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

Deciphering the global roles of Cold shock proteins in Listeria monocytogenes nutrient metabolism and stress tolerance

Listeria monocytogenes (Lm) accounts for serious public health and food safety problems owing to its stress resilience and pathogenicity. Based on their regulatory involvement in global gene expression events, cold-shock domain family proteins (Csps) are crucial in expression of various stress fitness and virulence phenotypes in bacteria. Lm possesses three Csps (CspA, CspB, and CspD) whose regulatory roles in the context of the genetic diversity of this bacterium are not yet fully understood. We examined the impacts of Csps deficiency on Lm nutrient metabolism and stress tolerance using a set of csp deletion mutants generated in different genetic backgrounds. Phenotype microarrays (PM) analysis showed that the absence of Csps in ∆cspABD reduces carbon (C-) source utilization capacity and increases Lm sensitivity to osmotic, pH, various chemical, and antimicrobial stress conditions. Single and double csp deletion mutants in different Lm genetic backgrounds were used to further dissect the roles of individual Csps in these phenotypes. Selected PM-based observations were further corroborated through targeted phenotypic assays, confirming that Csps are crucial in Lm for optimal utilization of various C-sources including rhamnose and glucose as well as tolerance against NaCl, β-phenyethylamine (PEA), and food relevant detergent stress conditions. Strain and genetic lineage background-based differences, division of labour, epistasis, and functional redundancies among the Csps were uncovered with respect to their roles in various processes including C-source utilization, cold, and PEA stress resistance. Finally, targeted transcriptome analysis was performed, revealing the activation of csp gene expression under defined stress conditions and the impact of Csps on expression regulation of selected rhamnose utilization genes. Overall, our study shows that Csps play important roles in nutrient utilization and stress responses in Lm strains, contributing to traits that are central to the public health and food safety impacts of this pathogen.

Different Shades of Listeria monocytogenes: Strain, Serotype, and Lineage-Based Variability in Virulence and Stress Tolerance Profiles

Listeria monocytogenes is a public health and food safety challenge due to its virulence and natural stress resistance phenotypes. The variable distribution of L. monocytogenes molecular subtypes with respect to food products and processing environments and among human and animal clinical listeriosis cases is observed. Sixty-two clinical and food-associated L. monocytogenes isolates were examined through phenome and genome analysis. Virulence assessed using a zebrafish infection model revealed serotype and genotype-specific differences in pathogenicity. Strains of genetic lineage I serotype 4b and multilocus sequence type clonal complexes CC1, CC2, CC4, and CC6 grew and survived better and were more virulent than serotype 1/2a and 1/2c lineage II, CC8, and CC9 strains. Hemolysis, phospholipase activity, and lysozyme tolerance profiles were associated with the differences observed in virulence. Osmotic stress resistance evaluation revealed serotype 4b lineage I CC2 and CC4 strains as more osmotolerant, whereas serotype 1/2c lineage II CC9 strains were more osmo-sensitive than others. Variable tolerance to the widely used quaternary ammonium compound benzalkonium chloride (BC) was observed. Some outbreak and sporadic clinical case associated strains demonstrated BC tolerance, which might have contributed to their survival and transition in the food-processing environment facilitating food product contamination and ultimately outbreaks or sporadic listeriosis cases. Genome comparison uncovered various moderate differences in virulence and stress associated genes between the strains indicating that these differences in addition to gene expression regulation variations might largely be responsible for the observed virulence and stress sensitivity phenotypic differences. Overall, our study uncovered strain and genotype-dependent variation in virulence and stress resilience among clinical and food-associated L. monocytogenes isolates with potential public health risk implications. The extensive genome and phenotypic data generated provide a basis for developing improved Listeria control strategies and policies.

Cold Regulation of Genes Encoding Ion Transport Systems in the Oligotrophic Bacterium Caulobacter crescentus

In this study, we characterize the response of the free-living oligotrophic alphaproteobacterium Caulobacter crescentus to low temperatures by global transcriptomic analysis. Our results showed that 656 genes were upregulated and 619 were downregulated at least 2-fold after a temperature downshift. The identified differentially expressed genes (DEG) belong to several functional categories, notably inorganic ion transport and metabolism, and a subset of these genes had their expression confirmed by reverse transcription quantitative real-time PCR (RT-qPCR). Several genes belonging to the ferric uptake regulator (Fur) regulon were downregulated, indicating that iron homeostasis is relevant for adaptation to cold. Several upregulated genes encode proteins that interact with nucleic acids, particularly RNA: cspA, cspB, and the DEAD box RNA helicases rhlE, dbpA, and rhlB. Moreover, 31 small regulatory RNAs (sRNAs), including the cell cycle-regulated noncoding RNA (ncRNA) CcnA, were upregulated, indicating that posttranscriptional regulation is important for the cold stress response. Interestingly, several genes related to transport were upregulated under cold stress, including three AcrB-like cation/multidrug efflux pumps, the nitrate/nitrite transport system, and the potassium transport genes kdpFABC. Further characterization showed that kdpA is upregulated in a potassium-limited medium and at a low temperature in a SigT-independent way. kdpA mRNA is less stable in rho and rhlE mutant strains, but while the expression is positively regulated by RhlE, it is negatively regulated by Rho. A kdpA-deleted strain was generated, and its viability in response to osmotic, acidic, or cold stresses was determined. The implications of such variation in the gene expression for cold adaptation are discussed. IMPORTANCE Low-temperature stress is an important factor for nucleic acid stability and must be circumvented in order to maintain the basic cell processes, such as transcription and translation. The oligotrophic lifestyle presents further challenges to ensure the proper nutrient uptake and osmotic balance in an environment of slow nutrient flow. Here, we show that in Caulobacter crescentus, the expression of the genes involved in cation transport and homeostasis is altered in response to cold, which could lead to a decrease in iron uptake and an increase in nitrogen and high-affinity potassium transport by the Kdp system. This previously uncharacterized regulation of the Kdp transporter has revealed a new mechanism for adaptation to low temperatures that may be relevant for oligotrophic bacteria.

