Regulation of lipid A modifications by Salmonella typhimurium virulence genes phoP-phoQ

Role of Salmonella enterica lipopolysaccharide in activation of dendritic cell functions and bacterial containment

In contrast to nonpathogenic bacteria, the Gram-negative pathogen Salmonella enterica is not eradicated, but persists in murine dendritic cells (DC). The molecular basis of this phenotype is unknown. We set out to characterize bacterial and DC functions that are involved in Salmonella persistence. Our data prove that neither bacterial nor host cell de novo protein biosynthesis is required for Salmonella persistence in DC. We identified the Salmonella O-Ag of the LPS of Salmonella as an important factor for controlling the intracellular fate of Salmonella in DC. A Salmonella strain with entirely absent O-Ag showed an increased rate of uptake by DC, altered intracellular processing, and increased degradation, and also boosted the activation of immune functions of DC. These novel findings demonstrate that in addition to the multiple functions of the bacterial LPS in adaptation to the intestinal environment and protection against innate immune function, this molecule also has an important role in interaction of Salmonella with DC.

Masters of conquest and pillage: Xenorhabdus nematophila global regulators control transitions from virulence to nutrient acquisition

Invertebrate animal models are experimentally tractable and have immunity and disease symptoms that mirror those of vertebrates. Therefore they are of particular utility in understanding fundamental aspects of pathogenesis. Indeed, artificial models using human pathogens and invertebrate hosts have revealed conserved and novel molecular mechanisms of bacterial infection and host immune responses. Additional insights may be gained from investigating interactions between invertebrates and pathogens they encounter in their natural environments. For example, enteric bacteria in the genera Photorhabdus and Xenorhabdus are pathogens of insects that also mutualistically associate with nematodes in the genera Heterorhabditis and Steinernema respectively. These bacteria serve as models to understand naturally occurring symbiotic associations that result in disease in or benefit for animals. Xenorhabdus nematophila is the best-studied species of its genus with regard to the molecular mechanisms of its symbiotic associations. In this review, we summarize recent advances in understanding X. nematophila-host interactions. We emphasize regulatory cascades involved in coordinating transitions between various stages of the X. nematophila life cycle: infection, reproduction and transmission.

Bacterial delivery of siRNAs: a new approach to solid tumor therapy

RNAi is a powerful research tool for specific gene silencing and may also lead to promising novel therapeutic strategies. However, the development of RNAi-based therapies has been slow due to the lack of targeted delivery methods. The biggest challenge in the use of siRNA-based therapies is the delivery to target cells. There are many additional obstacles to in vivo delivery of siRNAs, such as degradation by endogenous enzymes and interaction with blood components leading to nonspecific uptake into cells, which govern biodistribution and availability of siRNA in the body. Naked unmodified synthetic siRNA including plasmid-carried-shRNA-expression constructs cannot penetrate cellular membranes, and therefore, systemic application is unlikely to be successful. The success of gene therapy by siRNAs relies on the development of safe, economical, and efficacious in vivo delivery systems into the target cells. Attenuated Salmonella have been employed recently as vectors to deliver silencing hairpin RNA (shRNA) expression plasmids into mammalian cells. This approach has achieved gene silencing in vitro and in vivo. The facultative anaerobic, invasive Salmonella have a natural tropism for solid tumors including metastatic tumors. Genetically modified, attenuated Salmonella have been used recently both as potential antitumor agents by themselves, and to deliver specific tumoricidal therapies. This chapter describes the use of attenuated bacteria as tumor-targeting delivery systems for cancer therapy.

Global systems-level analysis of Hfq and SmpB deletion mutants in Salmonella: implications for virulence and global protein translation

Using sample-matched transcriptomics and proteomics measurements it is now possible to begin to understand the impact of post-transcriptional regulatory programs in Enterobacteria. In bacteria post-transcriptional regulation is mediated by relatively few identified RNA-binding protein factors including CsrA, Hfq and SmpB. A mutation in any one of these three genes, csrA, hfq, and smpB, in Salmonella is attenuated for mouse virulence and unable to survive in macrophages. CsrA has a clearly defined specificity based on binding to a specific mRNA sequence to inhibit translation. However, the proteins regulated by Hfq and SmpB are not as clearly defined. Previous work identified proteins regulated by hfq using purification of the RNA-protein complex with direct sequencing of the bound RNAs and found binding to a surprisingly large number of transcripts. In this report we have used global proteomics to directly identify proteins regulated by Hfq or SmpB by comparing protein abundance in the parent and isogenic hfq or smpB mutant. From these same samples we also prepared RNA for microarray analysis to determine if alteration of protein expression was mediated post-transcriptionally. Samples were analyzed from bacteria grown under four different conditions; two laboratory conditions and two that are thought to mimic the intracellular environment. We show that mutants of hfq and smpB directly or indirectly modulate at least 20% and 4% of all possible Salmonella proteins, respectively, with limited correlation between transcription and protein expression. These proteins represent a broad spectrum of Salmonella proteins required for many biological processes including host cell invasion, motility, central metabolism, LPS biosynthesis, two-component regulatory systems, and fatty acid metabolism. Our results represent one of the first global analyses of post-transcriptional regulons in any organism and suggest that regulation at the translational level is widespread and plays an important role in virulence regulation and environmental adaptation for Salmonella.

