochloric acid. The addition of NaOH to attain pH values of 9.5, 10.0, 10.5 or 11.0 in the medium produced a dramatic increase of the lag phase at pH 10.5 and 11. Growth rates were also decreased while the maximal population increased with high pH values. These effects varied according to strains. Strains d and e were the most resistant to acidic and alkaline stresses, and e was the most affected by the addition of NaCl. A cold shock of 30 min at 0 degree C had limited effects on growth parameters. On the other hand, hyperthermal shocks (55 or 63 degrees C, 30 min) led to similar increased lag phases and to significant increases of the maximal population in all five strains.

Mechanism of osmotic activation of the quaternary ammonium compound transporter (QacT) of Lactobacillus plantarum

The accumulation of quaternary ammonium compounds in Lactobacillus plantarum is mediated via a single transport system with a high affinity for glycine betaine (apparent Km of 18 microM) and carnitine and a low affinity for proline (apparent Km of 950 microM) and other analogues. Mutants defective in the uptake of glycine betaine were generated by UV irradiation and selected on the basis of resistance to dehydroproline (DHP), a toxic proline analogue. Three independent DHP-resistant mutants showed reduced glycine betaine uptake rates and accumulation levels but behaved similarly to the wild type in terms of direct activation of uptake by high-osmolality conditions. Kinetic analysis of glycine betaine uptake and efflux in the wild-type and mutant cells is consistent with one uptake system for quaternary ammonium compounds in L. plantarum and a separate system(s) for their excretion. The mechanism of osmotic activation of the quaternary ammonium compound transport system (QacT) was studied. It was observed that the uptake rates were inhibited by the presence of internal substrate. Upon raising of the medium osmolality, the QacT system was rapidly activated (increase in maximal velocity) through a diminished inhibition by trans substrate as well as an effect that is independent of intracellular substrate. We also studied the effects of the cationic amphipath chlorpromazine, which inserts into the cytoplasmic membrane and thereby influences the uptake and efflux of glycine betaine. The results provide further evidence for the notion that the rapid efflux of glycine betaine upon osmotic downshock is mediated by a channel protein that is responding to membrane stretch or tension. The activation of QacT upon osmotic upshock seems to be brought about by a turgor-related parameter other than membrane stretch or tension.

Betaine and L-carnitine transport by Listeria monocytogenes Scott A in response to osmotic signals

The naturally occurring compatible solutes betaine and L-carnitine allow the food-borne pathogen Listeria monocytogenes to adjust to environments of high osmotic strength. Previously, it was demonstrated that L. monocytogenes possesses an ATP-dependent L-carnitine transporter (A. Verheul, F. M. Rombouts, R. R. Beumer, and T. Abee, J. Bacteriol. 177:3205-3212, 1995). The present study reveals that betaine and L-carnitine are taken up by separate highly specific transport systems and support a secondary transport mechanism for betaine uptake in L. monocytogenes. The initial uptake rates of betaine and L-carnitine are not influenced by an osmotic upshock, but the duration of transport of both osmolytes is directly related to the osmotic strength of the medium. Regulation of uptake of both betaine and L-carnitine is subject to inhibition by preaccumulated solute. Internal betaine inhibits not only transport of external betaine but also that of L-carnitine and, similarly, internal L-carnitine inhibits transport of both betaine and L-carnitine. The inhibition is alleviated upon osmotic upshock, which suggests that alterations in membrane structure are transmitted to the allosteric binding sites for betaine and L-carnitine of both transporters at the inner surface of the membrane. Upon osmotic downshock, betaine and L-carnitine are rapidly released by L. monocytogenes as a consequence of activation of a channel-like activity. The osmolyte-sensing mechanism described is new and is consistent with various unexplained observations of osmoregulation in other bacteria.

Protamine-induced permeabilization of cell envelopes of gram-positive and gram-negative bacteria

The inhibitory effect of the cationic peptide protamine on Listeria monocytogenes, Escherichia coli, and Shewanella putrefaciens has been studied in detail. The addition of protamine (10 to 1,000 micrograms/ml) resulted in inhibition of oxygen consumption after less than 1 min and loss of intracellular carboxyfluorescein and ATP after 2 to 5 min. Maximum antibacterial activity was reached at alkaline pH and in the absence of divalent cations. The efficient permeabilization of cell envelopes of both gram-positive and gram-negative bacteria suggests that protamine causes a general disruption of the cell envelope, leading to a rapid and nonspecific efflux of low- and high-molecular-weight compounds.

