Bacterial chitinases and chitin-binding proteins as virulence factors.

Bacterial chitinases (EC 3.2.1.14) and chitin-binding proteins (CBPs) play a fundamental role in the degradation of the ubiquitous biopolymer chitin, and the degradation products serve as an important nutrient source for marine- and soil-dwelling bacteria. However, it has recently become clear that representatives of both Gram-positive and Gram-negative bacterial pathogens encode chitinases and CBPs that support infection of non-chitinous mammalian hosts. This review addresses this biological role of bacterial chitinases and CBPs in terms of substrate specificities, regulation, secretion and involvement in cellular and animal infection.

Temporal expression of brain-derived neurotrophic factor (BDNF) mRNA in the rat hippocampus after treatment with selective and mixed monoaminergic antidepressants.

Strong evidence suggests that antidepressants work by induction of neuroplastic changes mediated through regulation of brain-derived neurotrophic factor (BDNF). This study was undertaken to investigate the time-course of the effect of three antidepressants; fluoxetine, imipramine and venlafaxine, which differentially affect monoamine reuptake, on BDNF mRNA expression in the hippocampus. The consequences of increased BDNF in the hippocampus are still indefinite. Here, we also determined the effects on the expression of two other genes (synaptophysin and growth-associated protein-43 (GAP-43)) known to be involved in synapse formation and axonal growth and likely regulated by BDNF. The effects were determined in rats after sub-chronic (7 days) and chronic (14 and 21 days) treatment using semi-quantitative in situ hybridisation. BDNF mRNA levels in the dentate gyrus (DG) were increased after treatment with venlafaxine (7, 14 and 21 days) and imipramine (14 and 21 days), but not after treatment with fluoxetine, indicating that stimulation of BDNF mRNA expression is dependent on the pharmacological profile and on the time-course of drug treatment. A transient increase in synaptophysin mRNA was observed after treatment with venlafaxine and fluoxetine whereas imipramine had no effect. In the CA3 region a reduction of GAP-43 mRNA was observed after treatment with imipramine (21 days) and fluoxetine (7 and 14 days). These results suggest that venlafaxine and imipramine, but not fluoxetine, induce neuroplastic effects in the hippocampus through stimulation of BDNF mRNA expression, and that the effect on BDNF is not directly translated into regulation of synaptophysin and GAP-43 mRNA.

Processing plant persistent strains of Listeria monocytogenes appear to have a lower virulence potential than clinical strains in selected virulence models.

Listeria monocytogenes is an important foodborne bacterial pathogen that can colonize food processing equipment. One group of genetically similar L. monocytogenes strains (RAPD type 9) was recently shown to reside in several independent fish processing plants. Persistent strains are likely to contaminate food products, and it is important to determine their virulence potential to evaluate risk to consumers. We compared the behaviour of food processing persistent and clinical L. monocytogenes strains in four virulence models: Adhesion, invasion and intracellular growth was studied in an epithelial cell line, Caco-2; time to death in a nematode model, Caenorhabditis elegans and in a fruit fly model, Drosophila melanogaster and fecal shedding in a guinea pig model. All strains adhered to and grew in Caco-2 cells in similar levels. When exposed to 10(6) CFU/ml, two strains representing the persistent RAPD type 9 invaded Caco-2 cells in lower numbers (10(2)-10(3) CFU/ml) as compared to the four other strains (10(4)-10(6) CFU/ml), including food and human clinical strains. In the D. melanogaster model, the two RAPD type 9 strains were among the slowest to kill. Similarly, the time to reach 50% killed C. elegans worms was longer (110 h) for the RAPD type 9 strains than for the other four strains (80 h). The Scott A strain and one RAPD type 9 strain were suspended in whipping cream before being fed to guinea pigs and the persistent RAPD type 9 strain was isolated from feces in a lower level (approximately 10(2) CFU/g) than the Scott A strain (approximately 10(5) CFU/g) (P<0.05). The addition of NaCl has been shown to cause autoaggregation and increases adhesion of L. monocytogenes to plastic. However, growth in the presence of NaCl did not alter the behaviour of the tested L. monocytogenes strains in the virulence models. Overall, the two strains representing a very common fish processing plant persistent group (RAPD type 9) appear to have a lower virulence potential in all four virulence models than Scott A and a strain isolated from a clinical case of listeriosis.

