Regulation of the mpt operon in Listeria innocua by the ManR protein

Carvacrol Selective Pressure Allows the Occurrence of Genetic Resistant Variants of Listeria monocytogenes EGD-e

Essential oils and their constituents, such as carvacrol, are potential food preservatives because of their great antimicrobial properties. However, the long-term effects of these compounds are unknown and raise the question of whether resistance to these antimicrobials could emerge. This work aims to evaluate the occurrence of genetic resistant variants (RVs) in Listeria monocytogenes EGD-e by exposure to carvacrol. Two protocols were performed for the RVs selection: (a) by continuous exposure to sublethal doses, where LmSCar was isolated, and (b) by reiterative exposure to short lethal treatments of carvacrol, where LmLCar was isolated. Both RVs showed an increase in carvacrol resistance. Moreover, LmLCar revealed an increased cross-resistance to heat treatments at acid conditions and to ampicillin. Whole-genome sequencing identified two single nucleotide variations in LmSCar and three non-silent mutations in LmLCar. Among them, those located in the genes encoding the transcriptional regulators RsbT (in LmSCar) and ManR (in LmLCar) could contribute to their increased carvacrol resistance. These results provide information regarding the mode of action of this antimicrobial and support the importance of knowing how RVs appear. Further studies are required to determine the emergence of RVs in food matrices and their impact on food safety.

PRD-Containing Virulence Regulators (PCVRs) in Pathogenic Bacteria

Bacterial pathogens rely on a complex network of regulatory proteins to adapt to hostile and nutrient-limiting host environments. The phosphoenolpyruvate phosphotransferase system (PTS) is a conserved pathway in bacteria that couples transport of sugars with phosphorylation to monitor host carbohydrate availability. A family of structurally homologous PTS-regulatory-domain-containing virulence regulators (PCVRs) has been recognized in divergent bacterial pathogens, including Streptococcus pyogenes Mga and Bacillus anthracis AtxA. These paradigm PCVRs undergo phosphorylation, potentially via the PTS, which impacts their dimerization and their activity. Recent work with predicted PCVRs from Streptococcus pneumoniae (MgaSpn) and Enterococcus faecalis (MafR) suggest they interact with DNA like nucleoid-associating proteins. Yet, Mga binds to promoter sequences as a homo-dimeric transcription factor, suggesting a bi-modal interaction with DNA. High-resolution crystal structures of 3 PCVRs have validated the domain structure, but also raised additional questions such as how ubiquitous are PCVRs, is PTS-mediated histidine phosphorylation via potential PCVRs widespread, do specific sugars signal through PCVRs, and do PCVRs interact with DNA both as transcription factors and nucleoid-associating proteins? Here, we will review known and putative PCVRs based on key domain and functional characteristics and consider their roles as both transcription factors and possibly chromatin-structuring proteins.

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.

Studies of the Listeria monocytogenes Cellobiose Transport Components and Their Impact on Virulence Gene Repression

BACKGROUND

Many bacteria transport cellobiose via a phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS). In Listeria monocytogenes, two pairs of soluble PTS components (EIIACel1/EIIBCel1 and EIIACel2/EIIBCel2) and the permease EIICCel1 were suggested to contribute to cellobiose uptake. Interestingly, utilization of several carbohydrates, including cellobiose, strongly represses virulence gene expression by inhibiting PrfA, the virulence gene activator.

RESULTS

The LevR-like transcription regulator CelR activates expression of the cellobiose-induced PTS operons celB1-celC1-celA1, celB2-celA2, and the EIIC-encoding monocistronic celC2. Phosphorylation by P∼His-HPr at His550 activates CelR, whereas phosphorylation by P∼EIIBCel1 or P∼EIIBCel2 at His823 inhibits it. Replacement of His823 with Ala or deletion of both celA or celB genes caused constitutive CelR regulon expression. Mutants lacking EIICCel1, CelR or both EIIACel exhibitedslow cellobiose consumption. Deletion of celC1 or celR prevented virulence gene repression by the disaccharide, but not by glucose and fructose. Surprisingly, deletion of both celA genes caused virulence gene repression even during growth on non-repressing carbohydrates. No cellobiose-related phenotype was found for the celC2 mutant.

