Interference of components of the phosphoenolpyruvate phosphotransferase system with the central virulence gene regulator PrfA of Listeria monocytogenes

The role of the universal sugar transport system components PtsI (EI) and PtsH (HPr) in Enterococcus faecium

Vancomycin-resistant enterococci (VRE) pose a serious threat to public health because of their limited treatment options. Therefore, there is an increasing need to identify novel targets to develop new drugs. Here, we examined the roles of the universal PTS components, PtsI and PtsH, in Enterococcus faecium to determine their roles in carbon metabolism, biofilm formation, stress response, and the ability to compete in the gastrointestinal tract. Clean deletion of ptsHI resulted in a significant reduction in the ability to import and metabolize simple sugars, attenuated growth rate, reduced biofilm formation, and decreased competitive fitness both in vitro and in vivo. However, no significant difference in stress survival was observed when compared with the wild type. These results suggest that targeting universal or specific PTS may provide a novel treatment strategy by reducing the fitness of E. faecium.

DegU-mediated suppression of carbohydrate uptake in Listeria monocytogenes increases adaptation to oxidative stress

The foodborne bacterial pathogen Listeria monocytogenes exhibits remarkable survival capabilities under challenging conditions, severely threatening food safety and human health. The orphan regulator DegU is a pleiotropic regulator required for bacterial environmental adaptation. However, the specific mechanism of how DegU participates in oxidative stress tolerance remains unknown in L. monocytogenes. In this study, we demonstrate that DegU suppresses carbohydrate uptake under stress conditions by altering global transcriptional profiles, particularly by modulating the transcription of the phosphoenolpyruvate-carbohydrate phosphotransferase system (PTS)-related genes, such as ptsH, ptsI, and hprK. Specifically, in the absence of degU, the transcripts of ptsI are significantly upregulated and those of hprK are significantly downregulated in response to copper ion-induced stress. Overexpression of ptsI significantly increases bacterial growth in vitro, while overexpression of hprK leads to a decrease in growth. We further demonstrate that DegU directly senses oxidative stress, downregulates ptsI transcription, and upregulates hprK transcription. Additionally, through an electrophoretic mobility shift assay, we demonstrate that DegU directly regulates the transcription of ptsI and hprK by binding to specific regions within their respective promoter sequences. Notably, the putative pivotal DegU binding sequence for ptsI is located from 38 to 68 base pairs upstream of the ptsH transcription start site (TSS), whereas for hprK, it is mapped from 36 to 124 base pairs upstream of the hprK TSS. In summary, we elucidate that DegU plays a significant role in suppressing carbohydrate uptake in response to oxidative stress through the direct regulation of ptsI and hprK.ImportanceUnderstanding the adaptive mechanisms employed by Listeria monocytogenes in harsh environments is of great significance. This study focuses on investigating the role of DegU in response to oxidative stress by examining global transcriptional profiles. The results highlight the noteworthy involvement of DegU in this stress response. Specifically, DegU acts as a direct sensor of oxidative stress, leading to the modulation of gene transcription. It downregulates ptsI transcription while it upregulates hprK transcription through direct binding to their promoters. Consequently, these regulatory actions impede bacterial growth, providing a defense mechanism against stress-induced damage. These findings gained from this study may have broader implications, serving as a reference for studying adaptive mechanisms in other pathogenic bacteria and aiding in the development of targeted strategies to control L. monocytogenes and ensure food safety.

Trehalose transport occurs via TreB in Listeria monocytogenes and it influences biofilm development and acid resistance

Listeria monocytogenes is a pathogenic bacterium that can inhabit a diverse range of environmental niches. This is largely attributed to the high proportion of carbohydrate-specific phosphotransferase system (PTS) genes in its genome. Carbohydrates can be assimilated as sources of energy but additionally they can serve as niche-specific cues for L. monocytogenes to shape its global gene expression, in order to cope with anticipated stresses. To examine carbon source utilization among wild L. monocytogenes isolates and to understand underlying molecular mechanisms, a diverse collection of L. monocytogenes strains (n = 168) with whole genome sequence (WGS) data available was screened for the ability to grow in chemically defined media with different carbon sources. The majority of the strains grew in glucose, mannose, fructose, cellobiose, glycerol, trehalose, and sucrose. Maltose, lactose, and rhamnose supported slower growth while ribose did not support any growth. In contrast to other strains, strain1386, which belonged to clonal complex 5 (CC5), was unable to grow on trehalose as a sole carbon source. WGS data revealed that it carried a substitution (N352K) in a putative PTS EIIBC trehalose transporter, TreB, while this asparagine residue is conserved in other strains in this collection. Spontaneous mutants of strain 1386 that could grow in trehalose were found to harbour a reversion of the substitution in TreB. These results provide genetic evidence that TreB is responsible for trehalose uptake and that the N352 residue is essential for TreB activity. Moreover, reversion mutants also restored other unusual phenotypes that strain 1386 displayed, i.e. altered colony morphology, impaired biofilm development, and reduced acid resistance. Transcriptional analysis at stationary phase with buffered BHI media revealed that trehalose metabolism positively influences the transcription of genes encoding amino acid-based acid resistance mechanisms. In summary, our results demonstrated that N352 is key to the function of the sole trehalose transporter TreB in L. monocytogenes and suggest that trehalose metabolism alters physiology to favour biofilm development and acid stress resistance. Moreover, since strain 1386 is among the strains recommended by the European Union Reference Laboratory for conducting food challenge studies in order to determine whether or not L. monocytogenes can grow in food, these findings have important implications for food safety.

Identification of genetic elements required for Listeria monocytogenes growth under limited nutrient conditions and virulence by a screening of transposon insertion library

Listeria monocytogenes, the causative agent of listeriosis, displays a lifestyle ranging from saprophytes in the soil to pathogenic as a facultative intracellular parasite in host cells. In the current study, a random transposon (Tn) insertion library was constructed in L. monocytogenes strain F2365 and screened to identify genes and pathways affecting in vitro growth and fitness in minimal medium (MM) containing different single carbohydrate as the sole carbon source. About 2,000 Tn-mutants were screened for impaired growth in MM with one of the following carbon sources: glucose, fructose, mannose, mannitol, sucrose, glycerol, and glucose 6-phosphate (G6P). Impaired or abolished growth of L. monocytogenes was observed for twenty-one Tn-mutants with disruptions in genes encoding purine biosynthesis enzymes (purL, purC, purA, and purM), pyrimidine biosynthesis proteins (pyrE and pyrC), ATP synthase (atpI and atpD2), branched-chain fatty acids (BCFA) synthesis enzyme (bkdA1), a putative lipoprotein (LMOF2365_2387 described as LP2387), dUTPase family protein (dUTPase), and two hypothetical proteins. All Tn-mutants, except the atpD2 mutant, grew as efficiently as wild-type strain in a nutrient rich media. The virulence of twenty-one Tn-mutants was assessed in mice at 72 h following intravenous (IV) infection. The most attenuated mutants had Tn insertions in purA, hypothetical protein (LMOf2365_0064 described as HP64), bkdA1, dUTPase, LP2387, and atpD2, confirming the important role of these genes in pathogenesis. Six Tn-mutants were then tested for ability to replicate intracellularly in murine macrophage J774.1 cells. Significant intracellular growth defects were observed in two Tn-mutants with insertions in purA and HP64 genes, suggesting that an intact purine biosynthesis pathway is important for intracellular growth of L. monocytogens. These findings may not be fully generalized to all of L. monocytogenes strains due to their genetic diversity. In conclusion, Tn-mutagenesis identified that biosynthesis of purines, pyrimidines, ATP, and BCFA are important for L. monocytogens pathogenesis. Purine and pyrimidine auxotrophs play an important role in the pathogenicity in other bacterial pathogens, but our study also revealed new proteins essential for both growth in MM and L. monocytogenes strain F2365 virulence.

