The alanine racemase gene alr is an alternative to antibiotic resistance genes in cloning systems for industrial Corynebacterium glutamicum strains

The Gram-positive eubacterium Streptomyces lividans contains four chromosomally encoded type I signal peptidases, SipW, SipX, SipY and SipZ, of which all but SipW have an unusual C-terminal membrane anchor. For in vitro characterisation of these signal peptidases, the S. lividans sip genes were expressed in Escherichia coli and the corresponding proteins were purified. The four enzymes had an optimum activity at an alkaline pH, notably pH 8-9 for SipW and SipY and pH 10-11 for SipX and SipZ. In contrast to SipW, the in vitro activities of SipX, SipY and SipZ significantly increased in the presence of detergent. Since none of the S. lividans Sip proteins contains the hydrophobic beta-barrel domain, which in E. coli LepB was proven to be requisite for detergent-dependent in vitro activity, we assume that for detergent dependence, the C-terminal transmembrane anchor can partly substitute for this domain. Finally, all Sip proteins were stimulated by added phospholipids, which strongly suggests that phospholipids play an important role in the catalytic mechanism.     

Membrane topology of the Streptomyces lividans type I signal peptidases

Most bacterial membranes contain one or two type I signal peptidases (SPases) for the removal of signal peptides from export proteins. For Streptomyces lividans, four different type I SPases (denoted SipW, SipX, SipY, and SipZ) were previously described. In this communication, we report the experimental determination of the membrane topology of these SPases. A protease protection assay of SPase tendamistat fusions confirmed the presence of the N- as well as the C-terminal transmembrane anchor for SipY. SipX and SipZ have a predicted topology similar to that of SipY. These three S. lividans SPases are currently the only known prokaryotic-type type I SPases of gram-positive bacteria with a C-terminal transmembrane anchor, thereby establishing a new subclass of type I SPases. In contrast, S. lividans SipW contains only the N-terminal transmembrane segment, similar to most type I SPases of gram-positive bacteria. Functional analysis showed that the C-terminal transmembrane anchor of SipY is important to enhance the processing activity, both in vitro as well as in vivo. Moreover, for the S. lividans SPases, a relation seems to exist between the presence or absence of the C-terminal anchor and the relative contributions to the total SPase processing activity in the cell. SipY and SipZ, two SPases with a C-terminal anchor, were shown to be of major importance to the cell. Accordingly, for SipW, missing the C-terminal anchor, a minor role in preprotein processing was found.     

Formation of D-alanyl-lipoteichoic acid is required for adhesion and virulence of Listeria monocytogenes

The dlt operon of Gram-positive bacteria comprises four genes (dltA, dltB, dltC and dltD) that catalyse the incorporation of D-alanine residues into the cell wall-associated lipoteichoic acids (LTAs). In this work, we characterized the dlt operon of Listeria monocytogenes and constructed a D-Ala-deficient LTA mutant by inactivating the first gene (dltA) of this operon. The DltA- mutant did not show any morphological alterations and its growth rate was similar to that of the wild-type strain. However, it exhibited an increased susceptibility to the cationic peptides colistin, nisin and polymyxin B. The virulence of the DltA- mutant was severely impaired in a mouse infection model (4 log increase in the LD50) and, in vitro, the adherence of the mutant to various cell lines (murine bone marrow-derived macrophages and hepatocytes and a human epithelial cell line) was strongly restricted, although the amounts of surface proteins implicated in virulence (ActA, InlA and InlB) remains unaffected. We suggest that the decreased adherence of the DltA- mutant to non-phagocytic and phagocytic cells might be as a result of the increased electronegativity of its charge surface and/or the presence at the bacterial surface of adhesins possessing altered binding activities. These results show that the D-alanylation of the LTAs contributes to the virulence of the intracellular pathogen L. monocytogenes.     