CONCLUSION

The two EIIA/BCel pairs are similarly efficient as phosphoryl donors in EIICCel1-catalyzed cellobiose transport and CelR regulation. The permanent virulence gene repression in the celA double mutant further supports a role of PTSCel components in PrfA regulation.

CodY-Mediated c-di-GMP-Dependent Inhibition of Mammalian Cell Invasion in Listeria monocytogenes

Elevated levels of the second messenger c-di-GMP suppress virulence in diverse pathogenic bacteria, yet mechanisms are poorly characterized. In the foodborne pathogen Listeria monocytogenes, high c-di-GMP levels inhibit mammalian cell invasion. Here, we show that invasion is impaired because of the decreased expression levels of internalin genes whose products are involved in invasion. We further show that at high c-di-GMP levels, the expression of the entire virulence regulon is suppressed, and so is the expression of the prfA gene encoding the master activator of the virulence regulon. Analysis of mechanisms controlling prfA expression pointed to the transcription factor CodY as a c-di-GMP-sensitive component. In high-c-di-GMP strains, codY gene expression is decreased, apparently due to the lower activity of CodY, which functions as an activator of codY transcription. We found that listerial CodY does not bind c-di-GMP in vitro and therefore investigated whether c-di-GMP levels affect two known cofactors of listerial CodY, branched-chain amino acids and GTP. Our manipulation of branched-chain amino acid levels did not perturb the c-di-GMP effect; however, our replacement of listerial CodY with the streptococcal CodY homolog, whose activity is GTP independent, abolished the c-di-GMP effect. The results of this study suggest that elevated c-di-GMP levels decrease the activity of the coordinator of metabolism and virulence, CodY, possibly via lower GTP levels, and that decreased CodY activity suppresses L. monocytogenes virulence by the decreased expression of the PrfA virulence regulon.IMPORTANCEListeria monocytogenes is a pathogen causing listeriosis, a disease responsible for the highest mortality rate among foodborne diseases. Understanding how the virulence of this pathogen is regulated is important for developing treatments to decrease the frequency of listerial infections in susceptible populations. In this study, we describe the mechanism through which elevated levels of the second messenger c-di-GMP inhibit listerial invasion in mammalian cells. Inhibition is caused by the decreased activity of the transcription factor CodY that coordinates metabolism and virulence.

Characterization of Class IIa Bacteriocin Resistance in Enterococcus faecium

Vancomycin-resistant enterococci, particularly resistant Enterococcus faecium, pose an escalating threat in nosocomial environments because of their innate resistance to many antibiotics, including vancomycin, a treatment of last resort. Many class IIa bacteriocins strongly target these enterococci and may offer a potential alternative for the management of this pathogen. However, E. faecium's resistance to these peptides remains relatively uncharacterized. Here, we explored the development of resistance of E. faecium to a cocktail of three class IIa bacteriocins: enterocin A, enterocin P, and hiracin JM79. We started by quantifying the frequency of resistance to these peptides in four clinical isolates of E. faecium We then investigated the levels of resistance of E. faecium 6E6 mutants as well as their fitness in different carbon sources. In order to elucidate the mechanism of resistance of E. faecium to class IIa bacteriocins, we completed whole-genome sequencing of resistant mutants and performed reverse transcription-quantitative PCR (qRT-PCR) of a suspected target mannose phosphotransferase (ManPTS). We then verified this ManPTS's role in bacteriocin susceptibility by showing that expression of the ManPTS in Lactococcus lactis results in susceptibility to the peptide cocktail. Based on the evidence found from these studies, we conclude that, in accord with other studies in E. faecalis and Listeria monocytogenes, resistance to class IIa bacteriocins in E. faecium 6E6 is likely caused by the disruption of a particular ManPTS, which we believe we have identified.