Listeria monocytogenes genes supporting growth under standard laboratory cultivation conditions and during macrophage infection

The Gram-positive bacterium Listeria monocytogenes occurs widespread in the environment and infects humans when ingested along with contaminated food. Such infections are particularly dangerous for risk group patients, for whom they represent a life-threatening disease. To invent novel strategies to control contamination and disease, it is important to identify those cellular processes that maintain pathogen growth inside and outside the host. Here, we have applied transposon insertion sequencing (Tn-Seq) to L. monocytogenes for the identification of such processes on a genome-wide scale. Our approach identified 394 open reading frames that are required for growth under standard laboratory conditions and 42 further genes, which become necessary during intracellular growth in macrophages. Most of these genes encode components of the translation machinery and act in chromosome-related processes, cell division, and biosynthesis of the cellular envelope. Several cofactor biosynthesis pathways and 29 genes with unknown functions are also required for growth, suggesting novel options for the development of antilisterial drugs. Among the genes specifically required during intracellular growth are known virulence factors, genes compensating intracellular auxotrophies, and several cell division genes. Our experiments also highlight the importance of PASTA kinase signaling for general viability and of glycine metabolism and chromosome segregation for efficient intracellular growth of L. monocytogenes.

The Global Regulator CcpA of Listeria monocytogenes Confers Sensitivity to Antimicrobial Fatty Acids

Free fatty acids (FFAs) are known to exhibit antimicrobial and anti-virulent properties against bacterial pathogens. Specific FFAs, such as lauric acid (LA; C12:0), exert both effects against the foodborne pathogen Listeria monocytogenes: at low levels, LA acts to inhibit the activity of the virulence regulator PrfA, whereas at higher levels, LA inhibits bacterial growth. Deletion of prfA is known to promote tolerance toward antimicrobial FFAs, suggesting that the response of L. monocytogenes to anti-virulent and antimicrobial FFAs could be linked. In this study, we explored the response of L. monocytogenes toward antimicrobial FFAs holding an anti-virulence activity by isolating strains that can grow at high concentrations of LA. We found that LA-tolerant isolates carry mutations in the gene encoding the global regulator CcpA. Importantly, we discovered that mutation or deletion of ccpA protect L. monocytogenes against the antimicrobial activity of FFAs, whereas the ccpA mutants remain sensitive toward FFA’s PrfA inhibitory effect. A regulatory link involving CcpA, connecting the response toward the antimicrobial and anti-virulence activities of FFAs, is therefore unlikely. To further study how deletion of ccpA promotes FFA tolerance, we performed a transcriptomic analysis of the response to LA. Our data indicated that the FFA-tolerant phenotype of the ∆ccpA strain is not induced upon LA exposure but appears to be an inherent phenotypic trait of the ccpA deletion mutation. Interestingly, we found that the bacterial surface of L. monocytogenes becomes more hydrophilic upon deletion of ccpA, and we demonstrate that CcpA plays a role in the response of L. monocytogenes to other stress conditions, including low pH and antibiotics. Altogether, our study revealed that regulatory activities of CcpA lead to an increased hydrophobicity of the bacterial surface, which may confer sensitivity of L. monocytogenes against the antimicrobial activity of FFAs. Notably, CcpA is not involved in responding to the PrfA inhibitory effect of FFAs, showing that FFA-tolerant strains can still be targeted by the anti-virulent activity of FFAs.

Genome-wide analysis of in vivo CcpA binding with and without its key co-factor HPr in the major human pathogen group A Streptococcus

Catabolite control protein A (CcpA) is a master regulator of carbon source utilization and contributes to the virulence of numerous medically important Gram‐positive bacteria. Most functional assessments of CcpA, including interaction with its key co‐factor HPr, have been performed in nonpathogenic bacteria. In this study we aimed to identify the in vivo DNA binding profile of CcpA and assess the extent to which HPr is required for CcpA‐mediated regulation and DNA binding in the major human pathogen group A Streptococcus (GAS). Using a combination RNAseq/ChIP‐seq approach, we found that CcpA affects transcript levels of 514 of 1667 GAS genes (31%) whereas direct DNA binding was identified for 105 GAS genes. Three of the directly regulated genes encode the key GAS virulence factors Streptolysin S, PrtS (IL‐8 degrading proteinase), and SpeB (cysteine protease). Mutating CcpA Val301 to Ala (strain 2221‐CcpA‐V301A) abolished interaction between CcpA and HPr and impacted the transcript levels of 205 genes (40%) in the total CcpA regulon. By ChIP‐seq analysis, CcpAV301A bound to DNA from 74% of genes bound by wild‐type CcpA, but generally with lower affinity. These data delineate the direct CcpA regulon and clarify the HPr‐dependent and independent activities of CcpA in a key pathogenic bacterium.

Chitin Attenuates Expression of Listeria monocytogenes Virulence Genes in vitro

External signals are crucial for bacteria to sense their immediate environment and fine-tune gene expression accordingly. The foodborne pathogen Listeria monocytogenes senses a range of environmental cues in order to activate or deactivate the virulence-inducing transcriptional factor PrfA during transition between infectious and saprophytic lifecycles. Chitin is an abundant biopolymer formed from linked β-(1-4)-N-acetyl-D-glucosamine residues associated with fungi, the exoskeleton of insects and often incorporated into foods as a thickener or stabilizer. L. monocytogenes evolved to hydrolyse chitin, presumably, to facilitate nutrient acquisition from competitive environments such as soil where the polymer is abundant. Since mammals do not produce chitin, we reasoned that the polymer could serve as an environmental signal contributing to repression of L. monocytogenes PrfA-dependent expression. This study shows a significant downregulation of the core PrfA-regulon during virulence-inducing conditions in vitro in the presence of chitin. Our data suggest this phenomenon occurs through a mechanism that differs from PTS-transport of oligosaccharides generated from either degradation or chitinase-mediated hydrolysis of the polymer. Importantly, an indication that chitin can repress virulence expression of a constitutively active PrfA^∗ mutant is shown, possibly mediated via a post-translational modification inhibiting PrfA^∗ activity. To our knowledge, this is the first time that chitin is reported as a molecule with anti-virulence properties against a pathogenic bacterium. Thus, our findings identify chitin as a signal which may downregulate the virulence potential of the pathogen and may provide an alternative approach toward reducing disease risk.