A sensitive liquid chromatography/mass spectrometry-based assay for quantitation of amino-containing moieties in lipid A

A novel sensitive liquid chromatography/mass spectrometry-based assay was developed for the quantitation of aminosugars, including 2-amino-2-deoxyglucose (glucosamine, GlcN), 2-amino-2-deoxygalactose (galactosamine, GalN), and 4-amino-4-deoxyarabinose (aminoarabinose, AraN), and for ethanolamine (EtN), present in lipid A. This assay enables the identification and quantitation of all amino-containing moieties present in lipopolysaccharide or lipid A from a single sample. The method was applied to the analysis of lipid A (endotoxin) isolated from a variety of biosynthetic and regulatory mutants of Salmonella enterica serovar Typhimurium and Francisella tularensis subspecies novicida. Lipid A is treated with trifluoroacetic acid to liberate and deacetylate individual aminosugars and mass tagged with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate, which reacts with primary and secondary amines. The derivatives are separated using reversed-phase chromatography and analyzed using a single quadrupole mass spectrometer to detect quantities as small as 20 fmol. GalN was detected only in Francisella and AraN only in Salmonella, while GlcN was detected in lipid A samples from both species of bacteria. Additionally, we found an approximately 10-fold increase in the level of AraN in lipid A isolated from Salmonella grown in magnesium-limited versus magnesium-replete conditions. Salmonella with defined mutations in lipid A synthesis and regulatory genes were used to further validate the assay. Salmonella with null mutations in the phoP, pmrE, and prmF genes were unable to add AraN to their lipid A, while Salmonella with constitutively active phoP and pmrA exhibited AraN modification of lipid A even in the normally repressive magnesium-replete growth condition. The described assay produces excellent repeatability and reproducibility for the detection of amino-containing moieties in lipid A from a variety of bacterial sources.

Treponema denticola does not induce production of common innate immune mediators from primary gingival epithelial cells

It has been hypothesized that the neutrophil chemoattractant interleukin-8 (IL-8) forms a gradient in the oral cavity, with the highest concentration of IL-8 produced closest to the bacterial biofilm. In periodontitis, this gradient is disrupted, impairing neutrophil chemotaxis to diseased sites. Treponema denticola is prominently associated with periodontal disease, yet little is known about its ability to modulate the production of inflammatory mediators by epithelial cells. Others have shown that dentilisin, the major outer membrane protease of T. denticola, degrades IL-8 in vitro. We now provide evidence that T. denticola also fails to induce IL-8 production from primary gingival epithelial cells (PGEC). The lack of IL-8 production is not explained by IL-8 degradation, because a protease mutant that does not degrade IL-8 does not induce IL-8 production with these stimuli either. The lack of innate immune mediator production may be a more global phenomenon because T. denticola fails to induce IL-6 or intercellular adhesion molecule 1 production from PGEC. T. denticola also fails to induce transcription of IL-8 and human beta-defensin-2 messenger RNA. The lack of immune mediator production is not explained by the failure of T. denticola to interact with Toll-like receptor 2 (TLR-2), as T. denticola stimulates nuclear factor-kappaB nuclear translocation in TLR-2-transfected HEK293 cells. Not only can T. denticola degrade the IL-8 present in the periodontal lesion, but this organism also fails to induce IL-8 production by PGEC. The lack of an epithelial cell response to T. denticola may contribute to the pathogenesis of periodontitis by failing to trigger chemotaxis of neutrophils into the periodontal pocket.

