SigmaB- and PrfA-dependent transcription of genes previously classified as putative constituents of the Listeria monocytogenes PrfA regulon

The Virulence and Infectivity of Listeria monocytogenes Are Not Substantially Altered by Elevated SigB Activity

Listeria monocytogenes is a bacterial pathogen capable of causing severe infections but also thriving outside the host. To respond to different stress conditions, L. monocytogenes mainly utilizes the general stress response regulon, which largely is controlled by the alternative sigma factor Sigma B (SigB). In addition, SigB is important for virulence gene expression and infectivity. Upon encountering stress, a large multicomponent protein complex known as the stressosome becomes activated, ultimately leading to SigB activation. RsbX is a protein needed to reset a "stressed" stressosome and prevent unnecessary SigB activation in nonstressed conditions. Consequently, absence of RsbX leads to constitutive activation of SigB even without prevailing stress stimulus. To further examine the involvement of SigB in the virulence of this pathogen, we investigated whether a strain with constitutively active SigB would be affected in virulence factor expression and/or infectivity in cultured cells and in a chicken embryo infection model. Our results suggest that increased SigB activity does not substantially alter virulence gene expression compared with the wild-type (WT) strain at transcript and protein levels. Bacteria lacking RsbX were taken up by phagocytic and nonphagocytic cells at a similar frequency to WT bacteria, both in stressed and nonstressed conditions. Finally, the absence of RsbX only marginally affected the ability of bacteria to infect chicken embryos. Our results suggest only a minor role of RsbX in controlling virulence factor expression and infectivity under these conditions.

Acid stress signals are integrated into the σB-dependent general stress response pathway via the stressosome in the food-borne pathogen Listeria monocytogenes

The general stress response (GSR) in Listeria monocytogenes plays a critical role in the survival of this pathogen in the host gastrointestinal tract. The GSR is regulated by the alternative sigma factor B (σ^B), whose role in protection against acid stress is well established. Here, we investigated the involvement of the stressosome, a sensory hub, in transducing low pH signals to induce the GSR. Mild acid shock (15 min at pH 5.0) activated σ^B and conferred protection against a subsequent lethal pH challenge. A mutant strain where the stressosome subunit RsbR1 was solely present retained the ability to induce σ^B activity at pH 5.0. The role of stressosome phosphorylation in signal transduction was investigated by mutating the putative phosphorylation sites in the core stressosome proteins RsbR1 (rsbR1-T175A, -T209A, -T241A) and RsbS (rsbS-S56A), or the stressosome kinase RsbT (rsbT-N49A). The rsbS S56A and rsbT N49A mutations abolished the response to low pH. The rsbR1-T209A and rsbR1-T241A mutants displayed constitutive σ^B activity. Mild acid shock upregulates invasion genes inlAB and stimulates epithelial cell invasion, effects that were abolished in mutants with an inactive or overactive stressosome. Overall, the results show that the stressosome is required for acid-induced activation of σ^B in L. monocytogenes. Furthermore, they show that RsbR1 can function independently of its paralogues and signal transduction requires RsbT-mediated phosphorylation of RsbS on S56 and RsbR1 on T209 but not T175. These insights shed light on the mechanisms of signal transduction that activate the GSR in L. monocytogenes in response to acidic environments, and highlight the role this sensory process in the early stages of the infectious cycle.

The stress sensing hub known as the stressosome, found in many bacterial and archaeal lineages, plays a crucial role in both stress tolerance and virulence in the food-borne pathogen Listeria monocytogenes. However, the mechanisms that lead to its activation and the subsequent activation of the general stress response have remained elusive. In this study, we examined the signal transduction mechanisms that operate in the stressosome in response to acid stress. We found that only one of the five putative sensory proteins present in L. monocytogenes, RsbR1, was required for effective transduction of acid tress signals. We further found that phosphorylation of RsbS and RsbR1, mediated by the RsbT kinase, is essential for signal transduction. Failure to phosphorylate RsbS on Serine 56 completely abolished acid sensing by the stressosome, which prevented the development of adaptive acid tolerance. The acid-induced activation of internalin gene expression was also abolished in mutants with defective stressosome signalling, suggesting a role for the stressosome in the invasion of host cells. Together the data provide new insights into the mechanisms that activate the stressosome in response to acid stress and highlight the role this sensory hub plays in virulence.

