Impact of osmotic stress on the phosphorylation and subcellular location of Listeria monocytogenes stressosome proteins

Stress Adaptation Responses of a Listeria monocytogenes 1/2a Strain via Proteome Profiling

Listeria monocytogenes is a foodborne pathogen that is ubiquitous and largely distributed in food manufacturing environments. It is responsible for listeriosis, a disease that can lead to significant morbidity and fatality in immunocompromised patients, pregnant women, and newborns. Few reports have been published about proteome adaptation when L. monocytogenes is cultivated in stress conditions. In this study, we applied one-dimensional electrophoresis and 2D-PAGE combined with tandem mass spectrometry to evaluate proteome profiling in the following conditions: mild acid, low temperature, and high NaCl concentration. The total proteome was analyzed, also considering the case of normal growth-supporting conditions. A total of 1,160 proteins were identified and those related to pathogenesis and stress response pathways were analyzed. The proteins involved in the expression of virulent pathways when L. monocytogenes ST7 strain was grown under different stress conditions were described. Certain proteins, particularly those involved in the pathogenesis pathway, such as Listeriolysin regulatory protein and Internalin A, were only found when the strain was grown under specific stress conditions. Studying how L. monocytogenes adapts to stress can help to control its growth in food, reducing the risk for consumers.

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.

Super-resolving microscopy reveals the localizations and movement dynamics of stressosome proteins in Listeria monocytogenes

The human pathogen Listeria monocytogenes can cope with severe environmental challenges, for which the high molecular weight stressosome complex acts as the sensing hub in a complicated signal transduction pathway. Here, we show the dynamics and functional roles of the stressosome protein RsbR1 and its paralogue, the blue-light receptor RsbL, using photo-activated localization microscopy combined with single-particle tracking and single-molecule displacement mapping and supported by physiological studies. In live cells, RsbR1 is present in multiple states: in protomers with RsbS, large clusters of stressosome complexes, and in connection with the plasma membrane via Prli42. RsbL diffuses freely in the cytoplasm but forms clusters upon exposure to light. The clustering of RsbL is independent of the presence of Prli42. Our work provides a comprehensive view of the spatial organization and intracellular dynamics of the stressosome proteins in L. monocytogenes, which paves the way towards uncovering the stress-sensing mechanism of this signal transduction pathway.

In vivo characterisation of the Vibrio vulnificus stressosome: A complex involved in reshaping glucose metabolism and motility regulation, in nutrient- and iron-limited growth conditions

Stressosomes are signal-sensing and integration hubs identified in many bacteria. At present, the role of the stressosome has only been investigated in Gram-positive bacteria. This work represents the first in vivo characterisation of the stressosome in a Gram-negative bacterium, Vibrio vulnificus. Previous in vitro characterisation of the complex has led to the hypothesis of a complex involved in iron metabolism and control of c-di-GMP levels. We demonstrate that the stressosome is probably involved in reshaping the glucose metabolism in Fe- and nutrient-limited conditions and mutations of the locus affect the activation of the glyoxylate shunt. Moreover, we show that the stressosome is needed for the transcription of fleQ and to promote motility, consistent with the hypothesis that the stressosome is involved in regulating c-di-GMP. This report highlights the potential role of the stressosome in a Gram-negative bacterium, with implications for the metabolism and motility of this pathogen.

Bacillus subtilis Stressosome Sensor Protein Sequences Govern the Ability To Distinguish among Environmental Stressors and Elicit Different σB Response Profiles

Bacteria use a variety of systems to sense stress and mount an appropriate response to ensure fitness and survival. Bacillus subtilis uses stressosomes-cytoplasmic multiprotein complexes-to sense environmental stressors and enact the general stress response by activating the alternative sigma factor σ^B. Each stressosome includes 40 RsbR proteins, representing four paralogous (RsbRA, RsbRB, RsbRC, and RsbRD) putative stress sensors. Population-level analyses suggested that the RsbR paralogs are largely redundant, while our prior work using microfluidics-coupled fluorescence microscopy uncovered differences among the RsbR paralogs' σ^B response profiles with respect to timing and intensity when facing an identical stressor. Here, we use a similar approach to address the question of whether the σ^B responses mediated by each paralog differ in the presence of different environmental stressors: can they distinguish among stressors? Wild-type cells (with all four paralogs) and RsbRA-only cells activate σ^B with characteristic transient response timing irrespective of stressor but show various response magnitudes. However, cells with other individual RsbR paralogs show distinct timing and magnitude in their responses to ethanol, salt, oxidative, and acid stress, implying that RsbR proteins can distinguish among stressors. Experiments with hybrid fusion proteins comprising the N-terminal half of one paralog and the C-terminal half of another argue that the N-terminal identity influences response magnitude and that determinants in both halves of RsbRA are important for its stereotypical transient σ^B response timing. IMPORTANCE Bacterial survival depends on appropriate responses to diverse stressors. The general stress-response system in the environmental model bacterium Bacillus subtilis is constantly poised for an immediate response and uses unusual stress-sensing protein complexes called stressosomes. Stressosomes typically contain four different types of putative sensing protein. We asked whether each type of sensor has a distinct role in mediating response dynamics to different environmental stressors. We find that one sensor type always mediates a transient response, while the others show distinct response magnitude and timing to different stressors. We also find that a transient response is exceptional, as several engineered hybrid proteins did not show strong transient responses. Our work reveals functional distinctions among subunits of the stressosome complex and represents a step toward understanding how the general stress response of B. subtilis ensures its survival in natural environmental settings.