Microbes in gastrointestinal health and disease

Most, if not all, animals coexist with a complement of prokaryotic symbionts that confer a variety of physiologic benefits. In humans, the interaction between animal and bacterial cells is especially important in the gastrointestinal tract. Technical and conceptual advances have enabled rapid progress in characterizing the taxonomic composition, metabolic capacity, and immunomodulatory activity of the human gut microbiota, allowing us to establish its role in human health and disease. The human host coevolved with a normal microbiota over millennia and developed, deployed, and optimized complex immune mechanisms that monitor and control this microbial ecosystem. These cellular mechanisms have homeostatic roles beyond the traditional concept of defense against potential pathogens, suggesting these pathways contribute directly to the well-being of the gut. During their coevolution, the bacterial microbiota has established multiple mechanisms to influence the eukaryotic host, generally in a beneficial fashion, and maintain their stable niche. The prokaryotic genomes of the human microbiota encode a spectrum of metabolic capabilities beyond that of the host genome, making the microbiota an integral component of human physiology. Gaining a fuller understanding of both partners in the normal gut-microbiota interaction may shed light on how the relationship can go awry and contribute to a spectrum of immune, inflammatory, and metabolic disorders and may reveal mechanisms by which this relationship could be manipulated toward therapeutic ends.

Determination of pyrophosphorylated forms of lipid A in Gram-negative bacteria using a multivaried mass spectrometric approach

Lipid A isolated from several bacteria (Escherichia coli, Pseudomonas aeruginosa, Salmonella enterica, and various strains of Yersinia) showed abundant formation of pyrophosphate anions upon ion dissociation. Pyrophosphate [H(3)P(2)O(7)](-) and/or [HP(2)O(6)](-) anions were observed as dominant fragments from diphosphorylated lipid A anions regardless of the ionization mode (matrix-assisted laser desorption ionization or electrospray ionization), excitation mode (collisional activation or infrared photoexcitation), or mass analyzer (time-of-flight/time-of-flight, tandem quadrupole, Fourier transform-ion cyclotron resonance mass spectrometry). Dissociations of anions from model lipid phosphate, pyrophosphate, and hexose diphosphates confirmed that pyrophosphate fragments were formed abundantly only in the presence of an intact pyrophosphate group in the analyte molecule and were not due to intramolecular rearrangement upon ionization, ion-molecule reactions, or rearrangement following activation. This indicated that pyrophosphate groups are present in diphosphorylated lipid A from a variety of Gram-negative bacteria.

Structural biology of membrane-intrinsic beta-barrel enzymes: sentinels of the bacterial outer membrane

The outer membranes of Gram-negative bacteria are replete with integral membrane proteins that exhibit antiparallel beta-barrel structures, but very few of these proteins function as enzymes. In Escherichia coli, only three beta-barrel enzymes are known to exist in the outer membrane; these are the phospholipase OMPLA, the protease OmpT, and the phospholipidColon, two colonslipid A palmitoyltransferase PagP, all of which have been characterized at the structural level. Structural details have also emerged for the outer membrane beta-barrel enzyme PagL, a lipid A 3-O-deacylase from Pseudomonas aeruginosa. Lipid A can be further modified in the outer membrane by two beta-barrel enzymes of unknown structure; namely, the Salmonella enterica 3'-acyloxyacyl hydrolase LpxR, and the Rhizobium leguminosarum oxidase LpxQ, which employs O(2) to convert the proximal glucosamine unit of lipid A into 2-aminogluconate. Structural biology now indicates how beta-barrel enzymes can function as sentinels that remain dormant when the outer membrane permeability barrier is intact. Host immune defenses and antibiotics that perturb this barrier can directly trigger beta-barrel enzymes in the outer membrane. The ensuing adaptive responses occur instantaneously and rapidly outpace other signal transduction mechanisms that similarly function to restore the outer membrane permeability barrier.

Cytolocalization of the PhoP response regulator in Salmonella enterica: modulation by extracellular Mg2+ and by the SCV environment

The PhoP/PhoQ two-component system plays an essential role regulating numerous virulence phenotypes in Salmonella enterica. Previous work showed that PhoQ, the sensor protein, switches between the kinase- and the phosphatase-dominant state in response to environmental Mg2+ availability. This switch defines the PhoP phosphorylation status and, as a result, the transcriptional activity of this regulator. In this work, using the FlAsH labelling technique, we examine PhoP cytolocalization in response to extracellular Mg2+ limitation in vitro and to the Salmonella-containing vacuole (SCV) environment in macrophage cells. We demonstrate that in these PhoP/PhoQ-inducing environments PhoP displays preferential localization to one cell pole, while being homogeneously distributed in the bacterial cytoplasm in repressing conditions. Polar localization is lost in the absence of PhoQ or when a non-phosphorylatable PhoP(D52A) mutant is expressed. However, when PhoP transcriptional activation is achieved in a Mg2+- and PhoQ-independent manner, PhoP regains asymmetric polar localization. In addition, we show that, in the analysed conditions, PhoQ cellular distribution does not parallel PhoP location pattern. These findings reveal that PhoP cellular location is dynamic and conditioned by its environmentally defined transcriptional status, showing a new insight in the PhoP/PhoQ system mechanism.