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

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

Transcriptomic and Phenotypic Analyses of the Sigma B-Dependent Characteristics and the Synergism between Sigma B and Sigma L in Listeria monocytogenes EGD-e

Numerous gene expression and stress adaptation responses in L. monocytogenes are regulated through alternative sigma factors σ^B and σ^L. Stress response phenotypes and transcriptomes were compared between L. monocytogenes EGD-e and its ΔsigB and ΔsigBL mutants. Targeted growth phenotypic analysis revealed that the ΔsigB and ΔsigBL mutants are impaired during growth under cold and organic-acid stress conditions. Phenotypic microarrays revealed increased sensitivity in both mutants to various antimicrobial compounds. Genes de-regulated in these two mutants were identified by genome-wide transcriptome analysis during exponential growth in BHI. The ΔsigB and ΔsigBL strains repressed 198 and 254 genes, respectively, compared to the parent EGD-e strain at 3 °C, whereas 86 and 139 genes, respectively, were repressed in these mutants during growth at 37 °C. Genes repressed in these mutants are involved in various cellular functions including transcription regulation, energy metabolism and nutrient transport functions, and viral-associated processes. Exposure to cold stress induced a significant increase in σ^B and σ^L co-dependent genes of L. monocytogenes EGD-e since most (62%) of the down-regulated genes uncovered at 3 °C were detected in the ΔsigBL double-deletion mutant but not in ΔsigB or ΔsigL single-deletion mutants. Overall, the current study provides an expanded insight into σ^B and σ^L phenotypic roles and functional interactions in L. monocytogenes. Besides previously known σ^B- and σ^L-dependent genes, the transcriptomes defined in ΔsigB and ΔsigBL mutants reveal several new genes that are positively regulated by σ^B alone, as well as those co-regulated through σ^B- and σ^L-dependent mechanisms during L. monocytogenes growth under optimal and cold-stress temperature conditions.

Listeria monocytogens - Amended understanding of its pathogenesis with a complete picture of its membrane vesicles, quorum sensing, biofilm and invasion

Listeria monocytogenes is a ubiquitous, intracellular foodborne pathogen that causes listeriosis in animals and humans. Pathogenic Listeria monocytogenes easily adapted to the conditions of human gastrointestinal tract and tolerate the counter changes such as acidity, bile, osmolarity, and antimicrobial peptides. They secrete specialized biologically active extra organ called membrane vesicles which comprises proteins, lipids, and lipopolysaccharides. Listerial vesicles possess functional versatility and play a significant role in pathogenesis by cell-free intercellular communication and toxin packaging. L. monocytogenes can attach promptly and decisively to inert substratum including intestinal mucosa, and forms biofilms and causes detrimental effects. Further, they invade the host cells through quorum sensing (QS) controlled virulence determinants and biofilms. The precise degree to which the bacterium retains the intracellular ambiance of host cells remains unknown. The machinery associated with intracellular survival, and the role of membrane vesicles, quorum sensing, and the Agr system in Listeria monocytogenes largely remains unclear. The current review focused to understand the role of membrane vesicles mediated pathogenesis biofilms, and delivers auxiliary impetus to understanding the potentials of virulence mediated invasion in Listeria monocytogenes.

Cross Talk between SigB and PrfA in Listeria monocytogenes Facilitates Transitions between Extra- and Intracellular Environments

The foodborne pathogen Listeria monocytogenes can modulate its transcriptome and proteome to ensure its survival during transmission through vastly differing environmental conditions. While L. monocytogenes utilizes a large array of regulators to achieve survival and growth in different intra- and extrahost environments, the alternative sigma factor σ^B and the transcriptional activator of virulence genes protein PrfA are two key transcriptional regulators essential for responding to environmental stress conditions and for host infection. Importantly, emerging evidence suggests that the shift from extrahost environments to the host gastrointestinal tract and, subsequently, to intracellular environments requires regulatory interplay between σ^B and PrfA at transcriptional, posttranscriptional, and protein activity levels. Here, we review the current evidence for cross talk and interplay between σ^B and PrfA and their respective regulons and highlight the plasticity of σ^B and PrfA cross talk and the role of this cross talk in facilitating successful transition of L. monocytogenes from diverse extrahost to diverse extra- and intracellular host environments.

