Functional impact of mutational activation on the Listeria monocytogenes central virulence regulator PrfA

Listeria monocytogenes requires phosphotransferase systems to facilitate intracellular growth and virulence

The metabolism of bacterial pathogens is exquisitely evolved to support virulence in the nutrient-limiting host. Many bacterial pathogens utilize bipartite metabolism to support intracellular growth by splitting carbon utilization between two carbon sources and dividing flux to distinct metabolic needs. For example, previous studies suggest that the professional cytosolic pathogen Listeria monocytogenes (L. monocytogenes) utilizes glycerol and hexose phosphates (e.g., Glucose-6-Phosphate) as catabolic and anabolic carbon sources in the host cytosol, respectively. However, the role of this putative bipartite metabolism in L. monocytogenes virulence has not been fully assessed. Here, we demonstrate that when L. monocytogenes is unable to consume either glycerol (ΔglpD/ΔgolD), hexose phosphates (ΔuhpT), or both (ΔglpD/ΔgolD/ΔuhpT), it is still able to grow in the host cytosol and is 10- to 100-fold attenuated in vivo suggesting that L. monocytogenes consumes alternative carbon source(s) in the host. An in vitro metabolic screen using BioLog's phenotypic microarrays unexpectedly demonstrated that WT and PrfA* (G145S) L. monocytogenes, a strain with constitutive virulence gene expression, use phosphotransferase system (PTS) mediated carbon sources. These findings contrast with the existing metabolic model that cytosolic L. monocytogenes expressing PrfA does not use PTS mediated carbon sources. We next demonstrate that two independent and universal phosphocarrier proteins (PtsI [EI] and PtsH [HPr]), essential for the function of all PTS, are critical for intracellular growth and virulence in vivo. Constitutive virulence gene expression using a PrfA* (G145S) allele in ΔglpD/ΔgolD/ΔuhpT and ΔptsI failed to rescue in vivo virulence defects suggesting phenotypes are due to metabolic disruption and not virulence gene regulation. Finally, in vivo attenuation of ΔptsI and ΔptsH was additive to ΔglpD/ΔgolD/ΔuhpT, suggesting that hexose phosphates and glycerol and PTS mediated carbon source are relevant metabolites. Taken together, these studies indicate that PTS are critical virulence factors for the cytosolic growth and virulence of L. monocytogenes.

RECON gene disruption enhances host resistance to enable genome-wide evaluation of intracellular pathogen fitness during infection

Transposon sequencing (Tn-seq) is a powerful genome-wide technique to assess bacterial fitness under varying growth conditions. However, screening via Tn-seq in vivo is challenging. Dose limitations and host restrictions create bottlenecks that diminish the transposon mutant pool being screened. Here, we have developed a murine model with a disruption in Akr1c13 that renders the resulting RECON-/- mouse resistant to high-dose infection. We leveraged this model to perform a Tn-seq screen of the human pathogen Listeria monocytogenes in vivo. We identified 135 genes which were required for L. monocytogenes growth in mice including novel genes not previously identified for host survival. We identified organ-specific requirements for L. monocytogenes survival and investigated the role of the folate enzyme FolD in L. monocytogenes liver pathogenesis. A mutant lacking folD was impaired for growth in murine livers by 2.5-log10 compared to wild type and failed to spread cell-to-cell in fibroblasts. In contrast, a mutant in alsR, which encodes a transcription factor that represses an operon involved in D-allose catabolism, was attenuated in both livers and spleens of mice by 4-log10 and 3-log10, respectively, but showed modest phenotypes in in vitro models. We confirmed that dysregulation of the D-allose catabolism operon is responsible for the in vivo growth defect, as deletion of the operon in the ∆alsR background rescued virulence. By undertaking an unbiased, genome-wide screen in mice, we have identified novel fitness determinants for L. monocytogenes host infection, which highlights the utility of the RECON-/- mouse model for future screening efforts.

IMPORTANCE

Listeria monocytogenes is the gram-positive bacterium responsible for the food-borne disease listeriosis. Although infections with L. monocytogenes are limiting in healthy hosts, vulnerable populations, including pregnant and elderly people, can experience high rates of mortality. Thus, understanding the breadth of genetic requirements for L. monocytogenes in vivo survival will present new opportunities for treatment and prevention of listeriosis. We developed a murine model of infection using a RECON-/- mouse that is restrictive to systemic L. monocytogenes infection. We utilized this model to screen for L. monocytogenes genes required in vivo via transposon sequencing. We identified the liver-specific gene folD and a repressor, alsR, that only exhibits an in vivo growth defect. AlsR controls the expression of the D-allose operon which is a marker in diagnostic techniques to identify pathogenic Listeria. A better understanding of the role of the D-allose operon in human disease may further inform diagnostic and prevention measures.

The major role of Listeria monocytogenes folic acid metabolism during infection is the generation of N-formylmethionine

IMPORTANCE

Folic acid is an essential vitamin for bacteria, plants, and animals. The lack of folic acid leads to various consequences such as a shortage of amino acids and nucleotides that are fundamental building blocks for life. Though antifolate drugs are widely used for antimicrobial treatments, the underlying mechanism of bacterial folate deficiency during infection is unclear. This study compares the requirements of different folic acid end-products during the infection of Listeria monocytogenes, a facultative intracellular pathogen of animals and humans. The results reveal the critical importance of N-formylmethionine, the amino acid used by bacteria to initiate protein synthesis. This work extends the current understanding of folic acid metabolism in pathogens and potentially provides new insights into antifolate drug development in the future.