Contributions of a LysR Transcriptional Regulator to Listeria monocytogenes Virulence and Identification of Its Regulons

The capacity of Listeria monocytogenes to adapt to environmental changes is facilitated by a large number of regulatory proteins encoded by its genome. Among these proteins are the uncharacterized LysR-type transcriptional regulators (LTTRs). LTTRs can work as positive and/or negative transcription regulators at both local and global genetic levels. Previously, our group determined by comparative genome analysis that one member of the LTTRs (NCBI accession no. WP_003734782) was present in pathogenic strains but absent from nonpathogenic strains. The goal of the present study was to assess the importance of this transcription factor in the virulence of L. monocytogenes strain F2365 and to identify its regulons. An L. monocytogenes strain lacking lysR (the F2365ΔlysR strain) displayed significant reductions in cell invasion of and adhesion to Caco-2 cells. In plaque assays, the deletion of lysR resulted in a 42.86% decrease in plaque number and a 13.48% decrease in average plaque size. Furthermore, the deletion of lysR also attenuated the virulence of L. monocytogenes in mice following oral and intraperitoneal inoculation. The analysis of transcriptomics revealed that the transcript levels of 139 genes were upregulated, while 113 genes were downregulated in the F2365ΔlysR strain compared to levels in the wild-type bacteria. lysR-repressed genes included ABC transporters, important for starch and sucrose metabolism as well as glycerolipid metabolism, flagellar assembly, quorum sensing, and glycolysis/gluconeogenesis. Conversely, lysR activated the expression of genes related to fructose and mannose metabolism, cationic antimicrobial peptide (CAMP) resistance, and beta-lactam resistance. These data suggested that lysR contributed to L. monocytogenes virulence by broad impact on multiple pathways of gene expression.IMPORTANCE Listeria monocytogenes is the causative agent of listeriosis, an infectious and fatal disease of animals and humans. In this study, we have shown that lysR contributes to Listeria pathogenesis and replication in cell lines. We also highlight the importance of lysR in regulating the transcription of genes involved in different pathways that might be essential for the growth and persistence of L. monocytogenes in the host or under nutrient limitation. Better understanding L. monocytogenes pathogenesis and the role of various virulence factors is necessary for further development of prevention and control strategies.

Metabolism of the Gram-Positive Bacterial Pathogen Listeria monocytogenes

Bacterial metabolism represents the biochemical space that bacteria can manipulate to produce energy, reducing equivalents and building blocks for replication. Gram-positive pathogens, such as Listeria monocytogenes, show remarkable flexibility, which allows for exploitation of diverse biological niches from the soil to the intracytosolic space. Although the human host represents a potentially rich source for nutrient acquisition, competition for nutrients with the host and hostile host defenses can constrain bacterial metabolism by various mechanisms, including nutrient sequestration. Here, we review metabolism in the model Gram-positive bacterium, L. monocytogenes, and highlight pathways that enable the replication, survival, and virulence of this bacterial pathogen.

Controlled branched-chain amino acids auxotrophy in Listeria monocytogenes allows isoleucine to serve as a host signal and virulence effector

Listeria monocytogenes (Lm) is a saprophyte and intracellular pathogen. Transition to the pathogenic state relies on sensing of host-derived metabolites, yet it remains unclear how these are recognized and how they mediate virulence gene regulation. We previously found that low availability of isoleucine signals Lm to activate the virulent state. This response is dependent on CodY, a global regulator and isoleucine sensor. Isoleucine-bound CodY represses metabolic pathways including branched-chain amino acids (BCAA) biosynthesis, however under BCAA depletion, as occurs during infection, BCAA biosynthesis is upregulated and isoleucine-unbound CodY activates virulence genes. While isoleucine was revealed as an important input signal, it was not identified how internal levels are controlled during infection. Here we show that Lm regulates BCAA biosynthesis via CodY and via a riboregulator located upstream to the BCAA biosynthesis genes, named Rli60. rli60 is transcribed when BCAA levels drop, forming a ribosome-mediated attenuator that cis-regulates the downstream genes according to BCAA supply. Notably, we found that Rli60 restricts BCAA production, essentially starving Lm, a mechanism that is directly linked to virulence, as it controls the internal isoleucine pool and thereby CodY activity. This controlled BCAA auxotrophy likely evolved to enable isoleucine to serve as a host signal and virulence effector.

Bacterial pathogens must adapt to their host environment to carry out a successful infection. Sensing host-derived signals precedes adaptation, and triggers switching to the virulent state. Within mammalian cells L. monocytogenes responds to branched-chain amino acids (BCAA) deficiency by inducing virulence gene expression. In this study, we provide compelling evidence that fine tuning BCAA biosynthesis in L. monocytogenes allows the bacteria to sense isoleucine as a host-specific signal. Tightly controlled BCAA production depends on Rli60, a riboregulator, which is transcribed upstream to the BCAA biosynthesis genes. Rli60 functions as a ribosome mediated attenuator that cis-regulates BCAA production under limiting conditions. This study highlights the remarkable cross-regulation of metabolism and virulence in bacterial pathogens.

Metabolic adaptation of intracellular bacteria and fungi to macrophages

The mature phagosome of macrophages is a hostile environment for the vast majority of phagocytosed microbes. In addition to active destruction of the engulfed microbes by antimicrobial compounds, restriction of essential nutrients in the phagosomal compartment contributes to microbial growth inhibition and killing. However, some pathogenic microorganisms have not only developed various strategies to efficiently withstand or counteract antimicrobial activities, but also to acquire nutrients within macrophages for intracellular replication. Successful intracellular pathogens are able to utilize host-derived amino acids, carbohydrates and lipids as well as trace metals and vitamins during intracellular growth. This requires sophisticated strategies such as phagosome modification or escape, efficient nutrient transporters and metabolic adaptation. In this review, we discuss the metabolic adaptation of facultative intracellular bacteria and fungi to the intracellular lifestyle inside macrophages.

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.

Listeria monocytogenes cytosolic metabolism promotes replication, survival, and evasion of innate immunity

Listeria monocytogenes, the causative agent of listeriosis, is an intracellular pathogen that is exquisitely evolved to survive and replicate in the cytosol of eukaryotic cells. Eukaryotic cells typically restrict bacteria from colonising the cytosol, likely through a combination of cell autonomous defences, nutritional immunity, and innate immune responses including induction of programmed cell death. This suggests that L. monocytogenes and other professional cytosolic pathogens possess unique metabolic adaptations, not only to support replication but also to facilitate resistance to host-derived stresses/defences and avoidance of innate immune activation. In this review, we outline our current understanding of L. monocytogenes metabolism in the host cytosol and highlight major metabolic processes which promote intracellular replication and survival.

Edwardsiella piscicida lacking the cyclic AMP receptor protein (Crp) is avirulent and immunogenic in fish

Edwardsiella piscicida is a Gram-negative pathogen that generally causes lethal septicemia in marine and freshwater fish. We generated a E. piscicida CK216 Δcrp mutant to investigate various biological roles related to this organism, including pathogenesis. Lack of Crp in CK216 was demonstrated by immunoblotting using a Crp-specific antibody. Compared to the parental strain, the mutant exhibited changes in three biochemical phenotypes, including ornithine decarboxylation, citrate utilization, and H₂S production. Complementation of crp deletion in trans rescued the phenotype of the parental strain. This study proved that hemolytic activity in E. piscicida is controlled by Crp. In addition, significantly reduced motility of E. piscicida CK216 was observed, which resulted from a lack of flagella synthesis. To examine the virulence in fish, E. piscicida cells were injected into the goldfish (Carassius auratus) via intraperitoneal route. The LD₅₀ of CK216 was 9.25 × 10⁸ CFU, while that of the CK108 parental strain was 9.24 × 10⁵ CFU, attenuated 1000 fold in goldfish. Fish immunized with CK216 elicited IgM responses. Moreover, 80% of goldfish immunized with 1 × 10⁶ CFU survived after administration of a lethal dose (1 × 10⁷ CFU) of virulent E. piscicida CK41, suggesting the potential for E. piscicida CK216 to serve as a live attenuated vaccine in aquaculture.