Regulation of Listeria monocytogenes Virulence

Whereas obligate human and animal bacterial pathogens may be able to depend upon the warmth and relative stability of their chosen replication niche, environmental bacteria such as Listeria monocytogenes that harbor the ability to replicate both within animal cells and in the outside environment must maintain the capability to manage life under a variety of disparate conditions. Bacterial life in the outside environment requires adaptation to wide ranges of temperature, available nutrients, and physical stresses such as changes in pH and osmolarity as well as desiccation. Following ingestion by a susceptible animal host, the bacterium must adapt to similar changes during transit through the gastrointestinal tract and overcome a variety of barriers associated with host innate immune responses. Rapid alteration of patterns of gene expression and protein synthesis represent one strategy for quickly adapting to a dynamic host landscape. Here, we provide an overview of the impressive variety of strategies employed by the soil-dwelling, foodborne, mammalian pathogen L. monocytogenes to straddle diverse environments and optimize bacterial fitness both inside and outside host cells.

Dual-species biofilm of Listeria monocytogenes and Escherichia coli on stainless steel surface

Listeria monocytogenes is a Gram-positive bacterium commonly associated with foodborne diseases. Due its ability to survive under adverse environmental conditions and to form biofilm, this bacterium is a major concern for the food industry, since it can compromise sanitation procedures and increase the risk of post-processing contamination. Little is known about the interaction between L. monocytogenes and Gram-negative bacteria on biofilm formation. Thus, in order to evaluate this interaction, Escherichia coli and L. monocytogenes were tested for their ability to form biofilms together or in monoculture. We also aimed to evaluate the ability of L. monocytogenes 1/2a and its isogenic mutant strain (ΔprfA ΔsigB) to form biofilm in the presence of E. coli. We assessed the importance of the virulence regulators, PrfA and σ^B, in this process since they are involved in many aspects of L. monocytogenes pathogenicity. Biofilm formation was assessed using stainless steel AISI 304 #4 slides immersed into brain heart infusion broth, reconstituted powder milk and E. coli preconditioned medium at 25 °C. Our results indicated that a higher amount of biofilm was formed by the wild type strain of L. monocytogenes than by its isogenic mutant, indicating that prfA and sigB are important for biofilm development, especially maturation under our experimental conditions. The presence of E. coli or its metabolites in preconditioned medium did not influence biofilm formation by L. monocytogenes. Our results confirm the possibility of concomitant biofilm formation by L. monocytogenes and E. coli, two bacteria of major significance in the food industry.

The Listeria monocytogenes Bile Stimulon under Acidic Conditions Is Characterized by Strain-Specific Patterns and the Upregulation of Motility, Cell Wall Modification Functions, and the PrfA Regulon

Listeria monocytogenes uses a variety of transcriptional regulation strategies to adapt to the extra-host environment, the gastrointestinal tract, and the intracellular host environment. While the alternative sigma factor SigB has been proposed to be a key transcriptional regulator that facilitates L. monocytogenes adaptation to the gastrointestinal environment, the L. monocytogenes' transcriptional response to bile exposure is not well-understood. RNA-seq characterization of the bile stimulon was performed in two L. monocytogenes strains representing lineages I and II. Exposure to bile at pH 5.5 elicited a large transcriptomic response with ~16 and 23% of genes showing differential transcription in 10403S and H7858, respectively. The bile stimulon includes genes involved in motility and cell wall modification mechanisms, as well as genes in the PrfA regulon, which likely facilitate survival during the gastrointestinal stages of infection that follow bile exposure. The fact that bile exposure induced the PrfA regulon, but did not induce further upregulation of the SigB regulon (beyond that expected by exposure to pH 5.5), suggests a model where at the earlier stages of gastrointestinal infection (e.g., acid exposure in the stomach), SigB-dependent gene expression plays an important role. Subsequent exposure to bile induces the PrfA regulon, potentially priming L. monocytogenes for subsequent intracellular infection stages. Some members of the bile stimulon showed lineage- or strain-specific distribution when 27 Listeria genomes were analyzed. Even though sigB null mutants showed increased sensitivity to bile, the SigB regulon was not found to be upregulated in response to bile beyond levels expected by exposure to pH 5.5. Comparison of wildtype and corresponding ΔsigB strains newly identified 26 SigB-dependent genes, all with upstream putative SigB-dependent promoters.