Listeria monocytogenes utilizes glutathione and limited inorganic sulfur compounds as a source of essential L-cysteine

Listeria monocytogenes ( Lm ) is a Gram-positive facultative intracellular pathogen that leads a biphasic lifecycle, transitioning its metabolism and selectively inducing virulence genes when it encounters mammalian hosts. Virulence gene expression is controlled by the master virulence regulator PrfA, which is allosterically activated by host- and bacterially-derived glutathione (GSH). The amino acid L-cysteine is the rate-limiting substrate for GSH synthesis in bacteria and is essential for bacterial growth. Unlike many bacteria, Lm is auxotrophic for L-cysteine and must import exogenous cysteine for growth and virulence. GSH is enriched in the host cytoplasm, and previous work suggests that Lm utilizes exogenous GSH for PrfA activation. Despite these observations, the import mechanism(s) for GSH remains elusive. Analysis of known GSH importers predicted a homologous importer in Lm comprised of the Ctp ABC transporter and the OppDF ATPases of the Opp oligopeptide importer. Here, we demonstrated that the Ctp complex is a high-affinity GSH/GSSG importer that is required for Lm growth at physiologically relevant concentrations. Further, we demonstrated that OppDF are required for GSH/GSSG import in an Opp-independent manner. These data support a model where Ctp and OppDF form a unique complex for GSH/GSSG import that supports growth and pathogenesis. Additionally, we show that Lm utilizes the inorganic sulfur sources thiosulfate and H 2 S for growth in a CysK-dependent manner in the absence of other L-cysteine sources. These findings suggest a pathoadaptive role for partial cysteine auxotrophy in Lm , where locally high GSH/GSSG or inorganic sulfur concentrations may signal arrival to distinct host niches.

para -Aminobenzoic Acid Biosynthesis Is Required for Listeria monocytogenes Growth and Pathogenesis

Biosyntheses of para -aminobenzoic acid (PABA) and its downstream folic acid metabolites are essential for one-carbon metabolism in all life forms and the targets of sulfonamide and trimethoprim antibiotics. In this study, we identified and characterized two genes ( pabA and pabBC ) required for PABA biosynthesis in Listeria monocytogenes .

Role of FruR transcriptional regulator in virulence of Listeria monocytogenes and identification of its regulon

Listeria monocytogenes is a ubiquitous opportunistic foodborne pathogen capable of survival in various adverse environmental conditions. Pathogenesis of L. monocytogenes is tightly controlled by a complex regulatory network of transcriptional regulators that are necessary for survival and adaptations to harsh environmental conditions both inside and outside host cells. Among these regulatory pathways are members of the DeoR-family transcriptional regulators that are known to play a regulatory role in sugar metabolism. In this study, we deciphered the role of FruR, a DeoR family protein, which is a fructose operon transcriptional repressor protein, in L. monocytogenes pathogenesis and growth. Following intravenous (IV) inoculation in mice, a mutant strain with deletion of fruR exhibited a significant reduction in bacterial burden in liver and spleen tissues compared to the parent strain. Further, the ΔfruR strain had a defect in cell-to-cell spread in L2 fibroblast monolayers. Constitutive activation of PrfA, a pleiotropic activator of L. monocytogenes virulence factors, did not restore virulence to the ΔfruR strain, suggesting that the attenuation was not a result of impaired PrfA activation. Transcriptome analysis revealed that FruR functions as a positive regulator for genes encoding enzymes involved in the pentose phosphate pathway (PPP) and as a repressor for genes encoding enzymes in the glycolysis pathway. These results suggested that FruR may function to facilitate NADPH regeneration, which is necessary for full protection from oxidative stress. Interestingly, deletion of fruR increased sensitivity of L. monocytogenes to H₂O₂, confirming a role for FruR in survival of L. monocytogenes during oxidative stress. Using anti-mouse neutrophil/monocyte monoclonal antibody RB6-8C5 (RB6) in an in vivo infection model, we found that FruR has a specific function in protecting L. monocytogenes from neutrophil/monocyte-mediated killing. Overall, this work clarifies the role of FruR in controlling L. monocytogenes carbon flow between glycolysis and PPP for NADPH homeostasis, which provides a new mechanism allowing metabolic adaptation of L. monocytogenes to oxidative stress.

Rhizobacteria Impact Colonization of Listeria monocytogenes on Arabidopsis thaliana Roots

In spite of its relevance as a foodborne pathogen, we have limited knowledge about Listeria monocytogenes in the environment. L. monocytogenes outbreaks have been linked to fruits and vegetables; thus, a better understanding of the factors influencing its ability to colonize plants is important. We tested how environmental factors and other soil- and plant-associated bacteria influenced L. monocytogenes' ability to colonize plant roots using Arabidopsis thaliana seedlings in a hydroponic growth system. We determined that the successful root colonization of L. monocytogenes 10403S was modestly but significantly enhanced by the bacterium being pregrown at higher temperatures, and this effect was independent of the biofilm and virulence regulator PrfA. We tested 14 rhizosphere-derived bacteria for their impact on L. monocytogenes 10403S, identifying one that enhanced and 10 that inhibited the association of 10403S with plant roots. We also characterized the outcomes of these interactions under both coinoculation and invasion conditions. We characterized the physical requirements of five of these rhizobacteria to impact the association of L. monocytogenes 10403S with roots, visualizing one of these interactions by microscopy. Furthermore, we determined that two rhizobacteria (one an inhibitor, the other an enhancer of 10403S root association) were able to similarly impact 10 different L. monocytogenes strains, indicating that the effects of these rhizobacteria on L. monocytogenes are not strain specific. Taken together, our results advance our understanding of the parameters that affect L. monocytogenes plant root colonization, knowledge that may enable us to deter its association with and, thus, downstream contamination of, food crops. IMPORTANCE Listeria monocytogenes is ubiquitous in the environment, being found in or on soil, water, plants, and wildlife. However, little is known about the requirements for L. monocytogenes' existence in these settings. Recent L. monocytogenes outbreaks have been associated with contaminated produce; thus, we used a plant colonization model to investigate factors that alter L. monocytogenes' ability to colonize plant roots. We show that L. monocytogenes colonization of roots was enhanced when grown at higher temperatures prior to inoculation but did not require a known regulator of virulence and biofilm formation. Additionally, we identified several rhizobacteria that altered the ability of 11 different strains of L. monocytogenes to colonize plant roots. Understanding the factors that impact L. monocytogenes physiology and growth will be crucial for finding mechanisms (whether chemical or microbial) that enable its removal from plant surfaces to reduce L. monocytogenes contamination of produce and eliminate foodborne illness.