Sodium-driven, osmotically activated glycine betaine transport in Listeria monocytogenes membrane vesicles

Transport of the osmoprotectant and cryoprotectant glycine betaine was investigated in membrane vesicles of Listeria monocytogenes. Uptake-driving transmembrane potentials ranging from 111 to 122 mV within the pH range of 5.5 to 7.5 could be generated by the electron donor system ascorbate-phenazine methosulfate but not by the electron donor system ascorbate-N,N,N',N'-tetramethyl-p-phenylenediamine. Transport was dependent on both high concentrations of sodium ion and the presence of a hypertonic solute gradient. Arrhenius-type temperature activation was observed. Lineweaver-Burk plots indicated a Km of 4.4 microM for glycine betaine and a Vmax of 700 pmol/min x mg of protein. The Michaelis constant for NaCl depended on the solute used to maintain a constant hyperosmotic pressure, and the Km values were 200 and 75 mM when KCl and sucrose were employed, respectively. Transport was 65% lower in vesicles derived from cells grown under stress provided by KCI rather than NaCl and approximately 94% lower in vesicles derived from cells that were not grown under osmotic stress. This porter appears to be specific for glycine betaine, since neither proline, carnitine, nor choline inhibited uptake effectively. Kinetic studies using ionophores and artificial gradients indicate that glycine betaine is cotransported with sodium ion.

Role of osmolytes in adaptation of osmotically stressed and chill-stressed Listeria monocytogenes grown in liquid media and on processed meat surfaces

Listeria monocytogenes is a food-borne pathogen that is widely distributed in nature and is found in many kinds of fresh and processed foods. The pervasiveness of this organism is due, in part, to its ability to tolerate environments with elevated osmolarity and reduced temperatures. Previously, we showed that L. monocytogenes adapts to osmotic and chill stress by transporting the osmolyte glycine betaine from the environment and accumulating it intracellularly (R. Ko, L. T. Smith, and G. M. Smith, J. Bacteriol. 176:426-431, 1994). In the present study, the influence of various environmental conditions on the accumulation of glycine betaine and another osmolyte, carnitine, was investigated. Carnitine was shown to confer both chill and osmotic tolerance to the pathogen but was less effective than glycine betaine. The absolute amount of each osmolyte accumulated by the cell was dependent on the temperature, the osmolarity of the medium, and the phase of growth of the culture. L. monocytogenes also accumulated high levels of osmolytes when grown on a variety of processed meats at reduced temperatures. However, the contribution of carnitine to the total intracellular osmolyte concentration was much greater in samples grown on meat than in those grown in liquid media. While the amount of each osmolyte in meat was less than 1 nmol/mg (fresh weight), the overall levels of osmolytes in L. monocytogenes grown on meat were about the same as those in liquid samples, from about 200 to 1,000 nmol/mg of cell protein for each osmolyte. This finding suggests that the accumulation of osmolytes is as important in the survival of L. monocytogenes in meat as it is in liquid media.

Influence of Osmolarity on Phase Shift in Photorhabdus luminescens

The influence of osmolarity and other environmental factors like low oxygen levels, light, extreme pH values, and temperatures on phase variation of Photorhabdus luminescens, the symbiotic bacterium of entomopathogenic nematodes of the genus Heterorhabditis, was investigated. Only subculturing in low-osmolarity medium triggered a phase shift to secondary phase reliably.

An ATP-dependent L-carnitine transporter in Listeria monocytogenes Scott A is involved in osmoprotection