Home Alone: Elimination of All but One Alternative Sigma Factor in Listeria monocytogenes Allows Prediction of New Roles for σB

Among Listeria monocytogenes' four alternative σ factors, σ^B controls the largest regulon. As σ^B-dependent transcription of some genes may be masked by overlaps among regulons, and as some σ^B-dependent genes are expressed only under very specific conditions, we hypothesized that the σ^B regulon is not yet fully defined. To further extend our understanding of the σ^B regulon, we used RNA-seq to identify σ^B-dependent genes in an L. monocytogenes strain that expresses σ^B following rhamnose induction, and in which genes encoding the other alternative sigma factors have been deleted. Analysis of RNA-seq data with multiple bioinformatics approaches, including a sliding window method that detects differentially transcribed 5' untranslated regions (UTRs), identified 105 σ^B-dependent transcription units (TUs) comprising 201 genes preceded by σ^B-dependent promoters. Of these 105 TUs, 7 TUs comprising 15 genes had not been identified previously as σ^B-dependent. An additional 23 genes not reported previously as σ^B-dependent were identified in 9 previously recognized σ^B-dependent TUs. Overall, 38 of these 201 genes had not been identified previously as members of the L. monocytogenes σ^B regulon. These newly identified σ^B-dependent genes encode proteins annotated as being involved in transcriptional regulation, oxidative and osmotic stress response, and in metabolism of energy, carbon and nucleotides. In total, 18 putative σ^B-dependent promoters were newly identified. Interestingly, a number of genes previously identified as σ^B-dependent did not show significant evidence for σ^B-dependent transcription in our experiments. Based on promoter analyses, a number of these genes showed evidence for co-regulation by σ^B and other transcriptional factors, suggesting that some σ^B-dependent genes require additional transcriptional regulators along with σ^B for transcription. Over-expression of a single alternative sigma factor in the absence of all other alternative sigma factors allowed us to: (i) identify new σ^B-dependent functions in L. monocytogenes, such as regulation of genes involved in 1,2-propanediol utilization (LMRG_00594-LMRG_00611) and biosynthesis of pyrimidine nucleotides (LMRG_00978-LMRG_00985); and (ii) identify new σ^B-dependent genes involved in stress response and pathogenesis functions. These data further support that σ^B not only regulates stress response functions, but also plays a broad role in L. monocytogenes homeostasis and resilience.

The AirSR two-component system contributes to Staphylococcus aureus survival in human blood and transcriptionally regulates sspABC operon

To date, genes identified and transcriptionally regulated by the AirSR TCS have been involved in energy production and cellular homeostasis of the staphylococcal cell. It is well accepted that the state of cellular metabolism impacts the expression of virulence factors in Staphylococcus aureus. For this reason, we conducted experiments to determine if the AirSR TCS contributes to the pathogenesis of S. aureus using an antisense RNA interference technology, an inducible overexpression system, and gene deletions. Depletion of AirSR by antisense RNA expression or deletion of the genes, results in significant decrease in bacterial survival in human blood. Conversely, overexpression of AirR significantly promotes survival of S. aureus in blood. AirR promotes the secretion of virulence factors that inhibits opsonin-based phagocytosis. This enhanced survival is partially linked to the transcriptional regulation of the sspABC operon, encoding V8 protease (SspA), staphopain B (SspB) and staphostatin B (SspC). SspA and SspB are known virulence factors which proteolytically digest opsonins and inhibit killing of S. aureus by professional phagocytes. This is the first evidence linking the AirSR TCS to pathogenesis of S. aureus.