Expression of brain derived neurotrophic factor, activity-regulated cytoskeleton protein mRNA, and enhancement of adult hippocampal neurogenesis in rats after sub-chronic and chronic treatment with the triple monoamine re-uptake inhibitor tesofensine.

The changes of gene expression resulting from long-term exposure to monoamine antidepressant drugs in experimental animals are key to understanding the mechanisms of action of this class of drugs in man. Many of these genes and their products are either relevant biomarkers or directly involved in structural changes that are perhaps necessary for the antidepressant effect. Tesofensine is a novel triple monoamine reuptake inhibitor that acts to increase noradrenaline, serotonin, and dopamine neurotransmission. This study was undertaken to examine the effect of sub-chronic (5 days) and chronic (14 days) administration of Tesofensine on the expression of brain derived neurotrophic factor (BDNF) and activity-regulated cytoskeleton protein (Arc) in the rat hippocampus. Furthermore, hippocampi from the same animals were used to investigate the effect on cell proliferation by means of Ki-67- and NeuroD-immunoreactivity. We find that chronic, but not sub-chronic treatment with Tesofensine increases BDNF mRNA in the CA3 region of the hippocampus (35%), and Arc mRNA in the CA1 of the hippocampus (65%). Furthermore, the number of Ki-67- and neuroD-positive cells increased after chronic, but not sub-chronic treatment. This study shows that Tesofensine enhances hippocampal gene expression and new cell formation indicative for an antidepressant potential of this novel drug substance.

Sodium chloride enhances adherence and aggregation and strain variation influences invasiveness of Listeria monocytogenes strains.

Some subtypes of Listeria monocytogenes can persist in the food-processing industry, but the reasons for such persistence are not known. In the present study, 10 strains of L. monocytogenes representing known persistent randomly amplified polymorphic DNA (RAPD) types from fish processing plants were compared to eight strains of different RAPD type and origin (clinical, food, and animal). All 18 strains of L. monocytogenes had similar growth patterns at different temperatures (5 or 37 degrees C) or different salinities (0.5 or 5% NaCl), and all strains formed a thin layer of adhered cells on a plastic surface when cultured in tryptone soya broth (TSB) with a total of 1% glucose. Many ready-to-eat foods, such as cold-smoked fish, contain NaCl at concentrations of 2 to 5%, and NaCl is present in the processing environment. Adding NaCl to TSB changed the adhesion patterns of all strains, and all adhered better when NaCl was added. Also, the addition of NaCl caused a marked aggregation of 13 of the strains; however, 5 of the 18 strains did not aggregate in the presence of up to 5% NaCl. The aggregates stuck to the plastic surface, and this occurred in all but one of the persistent RAPD types. Four strains represented one particular RAPD type that has been isolated as a persistent RAPD type in several fish processing plants for up to 10 years. Because this RAPD type often can contaminate fish products, it is important to address its potential virulence. The 18 strains differed markedly in their ability to invade Caco-2 cells, and the four strains representing the universal persistent RAPD type were the least invasive (10(2) to 10(3) CFU/ml), whereas other strains invaded Caco-2 cells at levels of 10(4) to 10(5) CFU/ml. Five of the 18 strains belonged to the genetic lineage 1 and were the most invasive. Although the most commonly isolated persistent RAPD type was low invasive, it is important to understand why moderate salinity facilitates aggregation and biofilm formation, for this understanding can be beneficial in developing procedures to reduce processing plant contamination.

Effects of stress and adrenalectomy on activity-regulated cytoskeleton protein (Arc) gene expression.

Activity-regulated cytoskeletal-associated protein (Arc) is an effector immediate early gene induced by novelty and involved in consolidation of long-term memory. Since activation of glucocorticoid receptors is a prerequisite for memory consolidation, we therefore aimed to study the effect of acute restraint stress on Arc gene expression in adrenalectomized rats. Acute stress produced a significant increase in Arc gene expression in the medial prefrontal cortex, but not in the parietal cortex or in the pyramidal cell layer of the hippocampus. The basal level of Arc mRNA in adrenalectomized animals was high in the medial prefrontal cortex and unaffected by acute stress in these animals. These data are consistent with the role of Arc as an integrative modulator of synaptic plasticity by emphasizing the potential role of stress and glucocorticoids in the control of Arc gene expression.

Listeria monocytogenes has a functional chitinolytic system and an active lytic polysaccharide monooxygenase.