P. gingivalis and E. coli lipopolysaccharides exhibit different systemic but similar local induction of inflammatory markers

BACKGROUND

Porphyromonas gingivalis is a Gram-negative bacterium that is an important etiologic agent of human adult periodontitis. The goal of the study was to test the hypothesis that two isoforms of P. gingivalis lipopolysaccharide (PgLPS), PgLPS(1435)(/1449) and PgLPS(1690), exhibit differences in their capacity to stimulate systemic versus local responses compared to Escherichia coli lipopolysaccharide (LPS).

METHODS

LPS was inoculated into the scalp of mice, and the response was measured locally at the site of inoculation and systemically in the heart/aorta. Vascular cell adhesion molecule (VCAM)-1 was assessed at the protein level by enzyme-linked immunosorbent assay, and VCAM-1, E-selectin, and intercellular adhesion molecule (ICAM)-1 were assessed at the RNA level of the RNase protection assay. Serum tumor necrosis factor-alpha (TNF-alpha) levels were also measured.

RESULTS

E. coli LPS and both isoforms of P. gingivalis LPS were relatively potent in stimulating the expression of inflammatory markers, with E. coli LPS being more potent. In contrast, when the systemic response was measured in the heart/aorta, E. coli LPS, but not P. gingivalis LPS, significantly induced inflammatory markers. At moderate to low doses (1 and 10 microg per injection), serum TNF-alpha levels were minimally induced by P. gingivalis LPS compared to E. coli LPS.

CONCLUSIONS

Both forms of P. gingivalis LPS stimulated an inflammatory response when injected into connective tissue but were minimally stimulatory when a systemic response was measured. In contrast, E. coli LPS was a potent stimulus at the systemic and local levels.

Extracellular loops of lipid A 3-O-deacylase PagL are involved in recognition of aminoarabinose-based membrane modifications in Salmonella enterica serovar typhimurium

Salmonella enterica serovar Typhimurium modifies its lipopolysaccharide (LPS), including the lipid A portion, in response to changes in its environment including host tissues. The lipid A 3-O-deacylase PagL, the expression of which is promoted under a host-mimetic environment, exhibits latency in S. enterica; deacylation of lipid A is not usually observed in vivo, despite the expression of the outer membrane protein PagL. In contrast, PagL does not exhibit latency in S. enterica pmrA and pmrE mutants, both of which are deficient in the aminoarabinose-based modification of lipid A, indicating that aminoarabinose-modified LPS species were involved in the latency. In order to analyze the machinery for PagL's repression, we generated PagL mutants in which an amino acid residue located at four extracellular loops was replaced with alanine. Apparent lipid A 3-O deacylation was observed in S. enterica expressing the recombinant mutants PagL(R43A), PagL(R44A), PagL(C85A), and PagL(R135A), but not in S. enterica expressing wild-type PagL, suggesting that the point mutations released PagL from the latency. In addition, mutations at Arg-43, Arg-44, Cys-85, and Arg-135 did not affect lipid A 3-O-deacylase activity in an S. enterica pmrA mutant or in Escherichia coli BL21(DE3). These results, taken together, indicate that specific amino acid residues located at extracellular loops of PagL are involved in the recognition of aminoarabinose-modified LPS. Furthermore, S. enterica expressing the recombinant PagL(R43A) or PagL(R135A) mutant showed apparent growth arrest at 43 degrees C compared with S. enterica expressing wild-type PagL, indicating that the latency of PagL is important for bacterial growth.