Optimizing the balance between host and environmental survival skills: lessons learned from Listeria monocytogenes

Environmental pathogens - organisms that survive in the outside environment but maintain the capacity to cause disease in mammals - navigate the challenges of life in habitats that range from water and soil to the cytosol of host cells. The bacterium Listeria monocytogenes has served for decades as a model organism for studies of host-pathogen interactions and for fundamental paradigms of cell biology. This ubiquitous saprophyte has recently become a model for understanding how an environmental bacterium switches to life within human cells. This review describes how L. monocytogenes balances life in disparate environments with the help of a critical virulence regulator known as PrfA. Understanding L. monocytogenes survival strategies is important for gaining insight into how environmental microbes become pathogens.

Contributions of σ(B) and PrfA to Listeria monocytogenes salt stress under food relevant conditions

Listeria monocytogenes is well known to survive and grow under several stress conditions, including salt stress, which is important for growth in certain foods as well as for host infection. To characterize the contributions, to salt stress response, of transcriptional regulators important for stress response and virulence (i.e., σ(B) and PrfA), we analyzed three L. monocytogenes parent strains and isogenic mutants (ΔsigB, ΔprfA, and ΔsigBΔprfA), representing different serotypes and lineages, for their ability to grow, at 25°C, in BHI with 1.9 M NaCl. With regard to growth rate, only the lineage IV strain presented a significant difference between the parent strain and both of its respective mutants lacking prfA (ΔprfA and ΔsigBΔprfA). Conversely, the lineage I and II parent strains showed significantly shorter lag phase in comparison to their respective ΔsigB mutant strains. Intestinal epithelial cell invasion assay and hemolytic activity assays showed a significant role for σ(B) in the former and for PrfA in the latter. To explore the mechanism that may contribute to the extended lag phase in the ΔsigB mutant strain and survival and growth of the parent strain upon salt shock, whole genome transcription profiling was performed to compare transcript levels between the lineage I, serotype 1/2b, parent strain and its isogenic ΔsigB mutant after 30 min of lag phase growth at 25°C in the presence of 1.9M NaCl (salt shock) without aeration. Microarray data showed significantly higher transcript levels for 173 genes in the parent strain as compared to the ΔsigB strain. Overall, 102 of the 173 σ(B) up-regulated genes had been identified in previous studies, indicating that 71 genes were newly identified as being up-regulated by σ(B) in this study. We hypothesize that, among these genes newly identified as σ(B) up-regulated, four genes (lmo2174, lmo0530, lmo0527 and lmo0529) may play a major role in response to salt stress. Lmo2174 contains domains that facilitate sensing and producing a transduction signal in the form of cyclic di-GMP, which may activate the enzymes Lmo0527, Lmo0529 and Lmo0530, which encode proteins similar to those responsible for synthesis of exopolysaccharides that may protect the cell by changing the cell wall structure during salt stress. Overall, our data showed that σ(B), but not PrfA, contributes to growth under salt stress. Moreover, we show that the σ(B) regulon of a L. monocytogenes lineage I strain challenged with salt shock includes salt stress-specific as well as previously unidentified σ(B) up-regulated genes.