Do Shoot the Messenger: PASTA Kinases as Virulence Determinants and Antibiotic Targets

All domains of life utilize protein phosphorylation as a mechanism of signal transduction. In bacteria, protein phosphorylation was classically thought to be mediated exclusively by histidine kinases as part of two-component signaling systems. However, it is now well appreciated that eukaryotic-like serine/threonine kinases (eSTKs) control essential processes in bacteria. A subset of eSTKs are single-pass transmembrane proteins that have extracellular penicillin-binding-protein and serine/threonine kinase-associated (PASTA) domains which bind muropeptides. In a variety of important pathogens, PASTA kinases have been implicated in regulating biofilms, antibiotic resistance, and ultimately virulence. Although there are limited examples of direct regulation of virulence factors, PASTA kinases are critical for virulence due to their roles in regulating bacterial physiology in the context of stress. This review focuses on the role of PASTA kinases in virulence for a variety of important Gram-positive pathogens and concludes with a discussion of current efforts to develop kinase inhibitors as novel antimicrobials.

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

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

Metabolic Genetic Screens Reveal Multidimensional Regulation of Virulence Gene Expression in Listeria monocytogenes and an Aminopeptidase That Is Critical for PrfA Protein Activation

Listeria monocytogenes is an environmental saprophyte and intracellular bacterial pathogen. Upon invading mammalian cells, the bacterium senses abrupt changes in its metabolic environment, which are rapidly transduced to regulation of virulence gene expression. To explore the relationship between L. monocytogenes metabolism and virulence, we monitored virulence gene expression dynamics across a library of genetic mutants grown under two metabolic conditions known to activate the virulent state: charcoal-treated rich medium containing glucose-1-phosphate and minimal defined medium containing limiting concentrations of branched-chain amino acids (BCAAs). We identified over 100 distinct mutants that exhibit aberrant virulence gene expression profiles, the majority of which mapped to nonessential metabolic genes. Mutants displayed enhanced, decreased, and early and late virulence gene expression profiles, as well as persistent levels, demonstrating a high plasticity in virulence gene regulation. Among the mutants, one was noteworthy for its particularly low virulence gene expression level and mapped to an X-prolyl aminopeptidase (PepP). We show that this peptidase plays a role in posttranslational activation of the major virulence regulator, PrfA. Specifically, PepP mediates recruitment of PrfA to the cytoplasmic membrane, a step identified as critical for PrfA protein activation. This study establishes a novel step in the complex mechanism of PrfA activation and further highlights the cross regulation of metabolism and virulence.

L-glutamine Induces Expression of Listeria monocytogenes Virulence Genes

The high environmental adaptability of bacteria is contingent upon their ability to sense changes in their surroundings. Bacterial pathogen entry into host poses an abrupt and dramatic environmental change, during which successful pathogens gauge multiple parameters that signal host localization. The facultative human pathogen Listeria monocytogenes flourishes in soil, water and food, and in ~50 different animals, and serves as a model for intracellular infection. L. monocytogenes identifies host entry by sensing both physical (e.g., temperature) and chemical (e.g., metabolite concentrations) factors. We report here that L-glutamine, an abundant nitrogen source in host serum and cells, serves as an environmental indicator and inducer of virulence gene expression. In contrast, ammonia, which is the most abundant nitrogen source in soil and water, fully supports growth, but fails to activate virulence gene transcription. We demonstrate that induction of virulence genes only occurs when the Listerial intracellular concentration of L-glutamine crosses a certain threshold, acting as an on/off switch: off when L-glutamine concentrations are below the threshold, and fully on when the threshold is crossed. To turn on the switch, L-glutamine must be present, and the L-glutamine high affinity ABC transporter, GlnPQ, must be active. Inactivation of GlnPQ led to complete arrest of L-glutamine uptake, reduced type I interferon response in infected macrophages, dramatic reduction in expression of virulence genes, and attenuated virulence in a mouse infection model. These results may explain observations made with other pathogens correlating nitrogen metabolism and virulence, and suggest that gauging of L-glutamine as a means of ascertaining host localization may be a general mechanism.

A Multi-Serotype Approach Clarifies the Catabolite Control Protein A Regulon in the Major Human Pathogen Group A Streptococcus

Catabolite control protein A (CcpA) is a highly conserved, master regulator of carbon source utilization in gram-positive bacteria, but the CcpA regulon remains ill-defined. In this study we aimed to clarify the CcpA regulon by determining the impact of CcpA-inactivation on the virulence and transcriptome of three distinct serotypes of the major human pathogen Group A Streptococcus (GAS). CcpA-inactivation significantly decreased GAS virulence in a broad array of animal challenge models consistent with the idea that CcpA is critical to gram-positive bacterial pathogenesis. Via comparative transcriptomics, we established that the GAS CcpA core regulon is enriched for highly conserved CcpA binding motifs (i.e. cre sites). Conversely, strain-specific differences in the CcpA transcriptome seems to consist primarily of affected secondary networks. Refinement of cre site composition via analysis of the core regulon facilitated development of a modified cre consensus that shows promise for improved prediction of CcpA targets in other medically relevant gram-positive pathogens.

Transport and Catabolism of Pentitols by Listeria monocytogenes

Transposon insertion into Listeria monocytogenes lmo2665, which encodes an EIIC of the phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS), was found to prevent D-arabitol utilization. We confirm this result with a deletion mutant and show that Lmo2665 is also required for D-xylitol utilization. We therefore called this protein EIICAxl. Both pentitols are probably catabolized via the pentose phosphate pathway (PPP) because lmo2665 belongs to an operon, which encodes the three PTSAxl components, two sugar-P dehydrogenases, and most PPP enzymes. The two dehydrogenases oxidize the pentitol-phosphates produced during PTS-catalyzed transport to the PPP intermediate xylulose-5-P. L. monocytogenes contains another PTS, which exhibits significant sequence identity to PTSAxl. Its genes are also part of an operon encoding PPP enzymes. Deletion of the EIIC-encoding gene (lmo0508) affected neither D-arabitol nor D-xylitol utilization, although D-arabitol induces the expression of this operon. Both operons are controlled by MtlR/LicR-type transcription activators (Lmo2668 and Lmo0501, respectively). Phosphorylation of Lmo0501 by the soluble PTSAxl components probably explains why D-arabitol also induces the second pentitol operon. Listerial virulence genes are submitted to strong repression by PTS sugars, such as glucose. However, D-arabitol inhibited virulence gene expression only at high concentrations, probably owing to its less efficient utilization compared to glucose.