Modulation of hexa-acyl pyrophosphate lipid A population under Escherichia coli phosphate (Pho) regulon activation

Environmental phosphate is an important signal for microorganism gene regulation, and it has recently been shown to trigger some key bacterial virulence mechanisms. In many bacteria, the Pho regulon is the major circuit involved in adaptation to phosphate limitation. The Pho regulon is controlled jointly by the two-component regulatory system PhoR/PhoB and by the phosphate-specific transport (Pst) system, which both belong to the Pho regulon. We showed that a pst mutation results in virulence attenuation in extraintestinal pathogenic Escherichia coli (ExPEC) strains. Our results indicate that the bacterial cell surface of the pst mutants is altered. In this study, we show that pst mutants of ExPEC strains display an increased sensitivity to different cationic antimicrobial peptides and vancomycin. Remarkably, the hexa-acylated 1-pyrophosphate form of lipid A is significantly less abundant in pst mutants. Among differentially expressed genes in the pst mutant, lpxT coding for an enzyme that transfers a phosphoryl group to lipid A, forming the 1-diphosphate species, was found to be downregulated. Our results strongly suggest that the Pho regulon is involved in lipid A modifications, which could contribute to bacterial surface perturbations. Since the Pho regulon and the Pst system are conserved in many bacteria, such a lipid A modification mechanism could be widely distributed among gram-negative bacterial species.

The Salmonella PmrAB regulon: lipopolysaccharide modifications, antimicrobial peptide resistance and more

Microbes are able to sense and respond to their environment primarily through the use of two-component regulatory systems. Many of these systems activate virulence-factor expression and are regulated by host-derived signals, having evolved to control gene expression at the key time and place for optimal establishment and maintenance of infection. Salmonella spp. are enteric pathogens that are able to survive both within host macrophages during systemic spread and killing by innate immune factors at intestinal mucosal surfaces. This review focuses on a key mechanism of pathogenesis that involves the PmrA-PmrB two-component system, which is activated in vivo by direct or indirect means and regulates genes that modify lipopolysaccharide, aiding survival in host (and non-host) environments.

Acyl chain specificity of the acyltransferases LpxA and LpxD and substrate availability contribute to lipid A fatty acid heterogeneity in Porphyromonas gingivalis

Porphyromonas gingivalis lipid A is heterogeneous with regard to the number, type, and placement of fatty acids. Analysis of lipid A by matrix-assisted laser desorption ionization-time of flight mass spectrometry reveals clusters of peaks differing by 14 mass units indicative of an altered distribution of the fatty acids generating different lipid A structures. To examine whether the transfer of hydroxy fatty acids with different chain lengths could account for the clustering of lipid A structures, P. gingivalis lpxA (lpxA(Pg)) and lpxD(Pg) were cloned and expressed in Escherichia coli strains in which the homologous gene was mutated. Lipid A from strains expressing either of the P. gingivalis transferases was found to contain 16-carbon hydroxy fatty acids in addition to the normal E. coli 14-carbon hydroxy fatty acids, demonstrating that these acyltransferases display a relaxed acyl chain length specificity. Both LpxA and LpxD, from either E. coli or P. gingivalis, were also able to incorporate odd-chain fatty acids into lipid A when grown in the presence of 1% propionic acid. This indicates that E. coli lipid A acyltransferases do not have an absolute specificity for 14-carbon hydroxy fatty acids but can transfer fatty acids differing by one carbon unit if the fatty acid substrates are available. We conclude that the relaxed specificity of the P. gingivalis lipid A acyltransferases and the substrate availability account for the lipid A structural clusters that differ by 14 mass units observed in P. gingivalis lipopolysaccharide preparations.

Biological properties of the native and synthetic lipid A of Porphyromonas gingivalis lipopolysaccharide

INTRODUCTION AND METHODS

A pentaacyl and diphosphoryl lipid A molecule found in the lipid A isolated from Porphyromonas gingivalis lipopolysaccharide (LPS) was chemically synthesized, and its characteristics were evaluated to reconfirm its interesting bioactivities including low endotoxicity and activity against LPS-unresponsive C3H/HeJ mouse cells.

RESULTS

The synthesized P. gingivalis lipid A (synthetic Pg-LA) exhibited strong activities almost equivalent to those of Escherichia coli-type synthetic lipid A (compound 506) in all assays on LPS-responsive mice, and cells. LPS and native lipid A of P. gingivalis displayed overall endotoxic activities, but its potency was reduced in comparison to the synthetic analogs. In the assays using C3H/HeJ mouse cells, the LPS and native lipid A significantly stimulated splenocytes to cause mitosis, and peritoneal macrophages to induce tumor necrosis factor-alpha and interleukin-6 production. However, synthetic Pg-LA and compound 506 showed no activity on the LPS-unresponsive cells. Inhibition assays using some inhibitors including anti-human Toll-like receptor 2 (TLR2) and TLR4/MD-2 complex monoclonal antibodies showed that the biological activity of synthetic Pg-LA was mediated only through the TLR4 signaling pathway, which might act as a receptor for LPS, whereas TLR2, possibly together with CD14, was associated with the signaling cascade for LPS and native lipid A of P. gingivalis, in addition to the TLR4 pathway.