Listeria monocytogenes Cold Shock Proteins: Small Proteins with A Huge Impact

Listeria monocytogenes has evolved an extensive array of mechanisms for coping with stress and adapting to changing environmental conditions, ensuring its virulence phenotype expression. For this reason, L. monocytogenes has been identified as a significant food safety and public health concern. Among these adaptation systems are cold shock proteins (Csps), which facilitate rapid response to stress exposure. L. monocytogenes has three highly conserved csp genes, namely, cspA, cspB, and cspD. Using a series of csp deletion mutants, it has been shown that L. monocytogenes Csps are important for biofilm formation, motility, cold, osmotic, desiccation, and oxidative stress tolerance. Moreover, they are involved in overall virulence by impacting the expression of virulence-associated phenotypes, such as hemolysis and cell invasion. It is postulated that during stress exposure, Csps function to counteract harmful effects of stress, thereby preserving cell functions, such as DNA replication, transcription and translation, ensuring survival and growth of the cell. Interestingly, it seems that Csps might suppress tolerance to some stresses as their removal resulted in increased tolerance to stresses, such as desiccation for some strains. Differences in csp roles among strains from different genetic backgrounds are apparent for desiccation tolerance and biofilm production. Additionally, hierarchical trends for the different Csps and functional redundancies were observed on their influences on stress tolerance and virulence. Overall current data suggest that Csps have a wider role in bacteria physiology than previously assumed.

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.

Complete genome sequence of a piezophilic bacterium Salinimonas sediminis N102T, isolated from deep-sea sediment of the New Britain Trench

Salinimonas sediminis N102^T is a cold-adapted, slightly halophilic piezophile isolated from deep-sea sediment (4700 m) of the New Britain Trench. In this study, we report the complete genome sequence of S. sediminis N102^T, which is comprised of 4,440,293 base pairs with a mean G + C content of 48.2 mol%. The complete genome harbors 3851 predicted protein-coding genes, 70 tRNA genes and 15 rRNA genes. Abundant genes in the genome were predicted to be linked to bacterial deep-sea lifestyle. The complete genome sequence of S. sediminis N102^T provides insights into the microbial adaptation strategies to the deep-sea environment.

Initial Transcriptomic Response and Adaption of Listeria monocytogenes to Desiccation on Food Grade Stainless Steel

The foodborne pathogen Listeria monocytogenes survives exposure to a variety of stresses including desiccation in the food industry. Strand-specific RNA sequencing was applied to analyze changes in the transcriptomes of two strains of L. monocytogenes (Lm 568 and Lm 08-5578) during desiccation [15°C, 43% relative humidity (RH)] on food grade stainless steel surfaces over 48 h to simulate a weekend with no food production. Both strains showed similar survival during desiccation with a 1.8-2 Log CFU/cm2 reduction after 48 h. Analysis of differentially expressed (DE) genes (>twofold, adjusted p-value <0.05) revealed that the initial response to desiccation was established after 6 h and remained constant with few new genes being DE after 12, 24, and 48 h. A core of 81 up- and 73 down-regulated DE genes were identified as a shared, strain independent response to desiccation. Among common upregulated genes were energy and oxidative stress related genes e.g., qoxABCD (cytochrome aa3) pdhABC (pyruvate dehydrogenase complex) and mntABCH (manganese transporter). Common downregulated genes related to anaerobic growth, proteolysis and the two component systems lmo1172/lmo1173 and cheA/cheY, which are involved in cold growth and flagellin production, respectively. Both strains upregulated additional genes involved in combatting oxidative stress and reactive oxygen species (ROS), including sod (superoxide dismutase), kat (catalase), tpx (thiol peroxidase) and several thioredoxins including trxAB, lmo2390 and lmo2830. Osmotic stress related genes were also upregulated in both strains, including gbuABC (glycine betaine transporter) and several chaperones clpC, cspA, and groE. Significant strain differences were also detected with the food outbreak strain Lm 08-5578 differentially expressing 1.9 × more genes (726) compared to Lm 568 (410). Unique to Lm 08-5578 was a significant upregulation of the expression of the alternative transcription factor σB and its regulon. A number of long antisense transcripts (lasRNA) were upregulated during desiccation including anti0605, anti0936, anti1846, and anti0777, with the latter controlling flagellum biosynthesis and possibly the downregulation of motility genes observed in both strains. This exploration of the transcriptomes of desiccated L. monocytogenes provides further understanding of how this bacterium encounters and survives the stress faced when exposed to dry conditions in the food industry.