Identification of Surface Protein Biomarkers of Listeria monocytogenes via Bioinformatics and Antibody-Based Protein Detection Tools

The Gram-positive bacterium Listeria monocytogenes causes a significant percentage of the fatalities among foodborne illnesses in humans. Surface proteins specifically expressed in a wide range of L. monocytogenes serotypes under selective enrichment culture conditions could serve as potential biomarkers for detection and isolation of this pathogen via antibody-based methods. Our study aimed to identify such biomarkers. Interrogation of the L. monocytogenes serotype 4b strain F2365 genome identified 130 putative or known surface proteins. The homologues of four surface proteins, LMOf2365_0578, LMOf2365_0581, LMOf2365_0639, and LMOf2365_2117, were assessed as biomarkers due to the presence of conserved regions among strains of L. monocytogenes which are variable among other Listeria species. Rabbit polyclonal antibodies against the four recombinant proteins revealed the expression of only LMOf2365_0639 on the surface of serotype 4b strain LI0521 cells despite PCR detection of mRNA transcripts for all four proteins in the organism. Three of 35 monoclonal antibodies (MAbs) to LMOf2365_0639, MAbs M3643, M3644, and M3651, specifically recognized 42 (91.3%) of 46 L. monocytogenes lineage I and II isolates grown in nonselective brain heart infusion medium. While M3644 and M3651 reacted with 14 to 15 (82.4 to 88.2%) of 17 L. monocytogenes lineage I and II isolates, M3643 reacted with 22 (91.7%) of 24 lineage I, II, and III isolates grown in selective enrichment media (UVM1, modified Fraser, Palcam, and UVM2 media). The three MAbs exhibited only weak reactivities (the optical densities at 414 nm were close to the cutoff value) to some other Listeria species grown in selective enrichment media. Collectively, the data indicate the potential of LMOf2365_0639 as a surface biomarker of L. monocytogenes, with the aid of specific MAbs, for pathogen detection, identification, and isolation in clinical, environmental, and food samples.

IMPORTANCE

L. monocytogenes is traditionally divided into at least 12 serotypes. Currently, there are no monoclonal antibodies (MAbs) available that are capable of binding to the surface of L. monocytogenes strains representing all 12 serotypes. Such antibodies would be useful and are needed for the development of methods to detect and isolate L. monocytogenes from food samples. In our study, we aimed to identify surface proteins that possess regions of well-conserved amino acid sequences among various serotypes and then to employ them as antigen targets (biomarkers) for the development of MAbs. Through bioinformatics and protein expression analysis, we identified one of the four putative surface protein candidates, LMOf2365_0639, encoded by the genome of the L. monocytogenes serotype 4b strain F2365, as a useful surface biomarker. Extensive assessment of 35 MAbs raised against LMOf2365_0639 in our study revealed three MAbs (M3643, M3644, and M3651) that recognized a wide range of L. monocytogenes isolates.

Regulating the Intersection of Metabolism and Pathogenesis in Gram-positive Bacteria

Pathogenic bacteria must contend with immune systems that actively restrict the availability of nutrients and cofactors, and create a hostile growth environment. To deal with these hostile environments, pathogenic bacteria have evolved or acquired virulence determinants that aid in the acquisition of nutrients. This connection between pathogenesis and nutrition may explain why regulators of metabolism in nonpathogenic bacteria are used by pathogenic bacteria to regulate both metabolism and virulence. Such coordinated regulation is presumably advantageous because it conserves carbon and energy by aligning synthesis of virulence determinants with the nutritional environment. In Gram-positive bacterial pathogens, at least three metabolite-responsive global regulators, CcpA, CodY, and Rex, have been shown to coordinate the expression of metabolism and virulence genes. In this chapter, we discuss how environmental challenges alter metabolism, the regulators that respond to this altered metabolism, and how these regulators influence the host-pathogen interaction.

The phosphocarrier protein HPr of Neisseria meningitidis interacts with the transcription regulator CrgA and its deletion affects capsule production, cell adhesion, and virulence

The bacterial phosphotransferase system (PTS) transports and phosphorylates sugars, but also carries out numerous regulatory functions. The β-proteobacterium Neisseria meningitidis possesses an incomplete PTS unable to transport carbon sources because it lacks a membrane component. Nevertheless, the residual phosphorylation cascade is functional and the meningococcal PTS was therefore expected to carry out regulatory roles. Interestingly, a ΔptsH mutant (lacks the PTS protein HPr) exhibited reduced virulence in mice and after intraperitoneal challenge it was rapidly cleared from the bloodstream of BALB/c mice. The rapid clearance correlates with lower capsular polysaccharide production by the ΔptsH mutant, which is probably also responsible for its increased adhesion to Hec-1-B epithelial cells. In addition, compared to the wild-type strain more apoptotic cells were detected when Hec-1-B cells were infected with the ΔptsH strain. Coimmunoprecipitation revealed an interaction of HPr and P-Ser-HPr with the LysR type transcription regulator CrgA, which among others controls its own expression. Moreover, ptsH deletion caused increased expression of a ΦcrgA-lacZ fusion. Finally, the presence of HPr or phospho-HPr's during electrophoretic mobility shift assays enhanced the affinity of CrgA for its target sites preceding crgA and pilE, but HPr did not promote CrgA binding to the sia and pilC1 promoter regions.

Systems Level Analyses Reveal Multiple Regulatory Activities of CodY Controlling Metabolism, Motility and Virulence in Listeria monocytogenes

Bacteria sense and respond to many environmental cues, rewiring their regulatory network to facilitate adaptation to new conditions/niches. Global transcription factors that co-regulate multiple pathways simultaneously are essential to this regulatory rewiring. CodY is one such global regulator, controlling expression of both metabolic and virulence genes in Gram-positive bacteria. Branch chained amino acids (BCAAs) serve as a ligand for CodY and modulate its activity. Classically, CodY was considered to function primarily as a repressor under rich growth conditions. However, our previous studies of the bacterial pathogen Listeria monocytogenes revealed that CodY is active also when the bacteria are starved for BCAAs. Under these conditions, CodY loses the ability to repress genes (e.g., metabolic genes) and functions as a direct activator of the master virulence regulator gene, prfA. This observation raised the possibility that CodY possesses multiple functions that allow it to coordinate gene expression across a wide spectrum of metabolic growth conditions, and thus better adapt bacteria to the mammalian niche. To gain a deeper understanding of CodY's regulatory repertoire and identify direct target genes, we performed a genome wide analysis of the CodY regulon and DNA binding under both rich and minimal growth conditions, using RNA-Seq and ChIP-Seq techniques. We demonstrate here that CodY is indeed active (i.e., binds DNA) under both conditions, serving as a repressor and activator of different genes. Further, we identified new genes and pathways that are directly regulated by CodY (e.g., sigB, arg, his, actA, glpF, gadG, gdhA, poxB, glnR and fla genes), integrating metabolism, stress responses, motility and virulence in L. monocytogenes. This study establishes CodY as a multifaceted factor regulating L. monocytogenes physiology in a highly versatile manner.