Chitinases and chitin-active lytic polysaccharide monooxygenases (LPMOs) are most commonly associated with chitin metabolism, but are also reported as virulence factors in pathogenic bacteria. Listeria monocytogenes, a well-known virulent bacterium, possesses two chitinases (ChiA and ChiB) and a multi-modular lytic polysaccharide monooxygenase (LmLPMO10). These enzymes have been related to virulence and their role in chitin metabolism is poorly understood. It is thus of interest to functionally characterize the individual enzymes in order to shed light on their roles in vivo. Our results demonstrate that L. monocytogenes has a fully functional chitinolytic system. Both chitinases show substrate degradation rates similar to those of the nonprocessive endo-chitinase SmChiC from Serratia marcescens. Compared to the S. marcescens LPMO chitin-binding protein CBP21, LmLPMO10 shows a similar rate but different product profiles depending on the substrate. In LPMO-chitinase synergy experiments, CBP21 is able to boost the activity of both ChiA and ChiB more than LmLPMO10. Product analysis of the synergy assays revealed that the chitinases were unable to efficiently hydrolyse the LPMO products (chitooligosaccharide aldonic acids) with a degree of polymerization below four (ChiA and SmChiC) or three (ChiB). Gene transcription and protein expression analysis showed that LmLPMO10 is neither highly transcribed, nor abundantly secreted during the growth of L. monocytogenes in a chitin-containing medium. The chitinases on the other hand are both abundantly secreted in the presence of chitin. Although LmLPMO10 is shown to promote chitin degradation in tandem with the chitinases in vitro, the secretome and transcription data question whether this is the primary role of LmLPMO10 in vivo.

Does virulence assessment of Vibrio anguillarum using sea bass (Dicentrarchus labrax) larvae correspond with genotypic and phenotypic characterization?

BACKGROUND: Vibriosis is one of the most ubiquitous fish diseases caused by bacteria belonging to the genus Vibrio such as Vibrio (Listonella) anguillarum. Despite a lot of research efforts, the virulence factors and mechanism of V. anguillarum are still insufficiently known, in part because of the lack of standardized virulence assays. METHODOLOGY/PRINCIPAL FINDINGS: We investigated and compared the virulence of 15 V. anguillarum strains obtained from different hosts or non-host niches using a standardized gnotobiotic bioassay with European sea bass (Dicentrarchus labrax L.) larvae as model hosts. In addition, to assess potential relationships between virulence and genotypic and phenotypic characteristics, the strains were characterized by random amplified polymorphic DNA (RAPD) and repetitive extragenic palindromic PCR (rep-PCR) analyses, as well as by phenotypic analyses using Biolog's Phenotype MicroArray™ technology and some virulence factor assays. CONCLUSIONS/SIGNIFICANCE: Virulence testing revealed ten virulent and five avirulent strains. While some relation could be established between serotype, genotype and phenotype, no relation was found between virulence and genotypic or phenotypic characteristics, illustrating the complexity of V. anguillarum virulence. Moreover, the standardized gnotobiotic system used in this study has proven its strength as a model to assess and compare the virulence of different V. anguillarum strains in vivo. In this way, the bioassay contributes to the study of mechanisms underlying virulence in V. anguillarum.

Personality traits in rats predict vulnerability and resilience to developing stress-induced depression-like behaviors, HPA axis hyper-reactivity and brain changes in pERK1/2 activity.

Emerging evidence indicates that certain behavioral traits, such as anxiety, are associated with the development of depression-like behaviors after exposure to chronic stress. However, single traits do not explain the wide variability in vulnerability to stress observed in outbred populations. We hypothesized that a combination of behavioral traits might provide a better characterization of an individual's vulnerability to prolonged stress. Here, we sought to determine whether the characterization of relevant behavioral traits in rats could aid in identifying individuals with different vulnerabilities to developing stress-induced depression-like behavioral alterations. We also investigated whether behavioral traits would be related to the development of alterations in the hypothalamic-pituitary-adrenal axis and in brain activity - as measured through phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2)--in response to an acute stressor following either sub-chronic (2 weeks) or chronic (4 weeks) unpredictable stress (CUS). Sprague-Dawley rats were characterized using a battery of behavioral tasks, and three principal traits were identified: anxiety, exploration and activity. When combined, the first two traits were found to explain the variability in the stress responses. Our findings confirm the increased risk of animals with high anxiety developing certain depression-like behaviors (e.g., increased floating time in the forced swim test) when progressively exposed to stress. In contrast, the behavioral profile based on combined low anxiety and low exploration was resistant to alterations related to social behaviors, while the high anxiety and low exploration profile displayed a particularly vulnerable pattern of physiological and neurobiological responses after sub-chronic stress exposure. Our findings indicate important differences in animals' vulnerability and/or resilience to the effects of repeated stress, particularly during initial or intermediate levels of stress exposure, and they highlight that the behavioral inhibition profile of an animal provides a particular susceptibility to responding in a deleterious manner to stress.