Carotenoids are used as regulators for membrane fluidity by Staphylococcus xylosus

Carotenoids are associated with several important biological functions as antenna pigments in photosynthesis or protectives against oxidative stress. Occasionally they were also discussed as part of the cold adaptation mechanism of bacteria. For two Staphylococcus xylosus strains we demonstrated an increased content of staphyloxanthin and other carotenoids after growth at 10 °C but no detectable carotenoids after grow at 30 °C. By in vivo measurements of generalized polarization and anisotropy with two different probes Laurdan and TMA-DPH we detected a strong increase in membrane order with a simultaneous increase in membrane fluidity at low temperatures accompanied by a broadening of the phase transition. Increased carotenoid concentration was also correlated with an increased resistance of the cells against freeze-thaw stress. In addition, the fatty acid profile showed a moderate adaptation to low temperature by increasing the portion of anteiso-branched fatty acids. The suppression of carotenoid synthesis abolished the effects observed and thus confirmed the causative function of the carotenoids in the modulation of membrane parameters. A differential transcriptome analysis demonstrated the upregulation of genes involved in carotenoid syntheses under low temperature growth conditions. The presented data suggests that upregulated synthesis of carotenoids is a constitutive component in the cold adaptation strategy of Staphylococcus xylosus and combined with modifications of the fatty acid profile constitute the adaptation to grow under low temperature conditions.

Genome Sequencing of Mesonia algae K4-1 Reveals Its Adaptation to the Arctic Ocean

The special ecological environment of the Arctic has brought about a large number of salt-tolerant and psychrotolerant microorganisms. We isolated two culturable bacterial strains of the genus Mesonia; one from the Arctic ocean, Mesonia algae K4-1, and one from the tropical sea, Mesonia sp. HuA40. Our genome analysis and phenotypic experiments indicated that Mesonia algae K4-1 is a moderately halophilic and psychrophilic bacterium. Mesonia algae K4-1 can tolerate 3–14% NaCl and grow at a wide range of temperatures from 4 to 50°C. Mesonia sp. HuA40 is a mesophilic bacterium that can only grow with 3–9% NaCl. In addition, the salt adaptation strategy of Mesonia algae K4-1 accumulates organic osmolytes in the cell. RNA helicases, glutathione and organic compatible solutes may play important roles in maintaining the metabolism and physiological function of Mesonia algae K4-1 under cold stress. Moreover, the ability of Mesonia algae K4-1 to adapt to an oligotrophic marine environment is likely due to the synthesis of a large number of extracellular polysaccharides and the secretion of various families of extracellular proteases. This study systematically analyzed the relationship between genomic differentiation and environmental factors of the Mesonia genus and revealed the possible adaptation mechanism of Mesonia algae K4-1 in the extreme Arctic marine environment at the genomic level.

Listeria monocytogenes Biofilm Adaptation to Different Temperatures Seen Through Shotgun Proteomics

Listeria monocytogenes is a foodborne pathogen that can cause invasive severe human illness (listeriosis) in susceptible patients. Most human listeriosis cases appear to be caused by consumption of refrigerated ready-to-eat foods. Although initial contamination levels in foods are usually low, the ability of these bacteria to survive and multiply at low temperatures allows it to reach levels high enough to cause disease. This study explores the set of proteins that might have an association with L. monocytogenes adaptation to different temperatures. Cultures were grown in biofilm, the most widespread mode of growth in natural and industrial realms. Protein extractions were performed from three different growth temperatures (10, 25, and 37°C) and two growth phases (early stage and mature biofilm). L. monocytogenes subproteomes were targeted using three extraction methods: trypsin-enzymatic shaving, biotin-labeling and cell fractionation. The different subproteomes obtained were separated and analyzed by shotgun proteomics using high-performance liquid chromatography combined with tandem mass spectrometry (LC-OrbiTrap LTQVelos, ThermoFisher Scientific). A total of 141 (biotinylation), 98 (shaving) and 910 (fractionation) proteins were identified. Throughout the 920 unique proteins identified, many are connected to basic cell functions, but some are linked with thermoregulation. We observed some noteworthy protein abundance shifts associated with the major adaptation to cold mechanisms present in L. monocytogenes, namely: the role of ribosomes and the stressosome with a higher abundance of the general stress protein Ctc (Rl25) and the general stress transcription factor sigma B (σB), changes in cell fluidity and motility seen by higher levels of foldase protein PrsA2 and flagellin (FlaA), the uptake of osmolytes with a higher abundance of glycine betaine (GbuB) and carnitine transporters (OpucA), and the relevance of the overexpression of chaperone proteins such as cold shock proteins (CspLA and Dps). As for 37°C, we observed a significantly higher percentage of proteins associated with transcriptional or translational activity present in higher abundance upon comparison with the colder settings. These contrasts of protein expression throughout several conditions will enrich databases and help to model the regulatory circuitry that drives adaptation of L. monocytogenes to environments.

Draft Genome Sequence of Clostridium tagluense Strain A121T, Isolated from a Permafrost Core in the Canadian High Arctic

Clostridium tagluense strain A121^T was isolated from a permafrost core in the Canadian High Arctic. This endospore-forming strain is a strictly anaerobic, Gram-positive, and psychrotolerant bacterium.

Clostridium tagluense strain A121^T was isolated from a permafrost core in the Canadian High Arctic. This endospore-forming strain is a strictly anaerobic, Gram-positive, and psychrotolerant bacterium. This article describes the draft genome sequence of C. tagluense strain A121^T.