Characterisation of Listeria monocytogenes food-associated isolates to assess environmental fitness and virulence potential

The ability of Listeria monocytogenes isolates to survive within the food production environment (FPE), as well as virulence, varies greatly between strains. There are specific genetic determinants that have been identified which can strongly influence a strains ability to survive in the FPE and/or within human hosts. In this study, we assessed the FPE fitness and virulence potential, including efficacy of selected hygiene or treatment intervention, against 52 L. monocytogenes strains isolated from various food and food environment sources. Phenotypic tests were performed to determine the minimum inhibitory concentration of cadmium chloride and benzalkonium chloride and the sensitivities to five clinically relevant antibiotics. A genomic analysis was also performed to identify resistance genes correlating to the observed phenotypic resistance profiles, along with genetic determinants of interest which may elude to the FPE fitness and virulence potential. A transposon element containing a novel cadmium resistance gene, cadA7, a Tn916 variant insert in the hypervariable Listeria genomic island 1 region and an LGI2 variant were identified. Resistance to cadmium and disinfectants was prevalent among isolates in this study, although no resistance to clinically important antimicrobials was observed. Potential hypervirulent strains containing full length inlA, LIPI-1 and LIPI-3 were also identified in this study. Cumulatively, the results of this study show a vast array of FPE survival and pathogenicity potential among food production-associated isolates, which may be of concern for food processing operators and clinicians regarding L. monocytogenes strains colonising and persisting within the FPE, and subsequently contaminating food products then causing disease in at risk population groups.

Identification of Listeria monocytogenes Genes Contributing to Oxidative Stress Resistance under Conditions Relevant to Host Infection

The Gram-positive bacterium Listeria monocytogenes survives in environments ranging from the soil to the cytosol of infected host cells. Key to L. monocytogenes intracellular survival is the activation of PrfA, a transcriptional regulator that is required for the expression of multiple bacterial virulence factors. Mutations that constitutively activate prfA (prfA* mutations) result in high-level expression of multiple bacterial virulence factors as well as the physiological adaptation of L. monocytogenes for optimal replication within host cells. Here, we demonstrate that L. monocytogenes prfA* mutants exhibit significantly enhanced resistance to oxidative stress in comparison to that of wild-type strains. Transposon mutagenesis of L. monocytogenes prfA* strains resulted in the identification of three novel gene targets required for full oxidative stress resistance only in the context of PrfA activation. One gene, lmo0779, predicted to encode an uncharacterized protein, and two additional genes known as cbpA and ygbB, encoding a cyclic di-AMP binding protein and a 2-C-methyl-d-erythritol 2,4-cyclodiphosphate synthase, respectively, contribute to the enhanced oxidative stress resistance of prfA* strains while exhibiting no significant contribution in wild-type L. monocytogenes Transposon inactivation of cbpA and lmo0779 in a prfA* background led to reduced virulence in the liver of infected mice. These results indicate that L. monocytogenes calls upon specific bacterial factors for stress resistance in the context of PrfA activation and thus under conditions favorable for bacterial replication within infected mammalian cells.

Free Fatty Acids Interfere with the DNA Binding Activity of the Virulence Regulator PrfA of Listeria monocytogenes

Naturally occurring free fatty acids (FFAs) are recognized as potent antimicrobial agents that also affect the production of virulence factors in bacterial pathogens. In the foodborne pathogen Listeria monocytogenes, some medium- and long-chain FFAs act as antimicrobial agents as well as signaling compounds, causing a repression of transcription of virulence genes. We previously observed that the master virulence regulator PrfA is involved in both the antimicrobial and virulence-inhibitory response of L. monocytogenes to selected FFAs, but the underlying mechanisms are presently unknown. Here, we present a systematic analysis of the antimicrobial and PrfA-inhibitory activities of medium- and long-chain FFAs of various carbon chain lengths and degrees of saturation. We observed that exposure to specific antimicrobial and nonantimicrobial FFAs prevented PrfA-dependent activation of virulence gene transcription and reduced the levels of PrfA-regulated virulence factors. Thus, an antimicrobial activity was not compulsory for the PrfA-inhibitory ability of an FFA. In vitro binding experiments revealed that PrfA-inhibitory FFAs were also able to prevent the constitutively active variant PrfA* from binding to the PrfA box in the promoter region of the virulence gene hly, whereas noninhibitory FFAs did not affect its ability to bind DNA. Notably, the unsaturated FFAs inhibited the DNA binding activity of PrfA* most efficiently. Altogether, our findings support a model in which specific FFAs orchestrate a generalized reduction of the virulence potential of L. monocytogenes by directly targeting the key virulence regulator PrfA.IMPORTANCE Listeria monocytogenes is a Gram-positive pathogen able to cause foodborne infections in humans and animals. Key virulence genes in L. monocytogenes are activated by the transcription regulator PrfA, a DNA binding protein belonging to the CRP/FNR family. Various signals from the environment are known to affect the activity of PrfA, either positively or negatively. Recently, we found that specific medium- and long-chain free fatty acids act as antimicrobial agents as well as signaling compounds in L. monocytogenes Here, we show that both antimicrobial and nonantimicrobial free fatty acids inhibit PrfA-dependent activation of virulence gene transcription by interfering with the DNA binding activity of PrfA. Our findings suggest that free fatty acids could be candidates for alternative therapies against L. monocytogenes.

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.

Reactive oxygen species inhibit biofilm formation of Listeria monocytogenes

Although the level of reactive oxygen species (ROS) is altered upon the formation of bacterial biofilm, the relationship between ROS alteration and biofilm formation is still unclear. The aim of the present study is to use Listeria monocytogenes (L. monocytogenes) as a model organism to examine whether ROS have an effect on the biofilm formation. After eliminating ROS by treatment with NAD(P)H oxidase inhibitor Diphenyleneiodonium chloride (DPI) or scavenging reagents N-acetylcysteine (NAC), the biofilm formation of L. monocytogenes was examined. Our data demonstrate that DPI and NAC induced-reduction of ROS enhances the biofilm formation in L. monocytogenes without affecting bacterial growth and activity. These data provide the evidence that ROS produced by L. monocytogenes inhibit the biofilm formation.

Relative Roles of Listeriolysin O, InlA, and InlB in Listeria monocytogenes Uptake by Host Cells

Listeria monocytogenes is a facultative intracellular pathogen that infects a wide variety of cells, causing the life-threatening disease listeriosis. L. monocytogenes virulence factors include two surface invasins, InlA and InlB, known to promote bacterial uptake by host cells, and the secreted pore-forming toxin listeriolysin O (LLO), which disrupts the phagosome to allow bacterial proliferation in the cytosol.