Regulation of brain-derived neurotrophic factor (BDNF) in the chronic unpredictable stress rat model and the effects of chronic antidepressant treatment.

Chronic unpredictable stress (CUS) is a widely used animal model of depression. The present study was undertaken to investigate behavioral, physiological and molecular effects of CUS and/or chronic antidepressant treatment (venlafaxine or imipramine) in the same set of animals. Anhedonia, a core symptom of depression, was assessed by measuring consumption of a palatable solution. Exposure to CUS reduced intake of a palatable solution and this effect was prevented by chronic antidepressant treatment. Moreover, chronic antidepressant treatment decreased depressive-like behavior in a modified forced swim test in stressed rats. Present evidence suggests a role for brain-derived neurotrophic factor (BDNF) in depression. BDNF mRNA levels in the ventral and dorsal hippocampus were assessed by in situ hybridization. Exposure to CUS was not correlated with a decrease but rather with an increase in BDNF mRNA expression in both the dentate gyrus of the dorsal hippocampus and the CA3 region of the ventral hippocampus indicating that there is no simple link between depression-like behaviors per se and brain BDNF levels in rats. However, a significant increase in BDNF mRNA levels in the dentate gyrus of the dorsal hippocampus correlated with chronic antidepressant treatment emphasizing a role for BDNF in the mechanisms underlying antidepressant activity.

Neuronal precursor cell proliferation in the hippocampus after transient cerebral ischemia: a comparative study of two rat strains using stereological tools.

BACKGROUND: We are currently investigating microglial activation and neuronal precursor cell (NPC) proliferation after transient middle cerebral artery occlusion (tMCAo) in rats. This study aimed: (1) to investigate differences in hippocampal NPC proliferation in outbred male spontaneously hypertensive rats (SHRs) and Sprague-Dawley rats (SDs) one week after tMCAo; (2) to present the practical use of the optical fractionator and 2D nucleator in stereological brain tissue analyses; and (3) to report our experiences with an intraluminal tMCAo model where the occluding filament is advanced 22 mm beyond the carotid bifurcation and the common carotid artery is clamped during tMCAo. METHODS: Twenty-three SDs and twenty SHRs were randomized into four groups subjected to 90 minutes tMCAo or sham. BrdU (50 mg/kg) was administered intraperitoneally twice daily on Day 4 to 7 after surgery. On Day 8 all animals were euthanized. NeuN-stained tissue sections were used for brain and infarct volume estimation with the 2D nucleator and Cavalieri principle. Brains were studied for the presence of activated microglia (ED-1) and hippocampal BrdU incorporation using the optical fractionator. RESULTS: We found no significant difference or increase in post-ischemic NPC proliferation between the two strains. However, the response to remote ischemia may differ between SDs and SHRs. In three animals increased post-stroke NPC proliferation was associated with hippocampal ischemic injury. The mean infarct volume was 89.2 +/- 76.1 mm3 in SHRs and 16.9 +/- 22.7 mm3 in SDs (p < 0.005). Eight out of eleven SHRs had ischemic neocortical damage in contrast to only one out of 12 SDs. We observed involvement of the anterior choroidal and hypothalamic arteries in several animals from both strains and the anterior cerebral artery in two SHRs. CONCLUSIONS: We found no evidence of an early hippocampal NPC proliferation one week after tMCAo in both strains. Infarction within the anterior choroidal artery could induce hippocampal ischemia and increase NPC proliferation profoundly. NPC proliferation was not aggravated by the presence of activated microglia. Intraluminal tMCAo in SHRs gave a more reliable infarct with neocortical involvement, but affected territories supplied by the anterior cerebral, anterior choroidal and hypothalamic arteries.