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

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

Strand specific RNA-sequencing and membrane lipid profiling reveals growth phase-dependent cold stress response mechanisms in Listeria monocytogenes

The human pathogen Listeria monocytogenes continues to pose a challenge in the food industry, where it is known to contaminate ready-to-eat foods and grow during refrigerated storage. Increased knowledge of the cold-stress response of this pathogen will enhance the ability to control it in the food-supply-chain. This study utilized strand-specific RNA sequencing and whole cell fatty acid (FA) profiling to characterize the bacterium's cold stress response. RNA and FAs were extracted from a cold-tolerant strain at five time points between early lag phase and late stationary-phase, both at 4°C and 20°C. Overall, more genes (1.3×) were suppressed than induced at 4°C. Late stationary-phase cells exhibited the greatest number (n = 1,431) and magnitude (>1,000-fold) of differentially expressed genes (>2-fold, p<0.05) in response to cold. A core set of 22 genes was upregulated at all growth phases, including nine genes required for branched-chain fatty acid (BCFA) synthesis, the osmolyte transporter genes opuCBCD, and the internalin A and D genes. Genes suppressed at 4°C were largely associated with cobalamin (B12) biosynthesis or the production/export of cell wall components. Antisense transcription accounted for up to 1.6% of total mapped reads with higher levels (2.5×) observed at 4°C than 20°C. The greatest number of upregulated antisense transcripts at 4°C occurred in early lag phase, however, at both temperatures, antisense expression levels were highest in late stationary-phase cells. Cold-induced FA membrane changes included a 15% increase in the proportion of BCFAs and a 15% transient increase in unsaturated FAs between lag and exponential phase. These increases probably reduced the membrane phase transition temperature until optimal levels of BCFAs could be produced. Collectively, this research provides new information regarding cold-induced membrane composition changes in L. monocytogenes, the growth-phase dependency of its cold-stress regulon, and the active roles of antisense transcripts in regulating its cold stress response.

Biosynthesis and uptake of glycine betaine as cold-stress response to low temperature in fish pathogen Vibrio anguillarum

Fish pathogen Vibrio anguillarum, a mesophile bacterium, is usually found in estuarine and marine coastal ecosystems worldwide that pose a constant stress to local organism by its fluctuation in salinity as well as notable temperature change. Though V. anguillarum is able to proliferate while maintain its pathogenicity under low temperature (5-18°C), so far, coldadaption molecular mechanism of the bacteria is unknown. In this study, V. anguillarum was found possessing a putative glycine betaine synthesis system, which is encoded by betABI and synthesizes glycine betaine from its precursor choline. Furthermore, significant up-regulation of the bet gene at the transcriptional level was noted in log phase in response to cold-stress. Moreover, the accumulation of betaine glycine was only found appearing at low growth temperatures, suggesting that response regulation of both synthesis system and transporter system are cold-dependent. Furthermore, in-frame deletion mutation in the two putative ABC transporters and three putative BCCT family transporters associated with glycine betaine uptake could not block cellular accumulation of betaine glycine in V. anguillarum under coldstress, suggesting the redundant feature in V. anguillarum betaine transporter system. These findings confirmed that glycine betaine serves as an effective cold stress protectant and highlighted an underappreciated facet of the acclimatization of V. anguillarum to cold environments.

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.

Increased Biomass Production by Mesophilic Food-Associated Bacteria through Lowering the Growth Temperature from 30°C to 10°C

Five isolates from chilled food and refrigerator inner surfaces and closely related reference strains of the species Escherichia coli, Listeria monocytogenes, Staphylococcus xylosus, Bacillus cereus, Pedobacter nutrimenti, and Pedobacter panaciterrae were tested for the effect of growth temperature (30°C and 10°C) on biomass formation. Growth was monitored via optical density, and biomass formation was measured at the early stationary phase based on the following parameters in complex and defined media: viable cell count, total cell count, cell dry weight, whole-cell protein content, and cell morphology. According to the lack of growth at 1°C, all strains were assigned to the thermal class of mesophiles. Glucose and ammonium consumption related to cell yield were analyzed in defined media. Except for the protein content, temperature had a significant (t test, P < 0.05) effect on all biomass formation parameters for each strain. The results show a significant difference between the isolates and the related reference strains. Isolates achieved an increase in biomass production between 20% and 110% at the 10°C temperature, which is 15 to 25°C lower than their maximum growth rate temperatures. In contrast, reference strains showed a maximum increase of only about 25%, and some reference strains showed no increase or a decrease of approximately 25%. As expected, growth rates for all strains were higher at 30°C than at 10°C, while biomass production for isolates was higher at 10°C than at 30°C. In contrast, the reference strains showed similar growth yields at the two temperatures. This also demonstrates for mesophilic bacterial strains more efficient nutrient assimilation during growth at low temperatures. Until now, this characteristic was attributed only to psychrophilic microorganisms.

IMPORTANCE

For several psychrophilic species, increased biomass formation was described at temperatures lower than optimum growth temperatures, which are defined by the highest growth rate. This work shows increased biomass formation at low growth temperatures for mesophilic isolates. A comparison with closely related reference strains from culture collections showed a significantly smaller increase or no increase in biomass formation. This indicates a loss of specific adaptive mechanisms (e.g., cold adaptation) for mesophiles during long-term cultivation. The increased biomass production for mesophiles under low-temperature conditions opens new avenues for a more efficient biotechnological transformation of nutrients to microbial biomass. These findings may also be important for risk assessment of cooled foods since risk potential is often correlated with the cell numbers present in food samples.