A Genome-Wide Screen Reveals that the Vibrio cholerae Phosphoenolpyruvate Phosphotransferase System Modulates Virulence Gene Expression

Diverse environmental stimuli and a complex network of regulatory factors are known to modulate expression of Vibrio cholerae's principal virulence factors. However, there is relatively little known about how metabolic factors impinge upon the pathogen's well-characterized cascade of transcription factors that induce expression of cholera toxin and the toxin-coregulated pilus (TCP). Here, we used a transposon insertion site (TIS) sequencing-based strategy to identify new factors required for expression of tcpA, which encodes the major subunit of TCP, the organism's chief intestinal colonization factor. Besides identifying most of the genes known to modulate tcpA expression, the screen yielded ptsI and ptsH, which encode the enzyme I (EI) and Hpr components of the V. cholerae phosphoenolpyruvate phosphotransferase system (PTS). In addition to reduced expression of TcpA, strains lacking EI, Hpr, or the associated EIIA(Glc) protein produced less cholera toxin (CT) and had a diminished capacity to colonize the infant mouse intestine. The PTS modulates virulence gene expression by regulating expression of tcpPH and aphAB, which themselves control expression of toxT, the central activator of virulence gene expression. One mechanism by which PTS promotes virulence gene expression appears to be by modulating the amounts of intracellular cyclic AMP (cAMP). Our findings reveal that the V. cholerae PTS is an additional modulator of the ToxT regulon and demonstrate the potency of loss-of-function TIS sequencing screens for defining regulatory networks.

The bacterial phosphoenolpyruvate:carbohydrate phosphotransferase system: regulation by protein phosphorylation and phosphorylation-dependent protein-protein interactions

The bacterial phosphoenolpyruvate (PEP):carbohydrate phosphotransferase system (PTS) carries out both catalytic and regulatory functions. It catalyzes the transport and phosphorylation of a variety of sugars and sugar derivatives but also carries out numerous regulatory functions related to carbon, nitrogen, and phosphate metabolism, to chemotaxis, to potassium transport, and to the virulence of certain pathogens. For these different regulatory processes, the signal is provided by the phosphorylation state of the PTS components, which varies according to the availability of PTS substrates and the metabolic state of the cell. PEP acts as phosphoryl donor for enzyme I (EI), which, together with HPr and one of several EIIA and EIIB pairs, forms a phosphorylation cascade which allows phosphorylation of the cognate carbohydrate bound to the membrane-spanning EIIC. HPr of firmicutes and numerous proteobacteria is also phosphorylated in an ATP-dependent reaction catalyzed by the bifunctional HPr kinase/phosphorylase. PTS-mediated regulatory mechanisms are based either on direct phosphorylation of the target protein or on phosphorylation-dependent interactions. For regulation by PTS-mediated phosphorylation, the target proteins either acquired a PTS domain by fusing it to their N or C termini or integrated a specific, conserved PTS regulation domain (PRD) or, alternatively, developed their own specific sites for PTS-mediated phosphorylation. Protein-protein interactions can occur with either phosphorylated or unphosphorylated PTS components and can either stimulate or inhibit the function of the target proteins. This large variety of signal transduction mechanisms allows the PTS to regulate numerous proteins and to form a vast regulatory network responding to the phosphorylation state of various PTS components.

Interaction with enzyme IIBMpo (EIIBMpo) and phosphorylation by phosphorylated EIIBMpo exert antagonistic effects on the transcriptional activator ManR of Listeria monocytogenes

Listeriae take up glucose and mannose predominantly through a mannose class phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS(Man)), whose three components are encoded by the manLMN genes. The expression of these genes is controlled by ManR, a LevR-type transcription activator containing two PTS regulation domains (PRDs) and two PTS-like domains (enzyme IIA(Man) [EIIA(Man)]- and EIIB(Gat)-like). We demonstrate here that in Listeria monocytogenes, ManR is activated via the phosphorylation of His585 in the EIIA(Man)-like domain by the general PTS components enzyme I and HPr. We also show that ManR is regulated by the PTS(Mpo) and that EIIB(Mpo) plays a dual role in ManR regulation. First, yeast two-hybrid experiments revealed that unphosphorylated EIIB(Mpo) interacts with the two C-terminal domains of ManR (EIIB(Gat)-like and PRD2) and that this interaction is required for ManR activity. Second, in the absence of glucose/mannose, phosphorylated EIIB(Mpo) (P∼EIIB(Mpo)) inhibits ManR activity by phosphorylating His871 in PRD2. The presence of glucose/mannose causes the dephosphorylation of P∼EIIB(Mpo) and P∼PRD2 of ManR, which together lead to the induction of the manLMN operon. Complementation of a ΔmanR mutant with various manR alleles confirmed the antagonistic effects of PTS-catalyzed phosphorylation at the two different histidine residues of ManR. Deletion of manR prevented not only the expression of the manLMN operon but also glucose-mediated repression of virulence gene expression; however, repression by other carbohydrates was unaffected. Interestingly, the expression of manLMN in Listeria innocua was reported to require not only ManR but also the Crp-like transcription activator Lin0142. Unlike Lin0142, the L. monocytogenes homologue, Lmo0095, is not required for manLMN expression; its absence rather stimulates man expression.

IMPORTANCE

Listeria monocytogenes is a human pathogen causing the foodborne disease listeriosis. The expression of most virulence genes is controlled by the transcription activator PrfA. Its activity is strongly repressed by carbohydrates, including glucose, which is transported into L. monocytogenes mainly via a mannose/glucose-specific phosphotransferase system (PTS(Man)). Expression of the man operon is regulated by the transcription activator ManR, the activity of which is controlled by a second, low-efficiency PTS of the mannose family, which functions as glucose sensor. Here we demonstrate that the EIIB(Mpo) component plays a dual role in ManR regulation: it inactivates ManR by phosphorylating its His871 residue and stimulates ManR by interacting with its two C-terminal domains.

The metabolic regulator CodY links Listeria monocytogenes metabolism to virulence by directly activating the virulence regulatory gene prfA

Metabolic adaptations are critical to the ability of bacterial pathogens to grow within host cells and are normally preceded by sensing of host-specific metabolic signals, which in turn can influence the pathogen's virulence state. Previously, we reported that the intracellular bacterial pathogen Listeria monocytogenes responds to low availability of branched-chain amino acids (BCAAs) within mammalian cells by up-regulating both BCAA biosynthesis and virulence genes. The induction of virulence genes required the BCAA-responsive transcription regulator, CodY, but the molecular mechanism governing this mode of regulation was unclear. In this report, we demonstrate that CodY directly binds the coding sequence of the L. monocytogenes master virulence activator gene, prfA, 15 nt downstream of its start codon, and that this binding results in up-regulation of prfA transcription specifically under low concentrations of BCAA. Mutating this site abolished CodY binding and reduced prfA transcription in macrophages, and attenuated bacterial virulence in mice. Notably, the mutated binding site did not alter prfA transcription or PrfA activity under other conditions that are known to activate PrfA, such as during growth in the presence of glucose-1-phosphate. This study highlights the tight crosstalk between L. monocytogenes metabolism and virulence, while revealing novel features of CodY-mediated regulation.