A hypermutator phenotype attenuates the virulence of Listeria monocytogenes in a mouse model

The integrity of the genetic material of bacteria is guaranteed by a set of distinct repair mechanisms. The participation of these repair systems in bacterial pathogenicity has been addressed only recently. Here, we study for the first time the participation in virulence of the MutSL mismatch repair system of Listeria monocytogenes. The mutS and mutL genes, which are contiguous in the L. monocytogenes chromosome, were identified after in silico analysis. The deduced MutS shares 62% identity with MutS of Bacillus subtilis and 50% identity with HexA, its homologue in Streptococcus pneumoniae; MutL shares 59% identity with MutL of B. subtilis and 47% identity with HexB of S. pneumoniae. Functional analysis of the mutSL locus was studied by constructing a double knock-out mutant. We showed that the deletion DeltamutSL induces: (i) a 100- to 1000-fold increase in the spontaneous mutation rate; and (ii) a 10- to 15-fold increase in the frequency of transduction, thus demonstrating the role of mutSL of L. monocytogenes in both mismatch repair and homologous recombination. We found that the deletion DeltamutSL moderately affected bacterial virulence, with a 1-log increase in the lethal dose 50% (LD50) in the mouse. Strikingly, repeated passages of the mutant strain in mice reduced virulence further. Competition assays between wild-type and mutant strains showed that the deletion DeltamutSL reduced the capacity of L. monocytogenes to survive and multiply in mice. These results thus demonstrate that, for the intracellular pathogen L. monocytogenes, a hypermutator phenotype is more deleterious than profitable to its virulence.     

An iron-dependent mutant of Listeria monocytogenes of attenuated virulence

Abstract A bank of Tn 917 -insertional mutants from the facultative intracellular pathogen Listeria monocytogenes was screened by an original method based on bacterial growth on synthetic medium under iron-limiting conditions. One mutant, whose in vitro growth in synthetic medium was specifically dependent upon the availability of iron in its environment, was isolated and characterized. The insertional event occurred in a non-coding region, upstream of a rrn operon and located within a 1100-kb Not I fragment of the physical map, where the virulence genes already identified in L. monocytogenes were also present. Protein analysis by SDS-PAGE revealed a pleiotropic effect of the insertional event on cell-associated proteins, suggesting a polar effect of the transposon on adjacent unknown gene(s). The virulence in the mouse of this mutant was strongly impaired, although it was capable in vitro of growing intracellularly and of spreading from cell to cell, as shown by the production of lytic plaques on cell culture.     

Listeria monocytogenes: comparative interpretation of mouse virulence assay

Being an opportunistic bacterial pathogen, Listeria monocytogenes demonstrates significant strain variations in virulence and pathogenicity. The availability of laboratory procedures to ascertain the pathogenic potential of L. monocytogenes bacteria would greatly enhance the control and prevention of listerial infections. As a method that measures all virulent determinants, mouse virulence assay has been frequently used for assessing L. monocytogenes virulence. The pathogenic potential of a given L. monocytogenes strain as determined by mouse virulence assay is often calculated from mouse mortality data in combination with colony forming units (CFUs) derived from plate counts, and expressed by medium lethal dose (LD(50)). In this report, we describe an alternative method [i.e., relative virulence (%)] that does not involve CFU estimation, and is comparable to LD(50) for interpretation of mouse virulence assay for L. monocytogenes. The relative virulence (%) is obtained by dividing the number of dead mice with the total number of mice tested for a particular strain using a known virulent strain (e.g., L. monocytogenes EGD) as reference. Besides providing a more direct interpretation in comparison with LD(50) values for mouse virulence assay, this method requires fewer dosage groups per L. monocytogenes strain, and eliminates CFU estimation that is step subject to variations between runs and also between laboratories.     

Lipoproteins of Listeria monocytogenes are critical for virulence and TLR2-mediated immune activation