GABA regulates the rat hypothalamic-pituitary-adrenocortical axis via different GABA-A receptor alpha-subtypes.

The control of the corticotropin-releasing hormone (CRH) neurons in the hypothalamic paraventricular nucleus (PVN) is balanced by excitatory and inhibitory inputs. The GABA-A receptor, which is a major target for the inhibitory control, is composed of five subunits. The presence of an alpha(1)-, alpha(2)-, alpha(3)-, or alpha(5)-subunit in the GABA-A receptor protein complex is necessary for benzodiazepines to exert their potentiating effect on the receptor. We postulate that the effect of nonselective benzodiazepines on the hypothalamo-pituitary adrenocortical (HPA) axis is critically dependent on the composition of the GABA-A receptor subunits through which they act. We show here that positive modulators of alpha(1)-subtype containing GABA-A receptors with zolpidem (10 mg/kg) increase HPA activity in terms of increase in plasma corticosterone and induction of Fos in the PVN, whereas activation of non-alpha(1)-subtype GABA-A receptors using L-818,417 (10 mg/kg) likely inhibits the activity. We also show that the alpha(2)-subunit gene is highly expressed in the PVN, but its expression is not affected by chronic mild stress. These results show that the stimulatory effect on HPA activity after positive modulation of GABA-A receptors composed of alpha(1)-subunit(s) affects a selective afferent system than the PVN, which is distinct from another afferent system(s) activated by non alpha(1)-containing GABA-A receptors.

Rapid activation of the extracellular signal-regulated kinase 1/2 (ERK1/2) signaling pathway by electroconvulsive shock in the rat prefrontal cortex is not associated with TrkB neurotrophin receptor activation.

1. Emerging evidence indicates that brain-derived neurotrophic factor (BDNF) and its receptor TrkB play important roles in the mechanism of action of electroconvulsive shock (ECS) treatment. ECS produces a significant increase in brain BDNF synthesis together with a variety of neuroplastic changes including neurogenesis and axonal sprouting in the rodent brain, which is believed to be associated to the antidepressant effect of ECS. ERK1/2 (extracellular signal-regulated kinase-1/2) and Akt (protein kinase B), both intracellular signaling molecules being linked to neurotrophin signaling and synthesis, are important pathways triggered by TrkB autophosphorylation. 2. We have previously observed that chemical antidepressants induce a rapid activation of TrkB signaling in the rodent prefrontal cortex (PFC), which is likely a consequence of the stimulatory effect of antidepressants on BDNF synthesis. However, it is not known whether ECS triggers TrkB autophosphorylation and if any ECS-induced effect on TrkB function may be associated with the activation of the ERK1/2 and Akt pathways. 3. The present study assayed the phosphorylation levels of TrkB, ERK1/2, and Akt in the PFC of sham and ECS-treated rats. While the TrkB autophosphorylation (pTrkB) levels were decreased 30 min after both acute and chronic ECS, no change in pTrkB levels were observed at any other time points measured. In contrast, acute but not chronic ECS, transiently induced a very rapid and robust hyperphosphorylation of ERK1/2. Akt phosphorylation levels remained unchanged following acute or chronic ECS. Hence, although ECS effectively stimulates the ERK1/2 pathway in the PFC, this effect does not appear to involve upstream activation of TrkB.

The response regulator ResD modulates virulence gene expression in response to carbohydrates in Listeria monocytogenes.

Listeria monocytogenes is a versatile bacterial pathogen that is able to accommodate to diverse environmental and host conditions. Presently, we have identified a L. monocytogenes two-component response regulator, ResD that is required for the repression of virulence gene expression known to occur in the presence of easily fermentable carbohydrates not found inside host organisms. Structurally and functionally, ResD resembles the respiration regulator ResD in Bacillus subtilis as deletion of the L. monocytogenes resD reduces respiration and expression of cydA, encoding a subunit of cytochrome bd. The resD mutation also reduces expression of mptA encoding the EIIABman component of a mannose/glucose-specific PTS system, indicating that ResD controls sugar uptake. This notion was supported by the poor growth of resD mutant cells that was alleviated by excess of selected carbohydrates. Despite the growth deficient phenotype of the mutant in vitro the mutation did not affect intracellular multiplication in epithelial or macrophage cell lines. When examining virulence gene expression we observed traditional induction by charcoal in both mutant and wild-type cells whereas the repression observed in wild-type cells by fermentable carbohydrates did not occur in resD mutant cells. Thus, ResD is a central regulator of L. monocytogenes when present in the external environment.