Phosphotransferase system-dependent extracellular growth of listeria monocytogenes is regulated by alternative sigma factors σL and σH

Alternative sigma (σ) factors and phosphotransferase systems (PTSs) play pivotal roles in the environmental adaptation and virulence of Listeria monocytogenes. The growth of the L. monocytogenes parent strain 10403S and 15 isogenic alternative σ factor mutants was assessed in defined minimal medium (DM) with PTS-dependent or non-PTS-dependent carbon sources at 25°C or 37°C. Overall, our results suggested that the regulatory effect of alternative σ factors on the growth of L. monocytogenes is dependent on the temperature and the carbon source. One-way analysis of variance (one-way ANOVA) showed that the factor "strain" had a significant effect on the maximum growth rate (μmax), lag phase duration (λ), and maximum optical density (ODmax) in PTS-dependent carbon sources (P < 0.05) but not in a non-PTS-dependent carbon source. Also, the ODmax was not affected by strain for any of the three PTS-dependent carbon sources at 25°C but was affected by strain at 37°C. Monitoring by quantitative real-time PCR (qRT-PCR) showed that transcript levels for lmo0027, a glucose-glucoside PTS permease (PTS(Glc)-1)-encoding gene, were higher in the absence of σ(L), and lower in the absence of σ(H), than in the parent strain. Our data thus indicate that σ(L) negatively regulates lmo0027 and that the increased μmax observed for the ΔsigL strain in DM with glucose may be associated with increased expression of PTS(Glc)-1 encoded by lmo0027. Our findings suggest that σ(H) and σ(L) mediate the PTS-dependent growth of L. monocytogenes through complex transcriptional regulations and fine-tuning of the expression of specific pts genes, including lmo0027. Our findings also reveal a more important and complex role of alternative σ factors in the regulation of growth in different sugar sources than previously assumed.

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.

Fuchs T, Held C, Ehrenreich A, Scherer S. Identification of genes essential for anaerobic growth of Listeria monocytogenes

The facultative anaerobic bacterium Listeria monocytogenes encounters microaerophilic or anaerobic conditions in various environments, e.g. in soil, in decaying plant material, in food products and in the host gut. To elucidate the adaptation of Listeria monocytogenes to variations in oxygen tension, global transcription analyses using DNA microarrays were performed. In total, 139 genes were found to be transcribed differently during aerobic and anaerobic growth; 111 genes were downregulated and 28 genes were upregulated anaerobically. The oxygen-dependent transcription of central metabolic genes is in agreement with results from earlier physiological studies. Of those genes more strongly expressed under lower oxygen tension, 20 were knocked out individually. Growth analysis of these knock out mutants did not indicate an essential function for the respective genes during anaerobiosis. However, even if not essential, transcriptional induction of several genes might optimize the bacterial fitness of Listeria monocytogenes in anaerobic niches, e.g. during colonization of the gut. For example, expression of the anaerobically upregulated gene lmo0355, encoding a fumarate reductase α chain, supported growth on 10 mM fumarate under anaerobic but not under aerobic growth conditions. Genes essential for anaerobic growth were identified by screening a mutant library. Eleven out of 1360 investigated mutants were sensitive to anaerobiosis. All 11 mutants were interrupted in the atp locus. These results were further confirmed by phenotypic analysis of respective in-frame deletion and complementation mutants, suggesting that the generation of a proton motive force via F1F0-ATPase is essential for anaerobic proliferation of Listeria monocytogenes.

An improved susceptibility test based on Amberlite reveals the potential antilisterial activity of fosfomycin in vitro

Listeria monocytogenes is resistant to fosfomycin in vitro but is susceptible in vivo due to increased expression of positive regulator factor A (PrfA) and its dependent factor, hexose phosphate transporter (Hpt), upon infection of host cells. Amberlite, a polymeric adsorbent resin, could induce PrfA-dependent gene expression and thus, in theory, improve the sensitivity of L. monocytogenes to fosfomycin in vitro. In the current study, an improved susceptibility test based on Amberlite was developed using reference strains. Thirty-five clinical isolates were further examined to verify those preliminary results. Briefly, Amberlite increased in vitro fosfomycin sensitivity of all strains. Optimal Amberlite concentrations, as evaluated through the expression of phospholipase B (PlcB) and Hpt, were 10% and 15% (w/v) in agar media and 3% (w/v) in broth media. Mueller–Hinton (MH) medium, tryptone soya (TS) medium, and brain heart infusion (BHI) medium were used to verify the results in the control strains using agar dilution and broth micro- and macro-dilution methods. Better listerial growth was shown in TS and BHI than in MH. Both broth dilution methods yielded lower minimal inhibitory concentration (MIC) of fosfomycin than the agar dilution method. The MIC of fosfomycin for 35 clinical isolates was 2–32 μg/mL, suggesting improved susceptibility. In conclusion, in vitro sensitivity of L. monocytogenes to fosfomycin was substantially improved in the presence of 3% Amberlite-supplemented TSB or BHIB and the broth microdilution method. This improved method revealed the potential antilisterial activity of fosfomycin in vitro and could facilitate the therapy of listeriosis using fosfomycin.

CcpA-dependent carbohydrate catabolite repression regulates galactose metabolism in Streptococcus oligofermentans

Streptococcus oligofermentans is an oral commensal that inhibits the growth of the caries pathogen Streptococcus mutans by producing copious amounts of H(2)O(2) and that grows faster than S. mutans on galactose. In this study, we identified a novel eight-gene galactose (gal) operon in S. oligofermentans that was comprised of lacABCD, lacX, and three genes encoding a galactose-specific transporter. Disruption of lacA caused more growth reduction on galactose than mutation of galK, a gene in the Leloir pathway, indicating that the principal role of this operon is in galactose metabolism. Diauxic growth was observed in cultures containing glucose and galactose, and a luciferase reporter fusion to the putative gal promoter demonstrated 12-fold repression of the operon expression by glucose but was induced by galactose, suggesting a carbon catabolite repression (CCR) control in galactose utilization. Interestingly, none of the single-gene mutations in the well-known CCR regulators ccpA and manL affected diauxic growth, although the operon expression was upregulated in these mutants in glucose. A double mutation of ccpA and manL eliminated glucose repression of galactose utilization, suggesting that these genes have parallel functions in regulating gal operon expression and mediating CCR. Electrophoretic mobility shift assays demonstrated binding of CcpA to the putative catabolite response element motif in the promoter regions of the gal operon and manL, suggesting that CcpA regulates CCR through direct regulation of the transcription of the gal operon and manL. This provides the first example of oral streptococci using two parallel CcpA-dependent CCR pathways in controlling carbohydrate metabolism.

N-acetylglucosamine-6-phosphate deacetylase (NagA) of Listeria monocytogenes EGD, an essential enzyme for the metabolism and recycling of amino sugars

The main aim of our study was to determine the physiological function of NagA enzyme in the Listeria monocytogenes cell. The primary structure of the murein of L. monocytogenes is very similar to that of Escherichia coli, the main differences being amidation of diaminopimelic acid and partial de-N-acetylation of glucosamine residues. NagA is needed for the deacetylation of N-acetyl-glucosamine-6 phosphate to glucosamine-6 phosphate and acetate. Analysis of the L. monocytogenes genome reveals the presence of two proteins with NagA domain, Lmo0956 and Lmo2108, which are cytoplasmic putative proteins. We introduced independent mutations into the structural genes for the two proteins. In-depth characterization of one of these mutants, MN1, deficient in protein Lmo0956 revealed strikingly altered cell morphology, strongly reduced cell wall murein content and decreased sensitivity to cell wall hydrolase, mutanolysin and peptide antibiotic, colistin. The gene products of operon 150, consisting of three genes: lmo0956, lmo0957, and lmo0958, are necessary for the cytosolic steps of the amino-sugar-recycling pathway. The cytoplasmic de-N-acetylase Lmo0956 of L. monocytogenes is required for cell wall peptidoglycan and teichoic acid biosynthesis and is also essential for bacterial cell growth, cell division, and sensitivity to cell wall hydrolases and peptide antibiotics.