Numerous cell surface components of Listeria influence and regulate innate immune recognition and virulence. Here, we demonstrate that lipidation of prelipoproteins in Listeria monocytogenes is required to promote NF-kappaB activation via TLR2. In HeLa cells transiently expressing TLR2, L. monocytogenes and Listeria innocua mutants lacking the prolipoprotein diacylglyceryl transferase (lgt) gene are unable to induce TLR2-dependent activation of NF-kappaB, a property intrinsic to their isogenic parental strains. TLR2-dependent immune recognition is directed to secreted, soluble lipoproteins as evidenced by the sensitivity of the response to lipoprotein lipase. Studies of bone marrow-derived macrophages of C57BL/6 wild-type and TLR2-deficient mice infected with wild-type and lgt mutant strains indicate that the absence of host TLR2 receptor signaling has consequences similar to those of the absence of the bacterial TLR2 ligand, i.e., a delay in cellular immune responses directed toward the bacterium. Infection studies with the wild-type and TLR2(-/-) mice indicated attenuation of the lgt deletion mutant in both mouse strains, implying multiple roles of lipoproteins during infection. Further characterization of the Delta lgt mutant indicated that it is impaired for both invasion and intracellular survival and exhibits increased susceptibility to cationic peptides. Our studies identify lipoproteins as the immunologically active ligand of TLR2 and assign a critical role for this receptor in the recognition of these bacteria during infection, but they also reveal the overall importance of the lipoproteins for the pathogenicity of Listeria.     

The unrelated surface proteins ActA of Listeria monocytogenes and lcsA of Shigella flexneri are sufficient to confer actin-based motility on Listeria innocua and Escherichia coli respectively

Listeria monocytogenes and Shigella flexneri are two unrelated facultative intracellular pathogens which spread from cell to cell by using a similar mode of intracellular movement based on continuous actin assembly at one pole of the bacterium. This process requires the asymmetrical expression of the ActA surface protein in L. monocytogenes and the lcsA (VirG) surface protein in S. flexneri . ActA and lcsA share no sequence homology. To assess the role of the two proteins in the generation of actin-based movement, we expressed them in the genetic context of two non-actin polymerizing, non-pathogenic bacterial species, Listeria innocua and Escherichia coli . In the absence of any additional bacterial pathogenicity determinants, both proteins induced actin assembly and propulsion of the bacteria in cytoplasmic extracts from Xenopus eggs, as visualized by the formation of characteristic actin comet tails. E. coli expressing lcsA moved about two times faster than Listeria and displayed longer actin tails. However, actin dynamics (actin filament distribution and filament half-lives) were similar in lcsA- and ActA-induced actin tails suggesting that by using unrelated surface molecules, L. monocytogenes and S. flexneri move intracellularly by interacting with the same host cytoskeleton components or by interfering with the same host cell signal transduction pathway.     

Inactivation of the srtA gene in Listeria monocytogenes inhibits anchoring of surface proteins and affects virulence

During infection of their hosts, Gram-positive bacteria express surface proteins that serve multiple biological functions. Surface proteins harbouring a C-terminal sorting signal with an LPXTG motif are covalently linked to the cell wall peptidoglycan by a transamidase named sortase. Two genes encoding putative sortases, termed srtA and srtB, were identified in the genome of the intracellular pathogenic bacterium Listeria monocytogenes. Inactivation of srtA abolishes anchoring of the invasion protein InlA to the bacterial surface. It also prevents the proper sorting of several other peptidoglycan-associated LPXTG proteins. Three were identified by a mass spectrometry approach. The DeltasrtA mutant strain is defective in entering epithelial cells, similar to a DeltainlA mutant. In contrast to a DeltainlA mutant, the DeltasrtA mutant is impaired for colonization of the liver and spleen after oral inoculation in mice. Thus, L. monocytogenes srtA is required for the cell wall anchoring of InlA and, presumably, for the anchoring of other LPXTG-containing proteins that are involved in listerial infections.     

Maturation of lipoproteins by type II signal peptidase is required for phagosomal escape of Listeria monocytogenes

Lipoproteins of Gram-positive bacteria are involved in a broad range of functions such as substrate binding and transport, antibiotic resistance, cell signaling, or protein export and folding. Lipoproteins are also known to initiate both innate and adaptative immune responses. However, their role in the pathogenicity of intracellular microorganisms is yet poorly understood. In Listeria monocytogenes, a Gram-positive facultative intracellular human pathogen, surface proteins have important roles in the interactions of the microorganism with the host cells. Among the putative surface proteins of L. monocytogenes, lipoproteins constitute the largest family. Here, we addressed the role of the signal peptidase (SPase II), responsible for the maturation of lipoproteins in listerial pathogenesis. We identified a gene, lsp, encoding a SPase II in the genome of L. monocytogenes and constructed a deltalsp chromosomal deletion mutant. The mutant strain fails to process several lipoproteins demonstrating that lsp encodes a genuine SPase II. This defect is accompanied by a reduced efficiency of phagosomal escape during infection of eucaryotic cells, and leads to an attenuated virulence. We show that lsp gene expression is strongly induced when bacteria are still entrapped inside phagosomes of infected macrophages. The data presented establish, thus, that maturation of lipoproteins is critical for efficient phagosomal escape of L. monocytogenes, a process temporally controlled by the regulation of Lsp production in infected cells.     