Listeria monocytogenes is a facultative intracellular pathogen that infects a wide variety of cells, causing the life-threatening disease listeriosis. L. monocytogenes virulence factors include two surface invasins, InlA and InlB, known to promote bacterial uptake by host cells, and the secreted pore-forming toxin listeriolysin O (LLO), which disrupts the phagosome to allow bacterial proliferation in the cytosol. In addition, plasma membrane perforation by LLO has been shown to facilitate L. monocytogenes internalization into epithelial cells. In this work, we tested the host cell range and importance of LLO-mediated L. monocytogenes internalization relative to the canonical invasins, InlA and InlB. We measured the efficiencies of L. monocytogenes association with and internalization into several human cell types (hepatocytes, cytotrophoblasts, and endothelial cells) using wild-type bacteria and isogenic single, double, and triple deletion mutants for the genes encoding InlA, InlB and LLO. No role for InlB was detected in any tested cells unless the InlB expression level was substantially enhanced, which was achieved by introducing a mutation (prfA*) in the gene encoding the transcription factor PrfA. In contrast, InlA and LLO were the most critical invasion factors, although they act in a different manner and in a cell-type-dependent fashion. As expected, InlA facilitates both bacterial attachment and internalization in cells that express its receptor, E-cadherin. LLO promotes L. monocytogenes internalization into hepatocytes, but not into cytotrophoblasts and endothelial cells. Finally, LLO and InlA cooperate to increase the efficiency of host cell invasion by L. monocytogenes.

Comparison of listeriolysin O and phospholipases PlcA and PlcB activities, and initial intracellular growth capability among food and clinical strains of Listeria monocytogenes

AIMS

We investigated whether Listeria monocytogenes strains differ in their ability to escape from the primary phagosome after internalization into human intestinal epithelial cells.

METHODS AND RESULTS

Food and clinical strains were used to study specific alleles; the activities of listeriolysin O (LLO) and phospholipases PlcA and PlcB, which promote rupture of the phagocytic vacuole; and initial intracellular bacterial growth in Caco-2 cells. Results showed no difference in LLO activities between food and clinical strains or among serotypes. In contrast, the LLO truncation mutant lacked detectable haemolytic activity and intracellular growth. PlcA and PlcB produced by the strains of serotypes 4b/4e and 1/2b exhibited significantly lower activities than those of serotypes 1/2a and 1/2c. In contrast, the strains of serotype 1/2b grew significantly faster than those of serotypes 4b/4e and 1/2a. Moreover, the PrfA truncation mutants lacked LLO and phospholipases activities and did not show intracellular growth.

CONCLUSIONS

We determined that LLO and PrfA mutants exert a significant effect on intracellular growth, although it was unclear from this study whether PlcA and PlcB alleles affect escape from vacuoles.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study estimates that low-virulence L. monocytogenes strains associated with escape ability from the primary vacuoles are not widely distributed among food strains.

Activation of the Listeria monocytogenes Virulence Program by a Reducing Environment

Upon entry into the host cell cytosol, the facultative intracellular pathogen Listeria monocytogenes coordinates the expression of numerous essential virulence factors by allosteric binding of glutathione (GSH) to the Crp-Fnr family transcriptional regulator PrfA. Here, we report that robust virulence gene expression can be recapitulated by growing bacteria in a synthetic medium containing GSH or other chemical reducing agents. Bacteria grown under these conditions were 45-fold more virulent in an acute murine infection model and conferred greater immunity to a subsequent lethal challenge than bacteria grown in conventional media. During cultivation in vitro, PrfA activation was completely dependent on the intracellular levels of GSH, as a glutathione synthase mutant (ΔgshF) was activated by exogenous GSH but not reducing agents. PrfA activation was repressed in a synthetic medium supplemented with oligopeptides, but the repression was relieved by stimulation of the stringent response. These data suggest that cytosolic L. monocytogenes interprets a combination of metabolic and redox cues as a signal to initiate robust virulence gene expression in vivo.

Intracellular pathogens are responsible for much of the worldwide morbidity and mortality from infectious diseases. These pathogens have evolved various strategies to proliferate within individual cells of the host and avoid the host immune response. Through cellular invasion or the use of specialized secretion machinery, all intracellular pathogens must access the host cell cytosol to establish their replicative niches. Determining how these pathogens sense and respond to the intracellular compartment to establish a successful infection is critical to our basic understanding of the pathogenesis of each organism and for the rational design of therapeutic interventions. Listeria monocytogenes is a model intracellular pathogen with robust in vitro and in vivo infection models. Studies of the host-sensing and downstream signaling mechanisms evolved by L. monocytogenes often describe themes of pathogenesis that are broadly applicable to less tractable pathogens. Here, we describe how bacteria use external redox states as a cue to activate virulence.

Secretion Chaperones PrsA2 and HtrA Are Required for Listeria monocytogenes Replication following Intracellular Induction of Virulence Factor Secretion

The Gram-positive bacterium Listeria monocytogenes transitions from an environmental organism to an intracellular pathogen following its ingestion by susceptible mammalian hosts. Bacterial replication within the cytosol of infected cells requires activation of the central virulence regulator PrfA followed by a PrfA-dependent induction of secreted virulence factors. The PrfA-induced secreted chaperone PrsA2 and the chaperone/protease HtrA contribute to the folding and stability of select proteins translocated across the bacterial membrane. L. monocytogenes strains that lack both prsA2 and htrA exhibit near-normal patterns of growth in broth culture but are severely attenuated in vivo We hypothesized that, in the absence of PrsA2 and HtrA, the increase in PrfA-dependent protein secretion that occurs following bacterial entry into the cytosol results in misfolded proteins accumulating at the bacterial membrane with a subsequent reduction in intracellular bacterial viability. Consistent with this hypothesis, the introduction of a constitutively activated allele of prfA (prfA*) into ΔprsA2 ΔhtrA strains was found to essentially inhibit bacterial growth at 37°C in broth culture. ΔprsA2 ΔhtrA strains were additionally found to be defective for cell invasion and vacuole escape in selected cell types, steps that precede full PrfA activation. These data establish the essential requirement for PrsA2 and HtrA in maintaining bacterial growth under conditions of PrfA activation. In addition, chaperone function is required for efficient bacterial invasion and rapid vacuole lysis within select host cell types, indicating roles for PrsA2/HtrA prior to cytosolic PrfA activation and the subsequent induction of virulence factor secretion.