The Dps-like protein Fri of Listeria monocytogenes promotes stress tolerance and intracellular multiplication in macrophage-like cells.

Members of the ferritin-like Dps protein family are found in a number of bacterial species, where they demonstrate the potential to bind iron, and have been implicated in tolerance to oxidative stress. In this study of the food-borne pathogen Listeria monocytogenes, the fri gene encoding a Dps homologue was deleted, and, compared to wild-type cells, it was found that the resulting mutant was less resistant to hydrogen peroxide, and demonstrated reduced survival following long-term (7-11 days) incubation in laboratory media. In view of this, it is shown that fri gene expression is controlled by the hydrogen peroxide regulator PerR, as well as the general stress sigma factor sigmaB. When fri mutant cells were transferred to iron-limiting conditions, growth was retarded relative to wild-type cells, indicating that Fri may be required for iron storage. This notion is supported by the observation that the L. monocytogenes genome appears not to encode other ferritin-like proteins. Given the role of Fri in resistance to oxidative stress, and growth under iron-limiting conditions, the ability of the fri mutant to infect mice was examined. When injected by the intraperitoneal route, the fri mutant demonstrated a reduced capacity to proliferate in the organs of infected mice relative to the wild-type, whereas when the bacteria were supplied intravenously this effect was mitigated. In addition, the mutant was impaired in its ability to survive and grow in J774.A1 mouse macrophage cells. Thus, the data suggest that Fri contributes to the ability of L. monocytogenes to survive in environments where oxidative stress and low iron availability may impede bacterial proliferation.

Influences of temperature, salinity and starvation on the motility and chemotactic response of Vibrio anguillarum.

The role of growth factors for the motility and chemotaxis of the fish pathogen Vibrio anguillarum was determined. Cells of V. anguillarum were chemotactic to serine in the temperature range 5-25 degrees C and in 0.8-2.7 % NaCl. The chemotactic response was significantly higher at 25 degrees C than at 5 or 15 degrees C. Growth in medium with 1.5 % NaCl gave a higher response than growth with 3 % NaCl; when the salinity of the chemotaxis buffer was raised, the chemotactic response was reduced. The role of starvation was also studied; V. anguillarum showed a high chemotactic response after starvation for 2 and 8 days. Motility and chemotaxis are important virulence factors for this bacterium. Not only was the ability to perform chemotactic motility maintained after starvation, but also it was shown that starvation does not interfere with the ability of the organism to cause infection in rainbow trout after a bath challenge. The swimming speed was reduced at lower temperatures. Within the range of salinity and starvation studied, the motile cells swam with the same velocity, indicating that V. anguillarum under all the examined conditions has a functional flagellum and rotates it with constant speed. Phenamil, a specific inhibitor of Na(+)-driven flagella, reduced the motility of both starved and non-starved cells of V. anguillarum indicating that, in both cases, a Na(+) motive force drives the flagellum.

The RNA-binding protein Hfq of Listeria monocytogenes: role in stress tolerance and virulence.

In gram-negative bacteria, the RNA-binding protein Hfq has emerged as an important regulatory factor in a variety of physiological processes, including stress resistance and virulence. In Escherichia coli, Hfq modulates the stability or the translation of mRNAs and interacts with numerous small regulatory RNAs. Here, we studied the role of Hfq in the stress tolerance and virulence of the gram-positive food-borne human pathogen Listeria monocytogenes. We present evidence that Hfq is involved in the ability of L. monocytogenes to tolerate osmotic and ethanol stress and contributes to long-term survival under amino acid-limiting conditions. However, Hfq is not required for resistance to acid and oxidative stress. Transcription of hfq is induced under various stress conditions, including osmotic and ethanol stress and at the entry into the stationary growth phase, thus supporting the view that Hfq is important for the growth and survival of L. monocytogenes in harsh environments. The stress-inducible transcription of hfq depends on the alternative sigma factor sigmaB, which controls the expression of numerous stress- and virulence-associated genes in L. monocytogenes. Infection studies showed that Hfq contributes to pathogenesis in mice, yet plays no role in the infection of cultured cell lines. This study provides, for the first time, information on the role of Hfq in the stress tolerance and virulence of a gram-positive pathogen.