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.

Listeria monocytogenes exopolysaccharide: origin, structure, biosynthetic machinery and c-di-GMP-dependent regulation

Elevated levels of the second messenger c-di-GMP activate biosynthesis of an unknown exopolysaccharide (EPS) in the food-borne pathogen Listeria monocytogenes. This EPS strongly protects cells against disinfectants and desiccation, indicating its potential significance for listerial persistence in the environment and for food safety. We analyzed the potential phylogenetic origin of this EPS, determined its complete structure, characterized genes involved in its biosynthesis and hydrolysis and identified diguanylate cyclases activating its synthesis. Phylogenetic analysis of EPS biosynthesis proteins suggests that they have evolved within monoderms. Scanning electron microscopy revealed that L. monocytogenes EPS is cell surface-bound. Secreted carbohydrates represent exclusively cell-wall debris. Based on carbohydrate composition, linkage and NMR analysis, the structure of the purified EPS is identified as a β-1,4-linked N-acetylmannosamine chain decorated with terminal α-1,6-linked galactose. All genes of the pssA-E operon are required for EPS production and so is a separately located pssZ gene. We show that PssZ has an EPS-specific glycosylhydrolase activity. Exogenously added PssZ prevents EPS-mediated cell aggregation and disperses preformed aggregates, whereas an E72Q mutant in the presumed catalytic residue is much less active. The diguanylate cyclases DgcA and DgcB, whose genes are located next to pssZ, are primarily responsible for c-di-GMP-dependent EPS production.

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.

Transcription regulators controlled by interaction with enzyme IIB components of the phosphoenolpyruvate: sugar phosphotransferase system

Numerous bacteria possess transcription activators and antiterminators composed of regulatory domains phosphorylated by components of the phosphoenolpyruvate:sugar phosphotransferase system (PTS). These domains, called PTS regulation domains (PRDs), usually contain two conserved histidines as potential phosphorylation sites. While antiterminators possess two PRDs with four phosphorylation sites, transcription activators contain two PRDs plus two regulatory domains resembling PTS components (EIIA and EIIB). The activity of these transcription regulators is controlled by up to five phosphorylations catalyzed by PTS proteins. Phosphorylation by the general PTS components EI and HPr is usually essential for the activity of PRD-containing transcription regulators, whereas phosphorylation by the sugar-specific components EIIA or EIIB lowers their activity. For a specific regulator, for example the Bacillus subtilis mtl operon activator MtlR, the functional phosphorylation sites can be different in other bacteria and consequently the detailed mode of regulation varies. Some of these transcription regulators are also controlled by an interaction with a sugar-specific EIIB PTS component. The EIIBs are frequently fused to the membrane-spanning EIIC and EIIB-mediated membrane sequestration is sometimes crucial for the control of a transcription regulator. This is also true for the Escherichia coli repressor Mlc, which does not contain a PRD but nevertheless interacts with the EIIB domain of the glucose-specific PTS. In addition, some PRD-containing transcription activators interact with a distinct EIIB protein located in the cytoplasm. The phosphorylation state of the EIIB components, which changes in response to the presence or absence of the corresponding carbon source, affects their interaction with transcription regulators. This article is part of a Special Issue entitled: Inhibitors of Protein Kinases (2012).

Membrane sequestration by the EIIB domain of the mannitol permease MtlA activates the Bacillus subtilis mtl operon regulator MtlR