Listeria monocytogenes is a gram-positive, psychotrophic, food-borne pathogen which is able to grow in osmotically stressful environments. Carnitine (beta-hydroxy-L-tau-N-trimethyl aminobutyrate) can contribute significantly to growth of L. monocytogenes at high osmolarity (R. R. Beumer, M. C. te Giffel, L. J. Cox, F. M. Rombouts, and T. Abee, Appl. Environ. Microbiol. 60:1359-1363, 1994). Transport of L-[N-methyl-14C]carnitine in L. monocytogenes was shown to be energy dependent. Analysis of cell extracts revealed that L-carnitine was not further metabolized, which supplies evidence for its role as an osmoprotectant in L. monocytogenes. Uptake of L-carnitine proceeds in the absence of a proton motive force and is strongly inhibited in the presence of the phosphate analogs vanadate and arsenate. The L-carnitine permease is therefore most likely driven by ATP. Kinetic analysis of L-carnitine transport in glucose-energized cells revealed the presence of a high-affinity uptake system with a Km of 10 microM and a maximum rate of transport (Vmax) of 48 nmol min-1 mg of protein-1. L-[14C]carnitine transport in L. monocytogenes is significantly inhibited by a 10-fold excess of unlabelled L-carnitine, acetylcarnitine, and tau-butyrobetaine, whereas L-proline and betaine display, even at a 100-fold excess, only a weak inhibitory effect. In conclusion, an ATP-dependent L-carnitine transport system in L. monocytogenes is described, and its possible roles in cold adaptation and intracellular growth in mammalian cells are discussed.

Transport of glycine-betaine by Listeria monocytogenes

Uptake of [14C]glycine-betaine by Listeria monocytogenes was stimulated by NaCl with optimal stimulation at 0.4-0.5 M. The glycine-betaine transport system had a Km of 22 microM and a Vmax of 11.7 nmol-1 min-1 mg-1 protein when grown in the absence of NaCl. When grown in the presence of 0.8 M NaCl the Vmax increased to 27.0 nmol-1 min-1 mg-1 protein in 0.8 M NaCl. At NaCl concentrations above 0.5 M the uptake rate of glycine-betaine was reduced. Measurement of intracellular K+ concentrations and fluorescent dye quenching indicated that higher NaCl concentrations also led to a decrease in the electrochemical potential difference across the cytoplasmic membrane. Uptake of glycine was also observed, but this was not stimulated by NaCl.

Effect of exogenous proline, betaine, and carnitine on growth of Listeria monocytogenes in a minimal medium

Three Listeria monocytogenes strains isolated from food or food-processing environments were used to assess the response of this species to salinity in a chemically defined minimal medium. Growth in a minimal medium containing five essential amino acids and glucose as a carbon and energy source was comparable to growth in a rich medium (brain heart infusion broth). In the absence and presence of 3% NaCl the final cell numbers reached in minimal medium were 10(9) and 10(7) CFU/ml, respectively. Growth under the latter conditions could not be detected by spectrophotometry by measuring A660. Apparently, this technique was not suitable for these experiments since the detection level was > 10(7) CFU/ml. Exogenously added proline (10 mM), trimethylglycine (betaine) (1 mM), and beta-hydroxy-gamma-N-trimethyl aminobutyrate (carnitine) (1 mM) significantly stimulated growth under osmotic stress conditions in minimal medium at both 37 and 10 degrees C. Betaine and carnitine are present in foods derived from plants and animals, respectively. Therefore, these compounds can contribute significantly to growth of L. monocytogenes in various foods at high osmolarities.

Glycine betaine confers enhanced osmotolerance and cryotolerance on Listeria monocytogenes

Listeria monocytogenes is a gram-positive food-borne pathogen that is notably resistant to osmotic stress and can grow at refrigerator temperatures. These two characteristics make it an insidious threat to public health. Like several other organisms, L. monocytogenes accumulates glycine betaine, a ubiquitous and effective osmolyte, intracellularly when grown under osmotic stress. However, it also accumulates glycine betaine when grown under chill stress at refrigerator temperatures. Exogenously added glycine betaine enhances the growth rate of stressed but not unstressed cells, i.e., it confers both osmotolerance and cryotolerance. Both salt-stimulated and cold-stimulated accumulation of glycine betaine occur by transport from the medium rather than by biosynthesis. Direct measurement of glycine betaine uptake shows that cells transport betaine 200-fold faster at high salt concentration (4% NaCl) than without added salt and 15-fold faster at 7 than at 30 degrees C. The kinetics of glycine betaine transport suggest that the two transport systems are indistinguishable in terms of affinity for betaine and may be the same. Hyperosmotic shock and cold shock experiments suggest the transport system(s) to be constitutive; activation was not blocked by chloramphenicol. A cold-activated transport system is a novel observation and has intriguing implications concerning the physical state of the cell membrane at low temperature.