An In Vivo Selection Identifies Listeria monocytogenes Genes Required to Sense the Intracellular Environment and Activate Virulence Factor Expression

Listeria monocytogenes is an environmental saprophyte and facultative intracellular bacterial pathogen with a well-defined life-cycle that involves escape from a phagosome, rapid cytosolic growth, and ActA-dependent cell-to-cell spread, all of which are dependent on the master transcriptional regulator PrfA. The environmental cues that lead to temporal and spatial control of L. monocytogenes virulence gene expression are poorly understood. In this study, we took advantage of the robust up-regulation of ActA that occurs intracellularly and expressed Cre recombinase from the actA promoter and 5' untranslated region in a strain in which loxP sites flanked essential genes, so that activation of actA led to bacterial death. Upon screening for transposon mutants that survived intracellularly, six genes were identified as necessary for ActA expression. Strikingly, most of the genes, including gshF, spxA1, yjbH, and ohrA, are predicted to play important roles in bacterial redox regulation. The mutants identified in the genetic selection fell into three broad categories: (1) those that failed to reach the cytosolic compartment; (2) mutants that entered the cytosol, but failed to activate the master virulence regulator PrfA; and (3) mutants that entered the cytosol and activated transcription of actA, but failed to synthesize it. The identification of mutants defective in vacuolar escape suggests that up-regulation of ActA occurs in the host cytosol and not the vacuole. Moreover, these results provide evidence for two non-redundant cytosolic cues; the first results in allosteric activation of PrfA via increased glutathione levels and transcriptional activation of actA while the second results in translational activation of actA and requires yjbH. Although the precise host cues have not yet been identified, we suggest that intracellular redox stress occurs as a consequence of both host and pathogen remodeling their metabolism upon infection.

Perforin-2 Protects Host Cells and Mice by Restricting the Vacuole to Cytosol Transitioning of a Bacterial Pathogen

The host-encoded Perforin-2 (encoded by the macrophage-expressed gene 1, Mpeg1), which possesses a pore-forming MACPF domain, reduces the viability of bacterial pathogens that reside within membrane-bound compartments. Here, it is shown that Perforin-2 also restricts the proliferation of the intracytosolic pathogen Listeria monocytogenes Within a few hours of systemic infection, the massive proliferation of L. monocytogenes in Perforin-2(-/-)mice leads to a rapid appearance of acute disease symptoms. We go on to show in cultured Perforin-2(-/-)cells that the vacuole-to-cytosol transitioning of L. monocytogenesis greatly accelerated. Unexpectedly, we found that in Perforin-2(-/-)macrophages,Listeria-containing vacuoles quickly (≤ 15 min) acidify, and that this was coincident with greater virulence gene expression, likely accounting for the more rapid translocation of L. monocytogenes to its replicative niche in the cytosol. This hypothesis was supported by our finding that aL. monocytogenes strain expressing virulence factors at a constitutively high level replicated equally well in Perforin-2(+/+)and Perforin-2(-/-)macrophages. Our findings suggest that the protective role of Perforin-2 against listeriosis is based on it limiting the intracellular replication of the pathogen. This cellular activity of Perforin-2 may derive from it regulating the acidification of Listeria-containing vacuoles, thereby depriving the pathogen of favorable intracellular conditions that promote its virulence gene activity.

Characterization of specific alleles in InlA and PrfA of Listeria monocytogenes isolated from foods in Osaka, Japan and their ability to invade Caco-2 cells

Listeria monocytogenes expresses the surface protein internalin A (InlA), enabling the invasion of human intestinal epithelial cells to cause severe food-borne diseases. Full-length sequence analysis of inlA of 114 food isolates resulted in the detection of 29 isolates with a premature stop codon (PMSC) mutation and 6 isolates with 3-codon deletion mutations (aa 738 to 740) in inlA. The isolates with inlA PMSCs demonstrated a significantly lower level of invasion than the other food isolates in a Caco-2 cell invasion assay (P<0.01), but the isolates with the 3-codon deletion exhibited invasion comparable to the isolates with non-truncated InlA (P>0.05). According to analysis of the positive regulatory factor A (PrfA) sequences of 114 L. monocytogenes isolates, 7 isolates of serotype 1/2a from chicken samples contained a PrfA protein with a 5-nucleotide deletion from 712 to 716, including a stop codon. Although the isolates with a 5-nucleotide deletion in prfA demonstrated invasion comparable to the isolates with non-truncated InlA and PrfA after growth at 30 °C (P>0.05), they exhibited a significantly higher level of invasion than the other isolates after growth at 20 °C (P<0.01). To the authors' knowledge, this is the first report of L. monocytogenes isolates with the stop-codon deletion of PrfA.

Host actin polymerization tunes the cell division cycle of an intracellular pathogen

Growth and division are two of the most fundamental capabilities of a bacterial cell. While they are well described for model organisms growing in broth culture, very little is known about the cell division cycle of bacteria replicating in more complex environments. Using a D-alanine reporter strategy, we found that intracellular Listeria monocytogenes (Lm) spend a smaller proportion of their cell cycle dividing compared to Lm growing in broth culture. This alteration to the cell division cycle is independent of bacterial doubling time. Instead, polymerization of host-derived actin at the bacterial cell surface extends the non-dividing elongation period and compresses the division period. By decreasing the relative proportion of dividing Lm, actin polymerization biases the population toward cells with the highest propensity to form actin tails. Thus, there is a positive-feedback loop between the Lm cell division cycle and a physical interaction with the host cytoskeleton.

Identification of a peptide-pheromone that enhances Listeria monocytogenes escape from host cell vacuoles

Listeria monocytogenes is a Gram-positive facultative intracellular bacterial pathogen that invades mammalian cells and escapes from membrane-bound vacuoles to replicate within the host cell cytosol. Gene products required for intracellular bacterial growth and bacterial spread to adjacent cells are regulated by a transcriptional activator known as PrfA. PrfA becomes activated following L. monocytogenes entry into host cells, however the signal that stimulates PrfA activation has not yet been defined. Here we provide evidence for L. monocytogenes secretion of a small peptide pheromone, pPplA, which enhances the escape of L. monocytogenes from host cell vacuoles and may facilitate PrfA activation. The pPplA pheromone is generated via the proteolytic processing of the PplA lipoprotein secretion signal peptide. While the PplA lipoprotein is dispensable for pathogenesis, bacteria lacking the pPplA pheromone are significantly attenuated for virulence in mice and have a reduced efficiency of bacterial escape from the vacuoles of nonprofessional phagocytic cells. Mutational activation of PrfA restores virulence and eliminates the need for pPplA-dependent signaling. Experimental evidence suggests that the pPplA peptide may help signal to L. monocytogenes its presence within the confines of the host cell vacuole, stimulating the expression of gene products that contribute to vacuole escape and facilitating PrfA activation to promote bacterial growth within the cytosol.