Under the microscope: From pathogens to probiotics and back

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

Carnitine in bacterial physiology and metabolism

Carnitine is a quaternary amine compound found at high concentration in animal tissues, particularly muscle, and is most well studied for its contribution to fatty acid transport into mitochondria. In bacteria, carnitine is an important osmoprotectant, and can also enhance thermotolerance, cryotolerance and barotolerance. Carnitine can be transported into the cell or acquired from metabolic precursors, where it can serve directly as a compatible solute for stress protection or be metabolized through one of a few distinct pathways as a nutrient source. In this review, we summarize what is known about carnitine physiology and metabolism in bacteria. In particular, recent advances in the aerobic and anaerobic metabolic pathways as well as the use of carnitine as an electron acceptor have addressed some long-standing questions in the field.

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

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

σ(B) affects biofilm formation under the dual stress conditions imposed by adding salt and low temperature in Listeria monocytogenes

The food-borne pathogenic bacteria Listeria monocytogenes can form biofilms on various surfaces including food-processing equipment. Biofilms offer survival benefits to the organisms entrapped against environmental insults. Moreover, the σ(B) transcription factor of L. monocytogenes plays an important role in its survival under various stress conditions. In this study, we evaluated whether σ(B) contributes to biofilm formation when L. monocytogenes is grown under various temperatures and media. When the wild-type strain was grown under static biofilm culture below ambient temperature (15°C) for 72 h, the difference in viable cell number (in both planktonic and biofilm cells) between the wild-type and ΔsigB mutant increased by adding NaCl to BHI broth (9% salt BHI > 6% salt BHI > BHI, w/v), and the specific activity of β-galactosidase was highly induced in the wild-type strain grown in 6% salt containing BHI broth. Furthermore, we measured surface-adhered biofilm forming ability using the crystal violet staining method. The wild-type strain formed a four times larger biofilm than that of the ΔsigB mutant in 6% salt-BHI medium at 15°C over a 72 h incubation and also showed the highest level of β-galactosidase specific activity. However, both the wild-type and ΔsigB mutant L. monocytogenes were defective for forming a biofilm in 9% salt-BHI medium at 15°C. Our results suggest that σ(B) plays an enhanced role in surface-adhered biofilm formation when L. monocytogenes encounters dual stress conditions, such as 6% NaCl and low temperature.

Combined metagenomic and phenomic approaches identify a novel salt tolerance gene from the human gut microbiome

In the current study, a number of salt-tolerant clones previously isolated from a human gut metagenomic library were screened using Phenotype MicroArray (PM) technology to assess their functional capacity. PM's can be used to study gene function, pathogenicity, metabolic capacity and identify drug targets using a series of specialized microtitre plate assays, where each well of the microtitre plate contains a different set of conditions and tests a different phenotype. Cellular respiration is monitored colorimetrically by the reduction of a tetrazolium dye. One clone, SMG 9, was found to be positive for utilization/transport of L-carnitine (a well-characterized osmoprotectant) in the presence of 6% w/v sodium chloride (NaCl). Subsequent experiments revealed a significant growth advantage in minimal media containing NaCl and L-carnitine. Fosmid sequencing revealed putative candidate genes responsible for the phenotype. Subsequent cloning of two genes did not replicate the L-carnitine-associated phenotype, although one of the genes, a σ⁵⁴-dependent transcriptional regulator, did confer salt tolerance to Escherichia coli when expressed in isolation. The original clone, SMG 9, was subsequently found to have lost the original observed phenotype upon further investigation. Nevertheless, this study demonstrates the usefulness of a phenomic approach to assign a functional role to metagenome-derived clones.

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.

Transcription factor σB plays an important role in the production of extracellular membrane-derived vesicles in Listeria monocytogenes

Gram-negative bacteria produce extracellular outer membrane vesicles (OMVs) that interact with host cells. Unlike Gram-negative bacteria, less is known about the production and role of extracellular membrane vesicles (MVs) in Gram-positive bacteria. The food-borne pathogen Listeria monocytogenes can survive under extreme environmental and energy stress conditions and the transcription factor σ^B is involved in this survival ability. Here, we first determined the production of MVs from L. monocytogenes and evaluated whether general stress transcription factor σ^B affected production of MVs in L. monocytogenes. L. monocytogenes secreted MVs during in vitro broth culture. The wild-type strain actively produced MVs approximately nine times more and also produced more intact shapes of MVs than those of the isogenic ΔsigB mutant. A proteomic analysis showed that 130 and 89 MV proteins were identified in the wild-type and ΔsigB mutant strains, respectively. Wild-type strain-derived MVs contained proteins regulated by σ^B such as transporters (OpuCA and OpuCC), stress response (Kat), metabolism (LacD), translation (InfC), and cell division protein (FtsZ). Gene Ontology (GO) enrichment analysis showed that wild-type-derived MV proteins corresponded to several GO terms, including response to stress (heat, acid, and bile resistance) and extracellular polysaccharide biosynthetic process, but not the ΔsigB mutant. Internalin B (InlB) was almost three times more contained in MVs derived from the wild-type strain than in MVs derived from the ΔsigB mutant. Taken together, these results suggest that σ^B plays a pivotal role in the production of MVs and protein profiles contained in MVs. L. monocytogenes MVs may contribute to host infection and survival ability under various stressful conditions.