Development and optimization of an EGFP-based reporter for measuring the general stress response in Listeria monocytogenes

A characteristic of the food-borne pathogen Listeria monocytogenes is its tolerance to the harsh conditions found both in minimally processed foods and the human gastrointestinal tract. This trait is partly under the control of the alternative sigma factor sigma B (σ(B)). To study the mechanisms that trigger the activation of σ(B) , and hence the development of stress tolerance, we have developed a fluorescent reporter fusion that allows the real-time activity of σ(B) to be monitored. The reporter, designated Plmo2230::egfp, fuses the strong σ(B)-dependent promoter from the lmo2230 gene (which encodes a putative arsenate reductase) to a gene encoding enhanced green fluorescence protein (EGFP). The reporter was integrated into the genomes of the wild-type strain L. monocytogenes EGD-e as well as two mutant derivatives lacking either sigB or rsbV. The resulting strains were used to study σ(B) activation in response to growth phase and hyperosmotic stress. The wild-type was strongly fluorescent in stationary phase or in cultures with added NaCl and this fluorescence was abolished in both the sigB and rsbV backgrounds, consistent with the σ(B)-dependency of the lmo2230 promoter. During sudden osmotic upshock (addition of 0.5 M NaCl during growth) a real-time increase in fluorescence was observed microscopically, reaching maximal activation after 30 min. Flow cytometry was used to study the activation of σ(B) at a population level by hyperosmotic stress during exponential growth. A strong and proportional increase in fluorescence was observed as the salt concentration increased from 0 to 0.9 M NaCl. Interestingly, there was considerable heterogeneity within the population and a significant proportion of cells failed to induce a high level of fluorescence, suggesting that σ(B) activation occurs stochastically in response to hyperosmotic stress. Thus the Plmo2230::egfp is a powerful tool that will allow the stress response to be better studied in this important human pathogen.

Toward a Systemic Understanding of Listeria monocytogenes Metabolism during Infection

Listeria monocytogenes is a foodborne human pathogen that can cause invasive infection in susceptible animals and humans. For proliferation within hosts, this facultative intracellular pathogen uses a reservoir of specific metabolic pathways, transporter, and enzymatic functions whose expression requires the coordinated activity of a complex regulatory network. The highly adapted metabolism of L. monocytogenes strongly depends on the nutrient composition of various milieus encountered during infection. Transcriptomic and proteomic studies revealed the spatial-temporal dynamic of gene expression of this pathogen during replication within cultured cells or in vivo. Metabolic clues are the utilization of unusual C(2)- and C(3)-bodies, the metabolism of pyruvate, thiamine availability, the uptake of peptides, the acquisition or biosynthesis of certain amino acids, and the degradation of glucose-phosphate via the pentose phosphate pathway. These examples illustrate the interference of in vivo conditions with energy, carbon, and nitrogen metabolism, thus affecting listerial growth. The exploitation, analysis, and modeling of the available data sets served as a first attempt to a systemic understanding of listerial metabolism during infection. L. monocytogenes might serve as a model organism for systems biology of a Gram-positive, facultative intracellular bacterium.

Salt stress-induced transcription of σB- and CtsR-regulated genes in persistent and non-persistent Listeria monocytogenes strains from food processing plants

Listeria monocytogenes is a foodborne pathogen that can persist in food processing environments. Six persistent and six non-persistent strains from fish processing plants and one persistent strain from a meat plant were selected to determine if expression of genes in the regulons of two stress response regulators, σ(B) and CtsR, under salt stress conditions is associated with the ability of L. monocytogenes to persist in food processing environments. Subtype data were also used to categorize the strains into genetic lineages I or II. Quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) was used to measure transcript levels for two σ(B)-regulated genes, inlA and gadD3, and two CtsR-regulated genes, lmo1138 and clpB, before and after (t=10 min) salt shock (i.e., exposure of exponential phase cells to BHI+6% NaCl for 10 min at 37°C). Exposure to salt stress induced higher transcript levels relative to levels under non-stress conditions for all four stress and virulence genes across all wildtype strains tested. Analysis of variance (ANOVA) of induction data revealed that transcript levels for one gene (clpB) were induced at significantly higher levels in non-persistent strains compared to persistent strains (p=0.020; two-way ANOVA). Significantly higher transcript levels of gadD3 (p=0.024; two-way ANOVA) and clpB (p=0.053; two-way ANOVA) were observed after salt shock in lineage I strains compared to lineage II strains. No clear association between stress gene transcript levels and persistence was detected. Our data are consistent with an emerging model that proposes that establishment of L. monocytogenes persistence in a specific environment occurs as a random, stochastic event, rather than as a consequence of specific bacterial strain characteristics.