Expression of Vibrio vulnificus insulin-degrading enzyme is regulated by the cAMP-CRP complex

Components of the bacterial phosphoenolpyruvate (PEP) : carbohydrate phosphortransferase system (PTS) have multiple regulatory roles in addition to PEP-dependent transport/phosphorylation of numerous carbohydrates. We have recently shown that, in an opportunistic human pathogen, Vibrio vulnificus, enzyme IIA(Glc) (EIIA(Glc)) interacts with a peptidase that has high sequence similarity to mammalian insulin-degrading enzymes, called Vibrio insulin-degrading enzyme (vIDE). Although the vIDE-EIIA(Glc) interaction is independent of the phosphorylation state of EIIA(Glc), vIDE shows no peptidase activity unless complexed with the unphosphorylated form of EIIA(Glc). A deletion mutant of ideV, the gene encoding vIDE, shows remarkably lower degrees of survival and virulence than the wild-type strain in mice, implying that vIDE is a virulence factor. In this study, we investigated regulation of ideV expression at the transcriptional level. Primer extension analysis identified two different transcriptional start sites of ideV: P(L) for the longer transcript and P(S) for the shorter transcript. We performed ligand fishing experiments by using the promoter region of ideV and found that the cAMP receptor protein (CRP) specifically binds to the promoter. DNase I footprinting experiments revealed that CRP binds to a region between the two promoters. In vitro transcription assays showed that CRP activates ideV P(S) transcription in the presence of cAMP whose concentration is regulated by EIIA(Glc). These results suggest that EIIA(Glc) regulates the expression level of vIDE as well as its activity.

Analysis of the serine/threonine/tyrosine phosphoproteome of the pathogenic bacterium Listeria monocytogenes reveals phosphorylated proteins related to virulence

Phosphorylation is the most common and widely studied post-translational protein modification in bacteria. It plays an important role in all kinds of cellular processes and controls key regulatory mechanisms, including virulence in certain pathogens. To gain insight into the role of protein phosphorylation in the pathogen Listeria monocytogenes, the serine (Ser), threonine (Thr) and tyrosine (Tyr) phosphoproteome of this bacterium was determined. We used the "gel free" proteomic approach with high accuracy mass spectrometry after enrichment of phosphopeptides. A total of 143 sites of phosphorylation were clearly identified, on 155 unique peptides of 112 phosphoproteins. The Ser/Thr/Tyr phosphorylation site distribution was 93:43:7. All identified phosphopeptides are monophosphorylated, except one and many identified phosphoproteins are related to virulence, translation, phosphoenolpyruvate:sugar phosphotransferase system, glycolysis and stress response. A description of these phosphoproteins is provided together with a comparison of the phosphosites in the L. monocytogenes proteins and in their homologues of other bacteria for which the phosphoproteome has been determined. Compared with the previous studies, we noticed a more extended conservation of the phosphorylation sites in glycolytic enzymes as well as ribosomal proteins.

Brucella adaptation and survival at the crossroad of metabolism and virulence

"In vivo" bacterial nutrition, i.e. the nature of the metabolic network and substrate(s) used by bacteria within their host, is a fundamental aspect of pathogenic or symbiotic lifestyles. A typical example are the Brucella spp., facultative intracellular pathogens responsible for chronic infections of animals and humans. Their virulence relies on their ability to modulate immune response and the physiology of host cells, but the fine-tuning of their metabolism in the host during infection appears increasingly crucial. Here we review new insights on the links between Brucella virulence and metabolism, pointing out the need to investigate both aspects to decipher Brucella infectious strategies.

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.

The transcriptional response of Listeria monocytogenes during adaptation to growth on lactate and diacetate includes synergistic changes that increase fermentative acetoin production

The organic acids lactate and diacetate are commonly used in combination in ready-to-eat foods because they show synergistic ability to inhibit the growth of Listeria monocytogenes. Full-genome microarrays were used to investigate the synergistic transcriptomic responses of two L. monocytogenes strains, H7858 (serotype 4b) and F6854 (serotype 1/2a), to these two organic acids under conditions representing osmotic and cold stress encountered in foods. Strains were exposed to brain heart infusion (BHI) broth at 7°C with 4.65% water-phase (w.p.) NaCl at pH 6.1 with (i) 2% w.p. potassium lactate, (ii) 0.14% w.p. sodium diacetate, (iii) the combination of both at the same levels, or (iv) no organic acids as a control. RNA was extracted 8 h after exposure, during lag phase, to capture gene transcription changes during adaptation to the organic acid stress. Significant differential transcription of 1,041 genes in H7858 and 640 genes in F6854 was observed in at least one pair of the 4 different treatments. The effects of combined treatment with lactate and diacetate included (i) synergistic transcription differences for 474 and 209 genes in H7858 and F6854, respectively, (ii) differential transcription of genes encoding cation transporters and ABC transporters of metals, and (iii) altered metabolism, including induction of a nutrient-limiting stress response, reduction of menaquinone biosynthesis, and a shift from fermentative production of acetate and lactate to energetically less favorable, neutral acetoin. These data suggest that additional treatments that interfere with cellular energy generation processes could more efficiently inhibit the growth of L. monocytogenes.

Mutational analysis of glucose transport regulation and glucose-mediated virulence gene repression in Listeria monocytogenes

Listeria monocytogenes transports glucose/mannose via non-PTS permeases and phosphoenolpyruvate:carbohydrate phosphotransferase systems (PTS). Two mannose class PTS are encoded by the constitutively expressed mpoABCD and the inducible manLMN operons. The man operon encodes the main glucose transporter because manL or manM deletion significantly slows glucose utilization, whereas mpoA deletion has no effect. The PTS(Mpo) mainly functions as a constitutively synthesized glucose sensor controlling man operon expression by phosphorylating and interacting with ManR, a LevR-like transcription activator. EIIB(Mpo) plays a dual role in ManR regulation: P~EIIB(Mpo) prevailing in the absence of glucose phosphorylates and thereby inhibits ManR activity, whereas unphosphorylated EIIB(Mpo) prevailing during glucose uptake is needed to render ManR active. In contrast to mpoA, deletion of mpoB therefore strongly inhibits man operon expression and glucose consumption. A ΔptsI (EI) mutant consumes glucose at an even slower rate probably via GlcU-like non-PTS transporters. Interestingly, deletion of ptsI, manL, manM or mpoB causes elevated PrfA-mediated virulence gene expression. The PTS(Man) is the major player in glucose-mediated PrfA inhibition because the ΔmpoA mutant showed normal PrfA activity. The four mutants showing PrfA derepression contain no or only little unphosphorylated EIIAB(Man) (ManL), which probably plays a central role in glucose-mediated PrfA regulation.