An inter-order horizontal gene transfer event enables the catabolism of compatible solutes by Colwellia psychrerythraea 34H

Colwellia is a genus of mostly psychrophilic halophilic Gammaproteobacteria frequently isolated from polar marine sediments and sea ice. In exploring the capacity of Colwellia psychrerythraea 34H to survive and grow in the liquid brines of sea ice, we detected a duplicated 37 kbp genomic island in its genome based on the abnormally high G + C content. This island contains an operon encoding for heterotetrameric sarcosine oxidase and is located adjacent to several genes used in the serial demethylation of glycine betaine, a compatible solute commonly used for osmoregulation, to dimethylglycine, sarcosine, and glycine. Molecular clock inferences of important events in the adaptation of C. psychrerythraea 34H to compatible solute utilization reflect the geological evolution of the polar regions. Validating genomic predictions, C. psychrerythraea 34H was shown to grow on defined media containing either choline or glycine betaine, and on a medium with sarcosine as the sole organic source of carbon and nitrogen. Growth by 8 of 9 tested Colwellia species on a newly developed sarcosine-based defined medium suggested that the ability to catabolize glycine betaine (the catabolic precursor of sarcosine) is likely widespread in the genus Colwellia. This capacity likely provides a selective advantage to Colwellia species in cold, salty environments like sea ice, and may have contributed to the ability of Colwellia to invade these extreme niches.

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

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

Growth and filamentation of cold-adapted, log-phase Listeria monocytogenes exposed to salt, acid, or alkali stress at 3°C

In Canada, there is a zero tolerance for Listeria in a 125-g sample of product in which growth of Listeria monocytogenes can occur, and a limit of ≤100 CFU/g in ready-to-eat (RTE) food products that support limited growth during the stated shelf life and/or RTE refrigerated foods with a shelf life of ≤5 days. L. monocytogenes can form filaments in response to pH and osmotic, atmospheric, and temperature stress, which can result in an underestimation of the risk of RTE foods as filaments form single colonies on plate count agars but can divide into individual cells once the stress is removed. The objective was to investigate the filamentation characteristics of three strains of L. monocytogenes exposed to saline, acidic, basic, and simultaneous acidic and saline environments at 3°C. After 4 days at 3°C, log-phase cells grown in tryptic soy broth (TSB) were longer than cells grown at 15°C, and 68% of cells were below the reference value of the 90th percentile of control cultures. When cultures growing at 3°C were exposed to additional stresses, increases in the proportion and length of filaments in the population were observed, while increases in log CFU per milliliter were reduced. After 4 days of incubation at 3°C, the log CFU per milliliter of L. monocytogenes increased by 1.1 U in TSB and 0.4 to 0.5 U in TSB with 4% NaCl, TSB with a pH of 6.0 with 4% NaCl, and TSB with a pH of 5.5. Moreover, the longest 10% of cells were 6.4 to 8.5 times longer than control cells, and only 20 to 30% of cells were below the reference value. Cultures grown in TSB at pH 6.0 with 4% NaCl experienced more sustained filamentation than cultures grown in TSB with 4% NaCl, but less than cultures grown in TSB at pH 6.0. The mechanism involved in filamentation could be different for cells exposed to NaCl than exposed to acid, and additional stress might not necessarily result in more extensive filament formation. These findings contribute to a better understanding of the widespread potential of filament formation and the potential implications for food safety.

Listeria monocytogenes mutants with altered growth phenotypes at refrigeration temperature and high salt concentrations

Listeria monocytogenes can survive and grow in refrigerated temperatures and high-salt environments. In an effort to better understand the associated mechanisms, a library of ∼ 5,200 transposon mutants of LS411, a food isolate from the Jalisco cheese outbreak, were screened for their ability to grow in brain heart infusion (BHI) broth at 5°C or in the presence of 7% NaCl and two mutants with altered growth profiles were identified. The LS522 mutant has a transposon insertion between secA2 and iap and showed a significant reduction in growth in BHI broth at 5°C and in the presence of 7% NaCl. Reverse transcriptase quantitative PCR (RT-qPCR) revealed a substantial reduction in the expression of iap. Additionally, a hypothetical gene (met), containing a putative S-adenosylmethionine-dependent methyltransferase domain, downstream of iap had downregulated expression. In-frame deletion mutants of iap and met were created in LS411. The LS560 (LS411 Δiap) mutant showed reduced growth at 5°C and in the presence of 7% salt, confirming its role in cold and salt growth attenuation. Surprisingly, the LS655 (LS411 Δmet) mutant showed slightly increased growth during refrigeration, though no alteration was seen in salt growth relative to the wild-type strain. The LS527 mutant, containing an insertion 36 bp upstream of the gbu operon, showed reduced expression of the gbu transcript by RT-qPCR and also showed growth reduction at 5°C and in the presence of 7% salt. This attenuation was severely exacerbated when the mutant was grown under the combined stresses. Analysis of the gbu operon deletion mutant showed decreased growth in 7% salt and refrigeration, supporting the previously characterized role for this gene in cold and salt adaptation. These studies indicate the potential for an intricate relationship between environmental stress regulation and virulence in L. monocytogenes.

Rapid, transient, and proportional activation of σ(B) in response to osmotic stress in Listeria monocytogenes

The osmotic activation of sigma B (σ(B)) in Listeria monocytogenes was studied by monitoring expression of four known σ(B)-dependent genes, opuCA, lmo2230, lmo2085, and sigB. Activation was found to be rapid, transient, and proportional to the magnitude of the osmotic stress applied, features that underpin the adaptability of this pathogen.