In most firmicutes expression of the mannitol operon is regulated by MtlR. This transcription activator is controlled via phosphorylation of its regulatory domains by components of the phosphoenolpyruvate : carbohydrate phosphotransferase system (PTS). We found that activation of Bacillus subtilis MtlR also requires an interaction with the EIIB(Mtl) domain of the mannitol permease MtlA (EIICB(Mtl) ). The constitutive expression of the mtlAFD operon in an mtlF mutant was prevented when entire mtlA or only its 3' part (EIIB(Mtl) ) were deleted. Yeast two-hybrid experiments revealed a direct interaction of the EIIB(Mtl) domain with the two C-terminal domains of MtlR. Complementation of the Δ3'-mtlA ΔmtlF or ΔmtlAFD mutants with mtlA restored constitutive MtlR activity, whereas complementation with only 3'-mtlA had no effect. Moreover, synthesis of EIIB(Mtl) in strains producing constitutively active MtlR caused MtlR inactivation. Interestingly, EIIB(Mtl) fused to the trans-membrane protein YwqC restored constitutive MtlR activity in the above mutants. Replacing the phosphorylatable Cys with Asp in MtlA or soluble EIIB(Mtl) lowered MtlR activation, indicating that MtlR does not interact with phosphorylatyed EIIB(Mtl) . Induction of the B. subtilis mtl operon therefore follows a novel regulation mechanism where the transcription activator needs to be sequestered to the membrane by unphosphorylated EIICB(Mtl) in order to be functional.

Target recognition, resistance, immunity and genome mining of class II bacteriocins from Gram-positive bacteria

Due to their very potent antimicrobial activity against diverse food-spoiling bacteria and pathogens and their favourable biochemical properties, peptide bacteriocins from Gram-positive bacteria have long been considered promising for applications in food preservation or medical treatment. To take advantage of bacteriocins in different applications, it is crucial to have detailed knowledge on the molecular mechanisms by which these peptides recognize and kill target cells, how producer cells protect themselves from their own bacteriocin (self-immunity) and how target cells may develop resistance. In this review we discuss some important recent progress in these areas for the non-lantibiotic (class II) bacteriocins. We also discuss some examples of how the current wealth of genome sequences provides an invaluable source in the search for novel class II bacteriocins.

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.

Nisin and class IIa bacteriocin resistance among Listeria and other foodborne pathogens and spoilage bacteria

Food safety has been an important issue globally due to increasing foodborne diseases and change in food habits. To inactivate foodborne pathogens, various novel technologies such as biopreservation systems have been studied. Bacteriocins are ribosomally synthesized peptides or proteins with antimicrobial activity produced by different groups of bacteria, but the bacteriocins produced by many lactic acid bacteria offer potential applications in food preservation. The use of bacteriocins in the food industry can help reduce the addition of chemical preservatives as well as the intensity of heat treatments, resulting in foods that are more naturally preserved. However, the development of highly tolerant and/or resistant strains may decrease the efficiency of bacteriocins as biopreservatives. Several mechanisms of bacteriocin resistance development have been proposed among various foodborne pathogens. The acquiring of resistance to bacteriocins can significantly affect physiological activity profile of bacteria, alter cell-envelope lipid composition, and also modify the antibiotic susceptibility/resistance profile of bacteria. This article presents a brief review on the scientific research about the various possible mechanisms involved in the development of resistance to nisin and Class IIa bacteriocins among the foodborne pathogens.

Mechanisms of resistance to bacteriocins targeting the mannose phosphotransferase system

The membrane proteins IIC and IID of the mannose phosphotransferase system (Man-PTS) together form a membrane-located complex that serves as a receptor for several different bacteriocins, including the pediocin-like class IIa bacteriocins and the class IIc bacteriocin lactococcin A. Bacterial strains sensitive to class IIa bacteriocins readily give rise to resistant mutants upon bacteriocin exposure. In the present study, we have therefore investigated lactococcin A-resistant mutants of Lactococcus lactis as well as natural food isolates of Listeria monocytogenes with different susceptibilities to class IIa bacteriocins. We found two major mechanisms of resistance. The first involves downregulation of Man-PTS gene expression, which takes place both in spontaneous resistant mutants and in natural resistant isolates. The second involves normal expression of the Man-PTS system, but the underlying mechanism of resistance for these cells is unknown. In some cases, the resistant phenotype was linked to a shift in the metabolism; i.e., reduced growth on glucose due to reduction in Man-PTS expression was accompanied by enhanced growth on another sugar, such as galactose. The implications of these findings in terms of metabolic heterogeneity are discussed.