The stressosome is required to transduce low pH signals leading to increased transcription of the amino acid-based acid tolerance mechanisms in Listeria monocytogenes

Increasing proton concentration in the environment represents a potentially lethal stress for single-celled microorganisms. To survive in an acidifying environment, the foodborne pathogen Listeria monocytogenes quickly activates the alternative sigma factor B (σB), resulting in upregulation of the general stress response (GSR) regulon. Activation of σB is regulated by the stressosome, a multi-protein sensory complex involved in stress detection and signal transduction. In this study, we used L. monocytogenes strains harbouring two stressosome mutants to investigate the role of this complex in triggering expression of known amino acid-based resistance mechanisms in response to low pH. We found that expression of glutamate decarboxylase (gadD3) and arginine and agmatine deiminases (arcA and aguA1, respectively) were upregulated upon acid shock (pH 5 for 15 min) in a stressosome-dependent manner. In contrast, transcription of the arg operons (argGH and argCJBDF), which encode enzymes for the l-arginine biosynthesis pathway, were upregulated upon acid shock in a stressosome-independent manner. Finally, we found that transcription of argR, which encodes a transcriptional regulator of the arc and arg operons, was largely unaffected by acidic shock. Thus, our findings suggest that the stressosome plays a role in activating amino acid-based pH homeostatic mechanisms in L. monocytogenes . Additionally, we show that genes encoding the l-arginine biosynthesis pathway are highly upregulated under acidic conditions, suggesting that intracellular arginine can help withstand environmental acidification in this pathogen.

Listeria monocytogenes Pathogenesis: The Role of Stress Adaptation

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

The Xanthomonas citri Reverse Fitness Deficiency by Activating a Novel β-Glucosidase Under Low Osmostress

Bacteria can withstand various types of environmental osmostress. A sudden rise in osmostress affects bacterial cell growth that is countered by activating special genes. The change of osmostress is generally a slow process under the natural environment. However, the collective response of bacteria to low osmostress remains unknown. This study revealed that the deletion of phoP (ΔphoP) from X. citri significantly compromised the growth and virulence as compared to the wild-type strain. Interestingly, low osmostress reversed physiological deficiencies of X. citri phoP mutant related to bacterial growth and virulence. The results also provided biochemical and genetic evidence that the physiological deficiency of phoP mutant can be reversed by low osmostress induced β-glucosidase (BglS) expression. Based on the data, this study proposes a novel regulatory mechanism of a novel β-glucosidase activation in X. citri through low osmostress to reverse the fitness deficiency.

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.

The Signal Peptide and Chaperone UNC93B1 Both Influence TLR8 Ectodomain Intracellular Endosomal Localization

Toll-like receptor 8 (TLR8) is a single-stranded RNA sensing receptor and is localized in the cellular compartments, where it encounters foreign or self-nucleic acids and activates innate and adaptive immune responses. However, the mechanism controlling intracellular localization TLR8 is not completely resolved. We previously revealed the intracellular localization of TLR8 ectodomain (ECD), and in this study, we investigated the mechanism of the intracellular localization. Here we found that TLR8 ECDs from different species as well as ECDs from different TLRs are all intracellularly localized, similarly to the full-length porcine TLR8. Furthermore, porcine, bovine, and human TLR8 ECDs are all localized in cell endosomes, reflecting the cellular localization of TLR8. Intriguingly, none of post-translational modifications at single sites, including glycosylation, phosphorylation, ubiquitination, acetylation, and palmitoylation alter porcine TLR8-ECD endosomal localization. Nevertheless, the signal peptide of porcine TLR8-ECD determines its endosomal localization. On the other hand, signaling regulator UNC93B1 also decides the endosomal localization of porcine, bovine, and human TLR8 ECDs. The results from this study shed light on the mechanisms of not only TLR8 intracellular localization but also the TLR immune signaling.