Autophagy limits Listeria monocytogenes intracellular growth in the early phase of primary infection

Autophagy has been recently proposed to be a component of the innate cellular immune response against several types of intracellular microorganisms. However, other intracellular bacteria including Listeria monocytogenes have been thought to evade the autophagic cellular surveillance. Here, we show that cellular infection by L. monocytogenes induces an autophagic response, which inhibits the growth of both the wild-type and a DeltaactA mutant strain, impaired in cell-to-cell spreading. The onset of early intracellular growth is accelerated in autophagy-deficient cells, but the growth rate once bacteria begin to multiply in the cytosol does not change. Moreover, a significant fraction of the intracellular bacteria colocalize with autophagosomes at the early time-points after infection. Thus, autophagy targets L. monocytogenes during primary infection by limiting the onset of early bacterial growth. The bacterial expression of listeriolysin O but not phospholipases is necessary for the induction of autophagy, suggesting a possible role for permeabilization of the vacuole in the induction of autophagy. Interestingly, the growth of a DeltaplcA/B L. monocytogenes strain deficient for bacterial phospholipases is impaired in wild-type cells, but restored in the absence of autophagy, suggesting that bacterial phospholipases may facilitate the escape of bacteria from autophagic degradation. We conclude that L. monocytogenes are targeted for degradation by autophagy during the primary infection, in the early phase of the intracellular cycle, following listeriolysin O-dependent vacuole perforation but preceding active multiplication in the cytosol, and that expression of bacterial phospholipases is necessary for the evasion of autophagy.     

Identification of trkH,Encoding a Potassium Uptake Protein Required for Francisella tularensis Systemic Dissemination in Mice

Francisella tularensis is a highly infectious bacterium causing the zoonotic disease tularaemia. During its infectious cycle, F. tularensis is not only exposed to the intracellular environment of macrophages but also resides transiently in extracellular compartments, in particular during its systemic dissemination. The screening of a bank of F. tularensis LVS transposon insertion mutants on chemically defined medium (CDM) led us to identify a gene, designated trkH, encoding a homolog of the potassium uptake permease TrkH. Inactivation of trkH impaired bacterial growth in CDM. Normal growth of the mutant was only restored when CDM was supplemented with potassium at high concentration. Strikingly, although not required for intracellular survival in cell culture models, TrkH appeared to be essential for bacterial virulence in the mouse. In vivo kinetics of bacterial dissemination revealed a severe defect of multiplication of the trkH mutant in the blood of infected animals. The trkH mutant also showed impaired growth in blood ex vivo. Genome sequence analyses suggest that the Trk system constitutes the unique functional active potassium transporter in both tularensis and holarctica subspecies. Hence, the impaired survival of the trkH mutant in vivo is likely to be due to its inability to survive in the low potassium environment (1–5 mM range) of the blood. This work unravels thus the importance of potassium acquisition in the extracellular phase of the F. tularensis infectious cycle. More generally, potassium could constitute an important mineral nutrient involved in other diseases linked to systemic dissemination of bacterial pathogens.     