Glutathione activates virulence gene expression of an intracellular pathogen

Intracellular pathogens are responsible for much of the world-wide morbidity and mortality due to infectious diseases. To colonize their hosts successfully, pathogens must sense their environment and regulate virulence gene expression appropriately. Accordingly, on entry into mammalian cells, the facultative intracellular bacterial pathogen Listeria monocytogenes remodels its transcriptional program by activating the master virulence regulator PrfA. Here we show that bacterial and host-derived glutathione are required to activate PrfA. In this study a genetic selection led to the identification of a bacterial mutant in glutathione synthase that exhibited reduced virulence gene expression and was attenuated 150-fold in mice. Genome sequencing of suppressor mutants that arose spontaneously in vivo revealed a single nucleotide change in prfA that locks the protein in the active conformation (PrfA*) and completely bypassed the requirement for glutathione during infection. Biochemical and genetic studies support a model in which glutathione-dependent PrfA activation is mediated by allosteric binding of glutathione to PrfA. Whereas glutathione and other low-molecular-weight thiols have important roles in redox homeostasis in all forms of life, here we demonstrate that glutathione represents a critical signalling molecule that activates the virulence of an intracellular pathogen.

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.

Roles of a novel Crp/Fnr family transcription factor Lmo0753 in soil survival, biofilm production and surface attachment to fresh produce of Listeria monocytogenes

Listeria monocytogenes is a foodborne bacterial pathogen and the causative agent of an infectious disease, listeriosis. L. monocytogenes is ubiquitous in nature and has the ability to persist in food processing environments for extended periods of time by forming biofilms and resisting industrial sanitization. Human listeriosis outbreaks are commonly linked to contaminated dairy products, ready-to-eat meats, and in recent years, fresh produce such as lettuce and cantaloupes. We identified a putative Crp/Fnr family transcription factor Lmo0753 that is highly specific to human-associated genetic lineages of L. monocytogenes. Lmo0753 possesses two conserved functional domains similar to the major virulence regulator PrfA in L. monocytogenes. To determine if Lmo0753 is involved in environmental persistence-related mechanisms, we compared lmo0753 deletion mutants with respective wild type and complementation mutants of two fully sequenced L. monocytogenes genetic lineage II strains 10403S and EGDe for the relative ability of growth under different nutrient availability and temperatures, soil survival, biofilm productivity and attachment to select fresh produce surfaces including romaine lettuce leaves and cantaloupe rinds. Our results collectively suggested that Lmo0753 plays an important role in L. monocytogenes biofilm production and attachment to fresh produce, which may contribute to the environmental persistence and recent emergence of this pathogen in human listeriosis outbreaks linked to fresh produce.

PrfA-like transcription factor gene lmo0753 contributes to L-rhamnose utilization in Listeria monocytogenes strains associated with human food-borne infections

Listeria monocytogenes is a food-borne bacterial pathogen and the causative agent of human and animal listeriosis. Among the three major genetic lineages of L. monocytogenes (i.e., LI, LII, and LIII), LI and LII are predominantly associated with food-borne listeriosis outbreaks, whereas LIII is rarely implicated in human infections. In a previous study, we identified a Crp/Fnr family transcription factor gene, lmo0753, that was highly specific to outbreak-associated LI and LII but absent from LIII. Lmo0753 shares two conserved functional domains, including a DNA binding domain, with the well-characterized master virulence regulator PrfA in L. monocytogenes. In this study, we constructed lmo0753 deletion and complementation mutants in two fully sequenced L. monocytogenes LII strains, 10403S and EGDe, and compared the flagellar motility, phospholipase C production, hemolysis, and intracellular growth of the mutants and their respective wild types. Our results suggested that lmo0753 plays a role in hemolytic activity in both EGDe and 10403S. More interestingly, we found that deletion of lmo0753 led to the loss of l-rhamnose utilization in EGDe, but not in 10403S. RNA-seq analysis of EGDe Δ0753 incubated in phenol red medium containing l-rhamnose as the sole carbon source revealed that 126 (4.5%) and 546 (19.5%) out of 2,798 genes in the EGDe genome were up- and downregulated more than 2-fold, respectively, compared to the wild-type strain. Genes related to biotin biosynthesis, general stress response, and rhamnose metabolism were shown to be differentially regulated. Findings from this study collectively suggested varied functional roles of lmo0753 in different LII L. monocytogenes strain backgrounds associated with human listeriosis outbreaks.

Processing of recombinant Listeria monocytogenes proteins for MHC class I presentation follows a dedicated, high-efficiency pathway

CD8(+) T lymphocytes recognize short peptides of ∼8-10 aa bound to MHC class I molecules (pMHC) on the surface of APCs. These peptides can be generated from either endogenous proteins synthesized by the biosynthetic machinery of the presenting cell or from exogenously sourced proteins. Because much of the research characterizing the MHC class I processing pathway has focused on endogenously synthesized proteins, it is not known whether differences exist in the processing pathway followed by endogenously synthesized versus exogenously sourced proteins. To highlight potential differences in the processing of endogenous versus exogenous proteins, we developed a model system to measure the efficiency of pMHC generation from nearly identical recombinant proteins expressed from vaccinia virus and Listeria monocytogenes. In these experiments, we uncovered a striking difference in the way recombinant Listeria Ags are processed and presented when compared with endogenously synthesized viral proteins. Specifically, we find that pMHC production from secreted Listeria proteins occurs at the same rate, independent of the cellular half-life of the protein from which it is derived, whereas the rate of pMHC production from endogenously synthesized viral proteins is absolutely dependent on its protein half-life. Accordingly, our data demonstrate the existence of a distinct and highly efficient MHC class I presentation pathway used for the processing of at least some exogenously synthesized proteins.