Growth temperature-dependent contributions of response regulators, σB, PrfA, and motility factors to Listeria monocytogenes invasion of Caco-2 cells

Foodborne pathogens encounter rapidly changing environmental conditions during transmission, including exposure to temperatures below 37°C. The goal of this study was to develop a better understanding of the effects of growth temperatures and temperature shifts on regulation of invasion phenotypes and invasion-associated genes in Listeria monocytogenes. We specifically characterized the effects of L. monocytogenes growth at different temperatures (30°C vs. 37°C) on (i) the contributions to Caco-2 invasion of different regulators (including σ(B), PrfA, and 14 response regulators [RRs]) and invasion proteins (i.e., InlA and FlaA), and on (ii) gadA, plcA, inlA, and flaA transcript levels and their regulation. Overall, Caco-2 invasion efficiency was higher for L. monocytogenes grown at 30°C than for bacteria grown at 37°C (p = 0.0051 for the effect of temperature on invasion efficiency; analysis of variance); the increased invasion efficiency of the parent strain 10403S (serotype 1/2a) observed after growth at 30°C persisted for 2.5 h exposure to 37°C. For L. monocytogenes grown at 30°C, the motility RRs DegU and CheY and σ(B), but not PrfA, significantly contributed to Caco-2 invasion efficiency. For L. monocytogenes grown at 37°C, none of the 14 RRs tested significantly contributed to Caco-2 invasion, whereas σ(B) and PrfA contributed synergistically to invasion efficiency. At both growth temperatures there was significant synergism between the contributions to invasion of FlaA and InlA; this synergism was more pronounced after growth at 30°C than at 37°C. Our data show that growth temperature affects invasion efficiency and regulation of virulence-associated genes in L. monocytogenes. These data support increasing evidence that a number of environmental conditions can modulate virulence-associated phenotypes of foodborne bacterial pathogens, including L. monocytogenes.

Listeria monocytogenes - from saprophyte to intracellular pathogen

Listeria monocytogenes is a bacterium that lives in the soil as a saprophyte but is capable of making the transition into a pathogen following its ingestion by susceptible humans or animals. Recent studies suggest that L. monocytogenes mediates its saprophyte-to-cytosolic-parasite transition through the careful modulation of the activity of a virulence regulatory protein known as PrfA, using a range of environmental cues that include available carbon sources. In this Progress article we describe the regulation of PrfA and its role in the L. monocytogenes transition from the saprophytic stage to the virulent intracellular stage.

Listeria monocytogenes sigmaB modulates PrfA-mediated virulence factor expression

Listeria monocytogenes sigma(B) and positive regulatory factor A (PrfA) are pleiotropic transcriptional regulators that coregulate a subset of virulence genes. A positive regulatory role for sigma(B) in prfA transcription has been well established; therefore, observations of increased virulence gene expression and hemolytic activity in a DeltasigB strain initially appeared paradoxical. To test the hypothesis that L. monocytogenes sigma(B) contributes to a regulatory network critical for appropriate repression as well as induction of virulence gene expression, genome-wide transcript profiling and follow-up quantitative reverse transcriptase PCR (qRT-PCR), reporter fusion, and phenotypic experiments were conducted using L. monocytogenes prfA*, prfA* DeltasigB, DeltaprfA, and DeltaprfA DeltasigB strains. Genome-wide transcript profiling and qRT-PCR showed that in the presence of active PrfA (PrfA*), sigma(B) is responsible for reduced expression of the PrfA regulon. sigma(B)-dependent modulation of PrfA regulon expression reduced the cytotoxic effects of a PrfA* strain in HepG2 cells, highlighting the functional importance of regulatory interactions between PrfA and sigma(B). The emerging model of the role of sigma(B) in regulating overall PrfA activity includes a switch from transcriptional activation at the P2(prfA) promoter (e.g., in extracellular bacteria when PrfA activity is low) to posttranscriptional downregulation of PrfA regulon expression (e.g., in intracellular bacteria when PrfA activity is high).