Ⅲ型效应子GALA对青枯菌在两种寄主植物上致病性的影响

[目的]研究Ⅲ型效应子GALAs对青枯菌OE1-1在不同寄主植物致病性上的影响。[方法]构建青枯菌OE1-1的多种GALA缺失突变体,通过根切和叶片注射等方法研究GALAs对青枯菌OE1-1致病力和细胞内增殖能力的影响。[结果]GALA多基因缺失突变体对寄主烟草的致病力减弱,在烟草体内细菌繁殖能力较野生型明显降低,但在寄主番茄上不影响其致病性。[结论]GALA效应子对青枯菌OE1-1在烟草植株致病性上展现协同作用。     

A homolog of Bacillus subtilis trigger factor in Listeria monocytogenes is involved in stress tolerance and bacterial virulence

Molecular chaperones play an essential role in the folding of nascent chain polypeptides, as well as in the refolding and degradation of misfolded or aggregated proteins. They also assist in protein translocation and participate in stress functions. We identified a gene, designated tig, encoding a protein homologous to trigger factor (TF), a cytosolic ribosome-associated chaperone, in the genome of Listeria monocytogenes. We constructed a chromosomal Delta tig deletion and evaluated the impact of the mutation on bacterial growth in broth under various stress conditions and on pathogenesis. The Delta tig deletion did not affect cell viability but impaired survival in the presence of heat and ethanol stresses. We also identified the ffh gene, encoding a protein homologous to the SRP54 eukaryotic component of the signal recognition particle. However, a Delta ffh deletion was not tolerated, suggesting that Ffh is essential, as it is in Bacillus subtilis and Escherichia coli. Thus, although dispensable for growth, TF is involved in the stress response of L. monocytogenes. The Delta tig mutant showed no or very modest intracellular survival defects in eukaryotic cells. However, in vivo it showed a reduced capacity to persist in the spleens and livers of infected mice, revealing that TF has a role in the pathogenicity of L. monocytogenes.     

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

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

A putative P-type ATPase required for virulence and resistance to haem toxicity in Listeria monocytogenes

Regulation of iron homeostasis in many pathogens is principally mediated by the ferric uptake regulator, Fur. Since acquisition of iron from the host is essential for the intracellular pathogen Listeria monocytogenes, we predicted the existence of Fur-regulated systems that support infection. We examined the contribution of nine Fur-regulated loci to the pathogenicity of L. monocytogenes in a murine model of infection. While mutating the majority of the genes failed to affect virulence, three mutants exhibited a significantly compromised virulence potential. Most striking was the role of the membrane protein we designate FrvA (Fur regulated virulence factor A; encoded by frvA [lmo0641]), which is absolutely required for the systemic phase of infection in mice and also for virulence in an alternative infection model, the Wax Moth Galleria mellonella. Further analysis of the ΔfrvA mutant revealed poor growth in iron deficient media and inhibition of growth by micromolar concentrations of haem or haemoglobin, a phenotype which may contribute to the attenuated growth of this mutant during infection. Uptake studies indicated that the ΔfrvA mutant is unaffected in the uptake of ferric citrate but demonstrates a significant increase in uptake of haem and haemin. The data suggest a potential role for FrvA as a haem exporter that functions, at least in part, to protect the cell against the potential toxicity of free haem.     

Isolation of Listeria monocytogenes mutants with high-level in vitro expression of host cytosol-induced gene products

The facultative intracellular bacterial pathogen Listeria monocytogenes dramatically increases the expression of several key virulence factors upon entry into the host cell cytosol. actA, the protein product of which is required for cell-to-cell spread of the bacterium, is expressed at low to undetectable levels in vitro and increases in expression more than 200-fold after L. monocytogenes escape from the phagosome. To identify bacterial factors that participate in the intracellular induction of actA expression, L. monocytogenes mutants expressing high levels of actA during in vitro growth were selected after chemical mutagenesis. The resulting mutant isolates displayed a wide range of actA expression levels, and many were less sensitive to environmental signals that normally mediate repression of virulence gene expression. Several isolates contained mutations affecting actA gene expression that mapped at least 40 kb outside the PrfA regulon, supporting the existence of additional regulatory factors that contribute to virulence gene expression. Two actA in vitro expression mutants contained novel mutations within PrfA, a key regulator of L. monocytogenes virulence gene expression. PrfA E77K and PrfA G155S mutations resulted in high-level expression of PrfA-dependent genes, increased bacterial invasion of epithelial cells and increased virulence in mice. Both prfA mutant strains were significantly less motile than wild-type L. monocytogenes. These results suggest that, although constitutive activation of PrfA and PrfA-dependent gene expression may enhance L. monocytogenes virulence, it may conversely hamper the bacterium's ability to compete in environments outside host cells.