Class IIa bacteriocin resistance in Enterococcus faecalis V583: the mannose PTS operon mediates global transcriptional responses

BACKGROUND

The class IIa bacteriocin, pediocin PA-1, has clear potential as food preservative and in the medical field to be used against Gram negative pathogen species as Enterococcus faecalis and Listeria monocytogenes. Resistance towards class IIa bacteriocins appear in laboratory and characterization of these phenotypes is important for their application. To gain insight into bacteriocin resistance we studied mutants of E. faecalis V583 resistant to pediocin PA-1 by use of transcriptomic analyses.

RESULTS

Mutants of E. faecalis V583 resistant to pediocin PA-1 were isolated, and their gene expression profiles were analyzed and compared to the wild type using whole-genome microarray. Significantly altered transcription was detected from about 200 genes; most of them encoding proteins involved in energy metabolism and transport. Glycolytic genes were down-regulated in the mutants, but most of the genes showing differential expression were up-regulated. The data indicate that the mutants were relieved from glucose repression and putative catabolic responsive elements (cre) could be identified in the upstream regions of 70% of the differentially expressed genes. Bacteriocin resistance was caused by reduced expression of the mpt operon encoding the mannose-specific phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS), and the same transcriptional changes were seen in a mptD-inactivated mutant. This mutant also had decreased transcription of the whole mpt operon, showing that the PTS is involved in its own transcriptional regulation.

CONCLUSION

Our data confirm the important role of mannose PTS in class IIa bacteriocin sensitivity and we demonstrate its importance involving global carbon catabolite control.

The major PEP-phosphotransferase systems (PTSs) for glucose, mannose and cellobiose of Listeria monocytogenes, and their significance for extra- and intracellular growth

In this report we examine the PEP-dependent phosphotransferase systems (PTSs) of Listeria monocytogenes EGD-e, especially those involved in glucose and cellobiose transport. This L. monocytogenes strain possesses in total 86 pts genes, encoding 29 complete PTSs for the transport of carbohydrates and sugar alcohols, and several single PTS components, possibly supporting transport of these compounds. By a systematic deletion analysis we identified the major PTSs involved in glucose, mannose and cellobiose transport, when L. monocytogenes grows in a defined minimal medium in the presence of these carbohydrates. Whereas all four PTS permeases belonging to the PTS(Man) family may be involved in mannose transport, only two of these (PTS(Man)-2 and PTS(Man)-3), and in addition at least one (PTS(Glc)-1) of the five PTS permeases belonging to the PTS(Glc) family, are able to transport glucose, albeit with different efficiencies. Cellobiose is transported mainly by one (PTS(Lac)-4) of the six members belonging to the PTS(Lac) family. In addition, PTS(Glc)-1 appears to be also able to transport cellobiose. The transcription of the operons encoding PTS(Man)-2 and PTS(Lac)-4 (but not that of the operon for PTS(Man)-3) is regulated by LevR-homologous PTS regulation domain (PRD) activators. Whereas the growth rate of the mutant lacking PTS(Man)-2, PTS(Man)-3 and PTS(Glc)-1 is drastically reduced (compared with the wild-type strain) in the presence of glucose, and that of the mutant lacking PTS(Lac)-4 and PTS(Glc)-1 in the presence of cellobiose, replication of both mutants within epithelial cells or macrophages is as efficient as that of the wild-type strain.