Allosteric mutants show that PrfA activation is dispensable for vacuole escape but required for efficient spread and Listeria survival in vivo

The transcriptional regulator PrfA controls key virulence determinants of the facultative intracellular pathogen Listeria monocytogenes. PrfA-dependent gene expression is strongly induced within host cells. While the basis of this activation is unknown, the structural homology of PrfA with the cAMP receptor protein (Crp) and the finding of constitutively activated PrfA* mutants suggests it may involve ligand-induced allostery. Here, we report the identification of a solvent-accessible cavity within the PrfA N-terminal domain that may accommodate an activating ligand. The pocket occupies a similar position to the cAMP binding site in Crp but lacks the cyclic nucleotide-anchoring motif and has its entrance on the opposite side of the β-barrel. Site-directed mutations in this pocket impaired intracellular PrfA-dependent gene activation without causing extensive structural/functional alterations to PrfA. Two substitutions, L48F and Y63W, almost completely abolished intracellular virulence gene induction and thus displayed the expected phenotype for allosteric activation-deficient PrfA mutations. Neither PrfA(allo) substitution affected vacuole escape and initial intracellular growth of L. monocytogenes in epithelial cells and macrophages but caused defective cell-to-cell spread and strong attenuation in mice. Our data support the hypothesis that PrfA is allosterically activated during intracellular infection and identify the probable binding site for the effector ligand. They also indicate that PrfA allosteric activation is not required for early intracellular survival but is essential for full Listeria virulence and colonization of host tissues.

Posttranslocation chaperone PrsA2 regulates the maturation and secretion of Listeria monocytogenes proprotein virulence factors

PrsA2 is a conserved posttranslocation chaperone and a peptidyl prolyl cis-trans isomerase (PPIase) that contributes to the virulence of the Gram-positive intracellular pathogen Listeria monocytogenes. One of the phenotypes associated with a prsA2 mutant is decreased activity of the broad-range phospholipase C (PC-PLC). PC-PLC is made as a proenzyme whose maturation is mediated by a metalloprotease (Mpl). The proforms of PC-PLC and Mpl accumulate at the membrane-cell wall interface until a decrease in pH triggers their maturation and rapid secretion into the host cell. In this study, we examined the mechanism by which PrsA2 regulates the activity of PC-PLC. We observed that in the absence of PrsA2, the proenzymes are secreted at physiological pH and do not mature upon a decrease in pH. The sensitivity of the prsA2 mutant to cell wall hydrolases was modified. However, no apparent changes in cell wall porosity were detected. Interestingly, synthesis of PC-PLC in the absence of its propeptide lead to the secretion of a fully active enzyme in the cytosol of host cells independent of PrsA2, indicating that neither the propeptide of PC-PLC nor PrsA2 is required for native folding of the catalytic domain, although both influence secretion of the enzyme. Taken together, these results suggest that PrsA2 regulates compartmentalization of Mpl and PC-PLC, possibly by influencing cell wall properties and interacting with the PC-PLC propeptide. Moreover, the ability of these proproteins to respond to a decrease in pH during intracellular growth depends on their localization at the membrane-cell wall interface.

Listeria monocytogenes adapts to long-term stationary phase survival without compromising bacterial virulence

Bacteria withstand starvation during long-term stationary phase through the acquisition of mutations that increase bacterial fitness. The evolution of the growth advantage in stationary phase (GASP) phenotype results in the ability of bacteria from an aged culture to outcompete bacteria from a younger culture when the two are mixed together. The GASP phenotype was first described for Escherichia coli, but has not been examined for an environmental bacterial pathogen, which must balance long-term survival strategies that promote fitness in the outside environment with those that promote fitness within the host. Listeria monocytogenes is an environmental bacterium that lives as a saprophyte in soil, but is capable of replicating within the cytosol of mammalian cells. Herein, we demonstrate the ability of L. monocytogenes to express GASP via the acquisition of mutations during long-term stationary growth. Listeria monocytogenes GASP occurred through mechanisms that were both dependent and independent of the stress-responsive alternative sigma factor SigB. Constitutive activation of the central virulence transcriptional regulator PrfA interfered with the development of GASP; however, L. monocytogenes GASP cultures retained full virulence in mice. These results indicate that L. monocytogenes can accrue mutations that optimize fitness during long-term stationary growth without negatively impacting virulence.

Probing the role of protein surface charge in the activation of PrfA, the central regulator of Listeria monocytogenes pathogenesis

Listeria monocytogenes is a food-borne intracellular bacterial pathogen capable of causing serious human disease. L. monocytogenes survival within mammalian cells depends upon the synthesis of a number of secreted virulence factors whose expression is regulated by the transcriptional activator PrfA. PrfA becomes activated following bacterial entry into host cells where it induces the expression of gene products required for bacterial spread to adjacent cells. Activation of PrfA appears to occur via the binding of a small molecule cofactor whose identity remains unknown. Electrostatic modeling of the predicted PrfA cofactor binding pocket revealed a highly positively charged region with two lysine residues, K64 and K122, located at the edge of the pocket and another (K130) located deep within the interior. Mutational analysis of these residues indicated that K64 and K122 contribute to intracellular activation of PrfA, whereas a K130 substitution abolished protein activity. The requirement of K64 and K122 for intracellular PrfA activation could be bypassed via the introduction of the prfA G145S mutation that constitutively activates PrfA in the absence of cofactor binding. Our data indicate that the positive charge of the PrfA binding pocket contributes to intracellular activation of PrfA, presumably by facilitating binding of an anionic cofactor.

A novel C-terminal mutation resulting in constitutive activation of the Listeria monocytogenes central virulence regulatory factor PrfA

The environmental bacterium Listeria monocytogenes survives and replicates in a variety of diverse ecological niches that range from the soil to the cytosol of infected mammalian cells. The ability of L. monocytogenes to replicate within an infected host requires the expression of a number of secreted bacterial gene products whose expression is regulated by the transcriptional activator PrfA. PrfA becomes activated following bacterial entry into host cells; however, the mechanism by which this activation occurs remains unknown. Here we describe a novel C-terminal mutation that results in the high-level constitutive activation of PrfA and yet, in contrast with other described prfA* activation mutations, only modestly increases PrfA DNA binding affinity. L. monocytogenes strains containing the prfA P219S mutation exhibited high levels of PrfA-dependent virulence gene expression, were hyperinvasive in tissue culture models of infection, were fully motile and were hypervirulent in mice. In contrast with PrfA G145S and other mutationally activated PrfA proteins, the PrfA P219S protein readily formed homodimers and did not exhibit a dramatic increase in its DNA-binding affinity for target promoters. Interestingly, the prfA P219S mutation is located adjacent to the prfA K220 residue that has been previously reported to contribute to PrfA DNA binding activity. prfA P219S therefore appears to constitutively activate PrfA via a novel mechanism which minimally affects PrfA DNA binding in vitro.