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.

Listeria monocytogenes requires the RsbX protein to prevent SigB-activation under non-stressed conditions

The survival of microbial cells under changing environmental conditions requires an efficient reprogramming of transcription, often mediated by alternative sigma factors. The Gram-positive human pathogen Listeria monocytogenes senses and responds to environmental stress mainly through the alternative sigma factor σ^B (SigB), which controls expression of the general stress response regulon. SigB activation is achieved through a complex series of phosphorylation/dephosphorylation events culminating in the release of SigB from its anti-sigma factor RsbW. At the top of the signal transduction pathway lies a large multiprotein complex known as the stressosome that is believed to act as a sensory hub for stresses. Following signal detection, stressosome proteins become phosphorylated. Resetting of the stressosome is hypothesized to be exerted by a putative phosphatase, RsbX, which presumably removes phosphate groups from stressosome proteins poststress. We addressed the role of the RsbX protein in modulating the activity of the stressosome and consequently regulating SigB activity in L. monocytogenes. We show that RsbX is required to reduce SigB activation levels under nonstress conditions and that it is required for appropriate SigB-mediated stress adaptation. A strain lacking RsbX displayed impaired motility and biofilm formation and also an increased survival at low pH. Our results could suggest that absence of RsbX alters the multiprotein composition of the stressosome without dramatically affecting its phosphorylation status. Overall, the data show that RsbX plays a critical role in modulating the signal transduction pathway by blocking SigB activation under nonstressed conditions.

IMPORTANCE Pathogenic bacteria need to sense and respond to stresses to survive harsh environments and also to turn off the response when no longer facing stress. Activity of the stress sigma factor SigB in the human pathogen Listeria monocytogenes is controlled by a hierarchic system having a large stress-sensing multiprotein complex known as the stressosome at the top. Following stress exposure, proteins in the stressosome become phosphorylated, leading to SigB activation. We have studied the role of a putative phosphatase, RsbX, which is hypothesized to dephosphorylate stressosome proteins. RsbX is critical not only to switch off the stress response poststress but also to keep the activity of SigB low at nonstressed conditions to prevent unnecessary gene expression and save energy.

Activation of the Listeria monocytogenes Stressosome in the Intracellular Eukaryotic Environment

Listeria monocytogenes is a ubiquitous environmental bacterium and intracellular pathogen that responds to stress using predominantly the alternative sigma factor SigB. Stress is sensed by a multiprotein complex, the stressosome, extensively studied in bacteria grown in nutrient media. Following signal perception, the stressosome triggers a phosphorylation cascade that releases SigB from its anti-sigma factor. Whether the stressosome is activated during the intracellular infection is unknown. Here, we analyzed the subcellular distribution of stressosome proteins in L. monocytogenes located inside epithelial cells following their immunodetection in membrane and cytosolic fractions prepared from intracellular bacteria. Unlike bacteria in laboratory media, intracellular bacteria have a large proportion of the core stressosome protein RsbR1 associated with the membrane. However, another core protein, RsbS, is undetectable. Despite the absence of RsbS, a SigB-dependent reporter revealed that SigB activity increases gradually from early (1 h) to late (6 h) postinfection times. We also found that RsbR1 paralogues attenuate the intensity of the SigB response and that the miniprotein Prli42, reported to tether the stressosome to the membrane in response to oxidative stress, plays no role in associating RsbR1 to the membrane of intracellular bacteria. Altogether, these data indicate that, once inside host cells, the L. monocytogenes stressosome may adopt a unique configuration to sense stress and to activate SigB in the intracellular eukaryotic niche. IMPORTANCE The response to stress mediated by the alternative sigma factor SigB has been extensively characterized in Bacillus subtilis and Listeria monocytogenes. These bacteria sense stress using a supramacromolecular complex, the stressosome, which triggers a cascade that releases SigB from its anti-sigma factor. Despite the fact that many structural data on the complex are available and analyses have been performed in mutants lacking components of the stressosome or the signaling cascade, the integration of the stress signal and the dynamics of stressosome proteins following environmental changes remain poorly understood. Our study provides data at the protein level on essential stressosome components and SigB activity when L. monocytogenes, normally a saprophytic bacterium, adapts to an intracellular lifestyle. Our results support activation of the stressosome complex in intracellular bacteria. The apparent loss of the stressosome core protein RsbS in intracellular L. monocytogenes also challenges current models, favoring the idea of a unique stressosome architecture responding to intracellular host cues.