Regulation of mannose phosphotransferase system permease and virulence gene expression in Listeria monocytogenes by the EII(t)Man transporter

The EII(t)(Man) phosphotransferase system (PTS) permease encoded by the mpt operon is the principal glucose transporter in Listeria monocytogenes. EII(t)(Man) participates in glucose-mediated carbon catabolite repression (CCR) and downregulation of virulence gene expression, and it is the receptor for class IIa bacteriocins. The regulation of this important protein and its roles in gene control were examined using derivatives of strain EGD-e in which the mpt operon or its regulatory genes, manR and lmo0095, were deleted. Real-time reverse transcription-PCR analysis showed that the mpt mRNA level was 10- and 100-fold lower in the lmo0095 and manR deletion strains, respectively. The manR mRNA level was higher in the mpt deletion mutant in medium lacking glucose, possibly due to disruption of a regulatory process that normally downregulates manR transcription in the absence of this sugar. Analysis of the mpt deletion mutant also showed that EII(t)(Man) participates to various degrees in glucose-mediated CCR of PTS operons. CCR of the lmo0027 gene, which encodes a beta-glucoside PTS transporter, required expression of EII(t)(Man). In contrast, genes in two mannose PTS operons (lmo0024, lmo1997, and lmo2002) were repressed by glucose even when EII(t)(Man) was not synthesized. A third mannose PTS operon, mpo, was not regulated by glucose or by the level of EII(t)(Man). Finally, the mRNA levels for five genes in the prfA virulence gene cluster were two- to fourfold higher in the mpt deletion mutant. The results show that EII(t)(Man) participates to various extents in glucose-mediated CCR of PTS operons and makes a small, albeit significant, contribution to downregulation of virulence gene transcription by glucose in strain EGD-e.

Modulation of PrfA activity in Listeria monocytogenes upon growth in different culture media

PrfA is the major transcriptional activator of most virulence genes of Listeria monocytogenes. Its activity is modulated by a variety of culture conditions. Here, we studied the PrfA activity in the L. monocytogenes wild-type strain EGD and an isogenic prfA deletion mutant (EGDDeltaprfA) carrying multiple copies of the wild-type prfA or the mutant prfA* gene (strains EGDDeltaprfApPrfA and EGDDeltaprfApPrfA*) in response to growth in brain heart infusion (BHI), Luria-Bertani broth (LB) or a defined minimal medium (MM) supplemented with one of the three phosphotransferase system (PTS) carbohydrates, glucose, mannose and cellobiose, or the non-PTS carbon source glycerol. Low PrfA activity was observed in the wild-type strain in BHI and LB with all of these carbon sources, while PrfA activity was high in minimal medium in the presence of glycerol. EGDDeltaprfApPrfA*, expressing a large amount of PrfA* protein, showed high PrfA activity under all growth conditions. In contrast, strain EGDDeltaprfApPrfA, expressing an equally high amount of PrfA protein, showed high PrfA activity only when cultured in BHI, and not in LB or MM (in the presence of any of the carbon sources). A ptsH mutant (lacking a functional HPr) was able to grow in BHI but not in LB or MM, regardless of which of the four carbon sources was added, suggesting that in LB and MM the uptake of the used PTS carbohydrates and the catabolism of glycerol are fully dependent on the functional common PTS pathway. The BHI culture medium, in contrast, apparently contains carbon sources (supporting listerial growth) which are taken up and metabolized by L. monocytogenes independently of the common PTS pathway. The growth rates of L. monocytogenes were strongly reduced in the presence of large amounts of PrfA (or PrfA*) protein when growing in MM, but were less reduced in LB and only slightly reduced in BHI. The expression of the genes encoding the PTS permeases of L. monocytogenes was determined in the listerial strains under the applied growth conditions. The data obtained further support the hypothesis that PrfA activity correlates with the expression level and the phosphorylation state of specific PTS permeases.

Exogenous or L-rhamnose-derived 1,2-propanediol is metabolized via a pduD-dependent pathway in Listeria innocua

1,2-Propanediol (1,2-PD) added exogenously to cultures or produced endogenously from l-rhamnose is metabolized to n-propanol and propionate in Listeria innocua Lin11. The pduD gene, which encodes a diol dehydratase ss subunit homolog, is required for 1,2-PD catabolism. pduD and 16 other genes within the pduA-to-pduF region of a large gene cluster are induced in medium containing 1,2-PD.