Functional analysis of the Listeria monocytogenes secretion chaperone PrsA2 and its multiple contributions to bacterial virulence

As an organism that has evolved to live in environments ranging from soil to the cytosol of mammalian cells, Listeria monocytogenes must regulate the secretion and activity of protein products that promote survival within these habitats. The post-translocation chaperone PrsA2 has been adapted to assist in the folding and activity of L. monocytogenes secreted proteins required for bacterial replication within host cells. Here we present the first structure/function investigation of the contributions of PrsA2 to protein secretion and activity as well as to bacterial virulence. Domain swap experiments with the closely related L. monocytogenes PrsA1 protein combined with targeted mutagenesis indicate distinct functional roles for the PrsA2 peptidyl-prolyl isomerase (PPIase) and the N- and C-terminal domains in pathogenesis. In contrast to other PrsA-like proteins described thus far in the literature, an absolute in vivo requirement for PrsA2 PPIase activity is evident in mouse infection models. This work illustrates the diversity of function associated with L. monocytogenes PrsA2 that serves to promote bacterial life within the infected host.

Constitutive activation of PrfA tilts the balance of Listeria monocytogenes fitness towards life within the host versus environmental survival

PrfA is a key regulator of Listeria monocytogenes pathogenesis and induces the expression of multiple virulence factors within the infected host. PrfA is post-translationally regulated such that the protein becomes activated upon bacterial entry into the cell cytosol. The signal that triggers PrfA activation remains unknown, however mutations have been identified (prfA* mutations) that lock the protein into a high activity state. In this report we examine the consequences of constitutive PrfA activation on L. monocytogenes fitness both in vitro and in vivo. Whereas prfA* mutants were hyper-virulent during animal infection, the mutants were compromised for fitness in broth culture and under conditions of stress. Broth culture prfA*-associated fitness defects were alleviated when glycerol was provided as the principal carbon source; under these conditions prfA* mutants exhibited a competitive advantage over wild type strains. Glycerol and other three carbon sugars have been reported to serve as primary carbon sources for L. monocytogenes during cytosolic growth, thus prfA* mutants are metabolically-primed for replication within eukaryotic cells. These results indicate the critical need for environment-appropriate regulation of PrfA activity to enable L. monocytogenes to optimize bacterial fitness inside and outside of host cells.

Listeria monocytogenes PrsA2 is required for virulence factor secretion and bacterial viability within the host cell cytosol

In the course of establishing its replication niche within the cytosol of infected host cells, the facultative intracellular bacterial pathogen Listeria monocytogenes must efficiently regulate the secretion and activity of multiple virulence factors. L. monocytogenes encodes two predicted posttranslocation secretion chaperones, PrsA1 and PrsA2, and evidence suggests that PrsA2 has been specifically adapted for bacterial pathogenesis. PrsA-like chaperones have been identified in a number of Gram-positive bacteria, where they are reported to function at the bacterial membrane-cell wall interface to assist in the folding of proteins translocated across the membrane; in some cases, these proteins have been found to be essential for bacterial viability. In this study, the contributions of PrsA2 and PrsA1 to L. monocytogenes growth and protein secretion were investigated in vitro and in vivo. Neither PrsA2 nor PrsA1 was found to be essential for L. monocytogenes growth in broth culture; however, optimal bacterial viability was found to be dependent upon PrsA2 for L. monocytogenes located within the cytosol of host cells. Proteomic analyses of prsA2 mutant strains in the presence of a mutationally activated allele of the virulence regulator PrfA revealed a critical requirement for PrsA2 activity under conditions of PrfA activation, an event which normally takes place within the host cell cytosol. Despite a high degree of amino acid similarity, no detectable degree of functional overlap was observed between PrsA2 and PrsA1. Our results indicate a critical requirement for PrsA2 under conditions relevant to host cell infection.

The virulence regulator PrfA promotes biofilm formation by Listeria monocytogenes

Listeria monocytogenes is a food-borne facultative intracellular pathogen. It is widespread in the environment and has several distinct life-styles. The key transcriptional activator PrfA positively regulates L. monocytogenes virulence genes to mediate the transition from extracellular, flagellum-propelled cell to intracellular pathogen. Here we report the first evidence that PrfA also has a significant positive impact on extracellular biofilm formation. Mutants lacking prfA were defective in surface-adhered biofilm formation. The DeltaprfA mutant exhibited wild-type flagellar motility, and its biofilm defect occurred after initial surface adhesion. We also observed that mutations that led to the constitutive expression of PrfA-dependent virulence genes had a minimal impact on biofilm formation. Furthermore, biofilm development was enhanced in a mutant encoding a PrfA protein variant unable to fully transition from the extracellular form to the virulent, intracellular activity conformation. These results indicate that PrfA positively regulates biofilm formation and suggest that PrfA has a global role in modulating the life-style of L. monocytogenes. The requirement of PrfA for optimal biofilm formation may provide selective pressure to maintain this critical virulence regulator when L. monocytogenes is outside host cells in the environment.

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.

The posttranslocation chaperone PrsA2 contributes to multiple facets of Listeria monocytogenes pathogenesis

Listeria monocytogenes is an intracellular bacterial pathogen whose virulence depends on the regulated expression of numerous secreted bacterial factors. As for other gram-positive bacteria, many proteins secreted by L. monocytogenes are translocated across the bacterial membrane in an unfolded state to the compartment existing between the membrane and the cell wall. This compartment presents a challenging environment for protein folding due to its high density of negative charge, high concentrations of cations, and low pH. We recently identified PrsA2 as a gene product required for L. monocytogenes virulence. PrsA2 was identified based on its increased secretion by strains containing a mutationally activated form of prfA, the key regulator of L. monocytogenes virulence gene expression. The prsA2 gene product is one of at least two predicted peptidyl-prolyl cis/trans-isomerases encoded by L. monocytogenes; these proteins function as posttranslocation protein chaperones and/or foldases. In this study, we demonstrate that PrsA2 plays a unique and important role in L. monocytogenes pathogenesis by promoting the activity and stability of at least two critical secreted virulence factors: listeriolysin O (LLO) and a broad-specificity phospholipase. Loss of PrsA2 activity severely attenuated virulence in mice and impaired bacterial cell-to-cell spread in host cells. In contrast, mutants lacking prsA1 resembled wild-type bacteria with respect to intracellular growth and cell-to-cell spread as well as virulence in mice. PrsA2 is thus distinct from PrsA1 in its unique requirement for the stability and full activity of L. monocytogenes-secreted factors that contribute to host infection.