Role of flhA and motA in growth of Listeria monocytogenes at low temperatures

While temperature-dependent induction of flagella is a well-characterized phenomenon in Listeria monocytogenes, the essentiality of increased flagellum production during growth at low temperatures remains unclear. To study this relationship, we compared the relative expression levels of two motility genes, flhA and motA, at 3°C, 25°C and 37°C in L. monocytogenes strain EGD-e by using qRT-PCR, and compared the growth curves, motility, and flagellation between the wild-type and flhA and motA deletion mutants. The relative expression levels of flhA and motA at 3°C were significantly higher than at 37°C (p<0.01). At 3°C, the level of flhA transcripts was also significantly higher than at 25°C (p<0.01). Growth curve analysis showed that at 3°C both the growth rates and maximum optical densities of ΔflhA and ΔmotA strains at 600 nm were significantly lower than those of the wild-type (p<0.001), while no significant differences were observed between the wild-type and the mutants at 37°C, and 25°C. Mutant strains ΔflhA and ΔmotA were nonmotile at all three temperatures. At 25°C, the number of flagellated cells of ΔmotA was notably reduced compared with the wild-type, whereas ΔflhA appeared nonflagellated at all temperatures. The results suggest that flhA and motA play a role in the cold tolerance of L. monocytogenes strain EGD-e, and that motile flagella may be needed for optimal cold stress response of L. monocytogenes.

Protection of Bacillus subtilis against cold stress via compatible-solute acquisition

Accumulation of compatible solutes is a strategy widely employed by bacteria to achieve cellular protection against high osmolarity. These compounds are also used in some microorganisms as thermostress protectants. We found that Bacillus subtilis uses the compatible solute glycine betaine as an effective cold stress protectant. Glycine betaine strongly stimulated growth at 15°C and permitted cell proliferation at the growth-inhibiting temperature of 13°C. Initial uptake of glycine betaine at 15°C was low but led eventually to the buildup of an intracellular pool whose size was double that found in cells grown at 35°C. Each of the three glycine betaine transporters (OpuA, OpuC, and OpuD) contributed to glycine betaine accumulation in the cold. Protection against cold stress was also accomplished when glycine betaine was synthesized from its precursor choline. Growth of a mutant defective in the osmoadaptive biosynthesis for the compatible solute proline was not impaired at low temperature (15°C). In addition to glycine betaine, the compatible solutes and osmoprotectants l-carnitine, crotonobetaine, butyrobetaine, homobetaine, dimethylsulfonioactetate, and proline betaine all served as cold stress protectants as well and were accumulated via known Opu transport systems. In contrast, the compatible solutes and osmoprotectants choline-O-sulfate, ectoine, proline, and glutamate were not cold protective. Our data highlight an underappreciated facet of the acclimatization of B. subtilis to cold environments and allow a comparison of the characteristics of compatible solutes with respect to their osmotic, heat, and cold stress-protective properties for B. subtilis cells.

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

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

Proteomics for the elucidation of cold adaptation mechanisms in Listeria monocytogenes

Listeria monocytogenes, one of the major food-related pathogens, is the aetiological agent of listeriosis, a potentially life-threatening illness. It is able to survive in hostile environments and stress conditions such as those encountered in food-processing technologies (high salt concentration, wide range of pH and temperature, low water availability) and it also thrives at temperatures ranging from -0.4 to 45 °C. In this study, expression proteomics was applied to gain insight into key cellular events that allow L. monocytogenes to survive and multiply even at refrigeration temperatures. Interestingly, we observed that the adaptation processes mainly affect biochemical pathways related to protein synthesis and folding, nutrient uptake and oxidative stress. Furthermore, proteins implicated in metabolic pathways for energy production, such as glycolysis and Pta-AckA pathway, were present to a higher level in the cells grown at 4 °C. This suggests that, on the whole, cells exhibit an enhanced demand for energy to sustain cold growth. Proteomics may represent a key tool in deciphering specific mechanisms underlying cold adaptation response and, more widely, cell machinery.

Evaluation of cold growth and related gene transcription responses associated with Listeria monocytogenes strains of different origins

The cold growth phenotypes and transcriptional activation of cold stress adaptation genes was evaluated amongst Listeria monocytogenes strains from human listeriosis cases, food products and associated production environments. Significant cold growth phenotypic variation was observed during growth of such strains in rich (BHI) as well as chemically defined minimal (MDM) nutrient conditions. While all twenty analyzed strains grew in BHI at 4 degrees C, only eight of these strains, mostly those recovered from human listeriosis cases, were also able to grow in MDM under similar cold stress. The cold growth phenotypes observed in BHI were used to define two categories of five strains each, which either displayed enhanced and poor cold tolerance relative to the rest of the strain collection. The first group (GP1) consisted of strains characterized by short lag times, whilst the second group (GP2) comprised of strains displaying prolonged lag times before growth resumption during incubation in BHI cultures at 4 degrees C. Transcription level activation of sigB, cspA and pgpH gene expression associated with cold stress exposure in a selection of GP1 and GP2 strains was assessed. Despite similar cold dependent sigB transcript induction between these two strain groups, there were significant differences observed in cold stress dependent induction of cspA and pgpH transcripts. Cold tolerant GP1 strains displayed relatively higher transcriptional activation of cspA and pgpH after cold stress exposure compared to the cold sensitive GP2 strains. This study highlights strain variability in cold stress tolerance phenotypes, as well as in strain capacity to activate specific cold adaptation gene expression responses. In addition the study also shows that enhanced and poor cold growth phenotypes are associated with particular strain capacity to activate important cold stress gene expression responses upon transition of L. monocytogenes into low temperature environments.

Compatible solutes: A listerial passe-partout?

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

Probiotics and gastrointestinal disease: successes, problems and future prospects

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

Listeria monocytogenes CtaP is a multifunctional cysteine transport-associated protein required for bacterial pathogenesis

The bacterial pathogen Listeria monocytogenes survives under a myriad of conditions in the outside environment and within the human host where infections can result in severe disease. Bacterial life within the host requires the expression of genes with roles in nutrient acquisition as well as the biosynthesis of bacterial products required to support intracellular growth. A gene product identified as the substrate-binding component of a novel oligopeptide transport system (encoded by lmo0135) was recently shown to be required for L. monocytogenes virulence. Here we demonstrate that lmo0135 encodes a multifunctional protein that is associated with cysteine transport, acid resistance, bacterial membrane integrity and adherence to host cells. The lmo0135 gene product (designated CtaP, for cysteine transport associated protein) was required for bacterial growth in the presence of low concentrations of cysteine in vitro, but was not required for bacterial replication within the host cytosol. Loss of CtaP increased membrane permeability and acid sensitivity, and reduced bacterial adherence to host cells. ctaP deletion mutants were severely attenuated following intragastric and intravenous inoculation of mice. Taken together, the data presented indicate that CtaP contributes to multiple facets of L. monocytogenes physiology, growth and survival both inside and outside of animal cells.

The alternative sigma factor sigma(L) of L. monocytogenes promotes growth under diverse environmental stresses

Listeria monocytogenes are important foodborne pathogens that can cause outbreaks of serious human disease. These organisms frequently colonize and proliferate on preserved food products despite exposure to stress conditions induced by low storage temperatures, inclusion of organic acid-based preservatives, and high osmolarity. To assess alternative sigma factor sigma(L) contributions to such stress resistance of L. monocytogenes, quantitative RT-PCR assays and sigL gene deletion mutagenesis were applied in L. monocytogenes EGDe. Transcription of sigL was significantly induced by growth of EGDe under cold, organic acid, and elevated NaCl salt concentration stress conditions. The growth of a DeltasigL strain exposed to these stress conditions was also found to be significantly impaired in comparison to that of its isogenic wild-type strain. The contribution of sigma(L) to transcription control of cold and NaCl stress adaptation genes, oppA, cspD, and clpP, was also comparatively assessed in DeltasigL and wild-type EGDe cells. Transcription of the oppA gene, which encodes the OppA protein that also promotes L. monocytogenes cold growth, was significantly reduced in cold stress-grown DeltasigL cells compared to levels of the wild-type EGDe strain. These findings therefore suggest important roles of sigma(L) regulatory pathways in facilitating resistance of L. monocytogenes organisms against stress conditions associated with low storage temperatures, exposure to organic acid, and elevated NaCl salt concentrations.

Carnitine enhances the growth of Listeria monocytogenes in infant formula at 7 degrees C

Carnitine has beneficial vitamin-like qualities and functions as a compatible solute. The presence of L-carnitine in infant formula significantly promoted the growth of Listeria monocytogenes at 7 degrees C, whereas disruption of the genes for carnitine and glycine betaine uptake compromised growth of the L. monocytogenes mutant in formula containing carnitine that was stored at 7 degrees C. Thus, addition of carnitine to baby formula may pose a potential food safety risk.

Phenotypic and transcriptomic analyses of mildly and severely salt-stressed Bacillus cereus ATCC 14579 cells

Bacteria are able to cope with the challenges of a sudden increase in salinity by activating adaptation mechanisms. In this study, exponentially growing cells of the pathogen Bacillus cereus ATCC 14579 were exposed to both mild (2.5% [wt/vol] NaCl) and severe (5% [wt/vol] NaCl) salt stress conditions. B. cereus continued to grow at a slightly reduced growth rate when it was shifted to mild salt stress conditions. Exposure to severe salt stress resulted in a lag period, and after 60 min growth had resumed, with cells displaying a filamentous morphology. Whole-genome expression analyses of cells exposed to 2.5% salt stress revealed that the expression of these cells overlapped with the expression of cells exposed to 5% salt stress, suggesting that the corresponding genes were involved in a general salt stress response. Upregulation of osmoprotectant, Na(+)/H(+), and di- and tripeptide transporters and activation of an oxidative stress response were noticeable aspects of the general salt stress transcriptome response. Activation of this response may confer cross-protection against other stresses, and indeed, increased resistance to heat and hydrogen peroxide could be demonstrated after preexposure to salt. A temporal shift between the transcriptome response and several phenotypic responses of severely salt-stressed cells was observed. After resumption of growth, these cells showed cellular filamentation, reduced chemotaxis, increased catalase activity, and optimal oxidative stress resistance, which corresponded to the transcriptome response displayed in the initial lag period. The linkage of transcriptomes and phenotypic characteristics can contribute to a better understanding of cellular stress adaptation strategies and possible cross-protection mechanisms.

Listeria as an enteroinvasive gastrointestinal pathogen

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