CONCLUSION

These results suggested that the moderated and reduced biological activity of P. gingivalis LPS and native lipid A, including their activity on C3H/HeJ mouse cells via the TLR2-mediated pathway, may be mediated by bioactive contaminants or low acylated molecules present in the native preparations having multiple lipid A moieties.

Differential regulation by magnesium of the two MsbB paralogs of Shigella flexneri

Shigella flexneri, a gram-negative enteric pathogen, is unusual in that it contains two nonredundant paralogous genes that encode the myristoyl transferase MsbB (LpxM) that catalyzes the final step in the synthesis of the lipid A moiety of lipopolysaccharide. MsbB1 is encoded on the chromosome, and MsbB2 is encoded on the large virulence plasmid present in all pathogenic shigellae. We demonstrate that myristoyl transferase activity due to MsbB2 is detected in limited magnesium medium, but not in replete magnesium medium, whereas that due to MsbB1 is detected under both conditions. MsbB2 increases overall hexa-acylation of lipid A under limited magnesium conditions. Regulation of MsbB2 by magnesium occurs at the level of transcription and is dependent on the conserved magnesium-inducible PhoPQ two-component regulatory pathway. Direct hexanucleotide repeats within the promoter upstream of msbB2 were identified as a putative PhoP binding site, and mutations within the repeats led to diminished PhoP-dependent expression of a transcriptional fusion of lacZ to this promoter. Thus, the virulence plasmid-encoded paralog of msbB is induced under limited magnesium in a PhoPQ-dependent manner. PhoPQ regulates the response of many Enterobacteriaceae to environmental signals, which include modifications of lipid A that confer increased resistance of the organism to stressful environments and antimicrobial peptides. The findings reported here are the first example of gene duplication in which one paralog has selectively acquired the mechanism for differential regulation by PhoPQ. Our findings provide molecular insight into the mechanisms by which each of the two MsbB proteins of S. flexneri likely contributes to pathogenesis.

Intravaginal immunization of mice with recombinant Salmonella enterica serovar Typhimurium expressing human papillomavirus type 16 antigens as a potential route of vaccination against cervical cancer

Cervical cancer, the second leading cause of cancer deaths in women, is the consequence of high-risk human papillomavirus (HPV) infections. Toward the development of therapeutic vaccines that can induce both innate and adaptive mucosal immune responses, we analyzed intravaginal (ivag) vaccine delivery of live attenuated Salmonella enterica serovar Typhimurium expressing HPV16L1 as a model antigen. Innate immune responses were examined in cervicovaginal tissues by determining gene expression patterns by microarray analysis using nylon membranes imprinted with cDNA fragments coding for inflammation-associated genes. At 24 h, a wide range of genes, including those for chemokines and Th1- and Th2-type cytokine and chemokine receptors were up-regulated in mice ivag immunized with Salmonella compared to control mice. However, the majority of transcripts returned to their steady-state levels 1 week after immunization, suggesting a transient inflammatory response. Indeed, cervicovaginal histology of immunized mice showed a massive, but transient, infiltration of macrophages and neutrophils, while T cells were still increased after 7 days. Ivag immunization also induced humoral and antitumor immune responses, i.e., serum and vaginal anti-HPV16VLP antibody titers similar to those induced by oral immunization, and significant protection in tumor protection experiments using HPV16-expressing C3 tumor cells. These results show that ivag immunization with live attenuated Salmonella expressing HPV16 antigens modulates the local mucosal gene expression pattern into a transient proinflammatory profile, elicits strong systemic and mucosal immunity against HPV16, and confers protection against HPV16 tumor cells subcutaneously implanted in mice. Examination of the efficacy with which ivag HPV16E7E6 Salmonella induces regression of tumors located in cervicovaginal tissue is warranted.

An inner membrane dioxygenase that generates the 2-hydroxymyristate moiety of Salmonella lipid A

The lipid A residues of certain Gram-negative bacteria, including most strains of Salmonella and Pseudomonas, are esterified with one or two secondary S-2-hydroxyacyl chains. The S-2 hydroxylation process is O 2-dependent in vivo, but the relevant enzymatic pathways have not been fully characterized because in vitro assays have not been developed. We previously reported that expression of the Salmonella lpxO gene confers upon Escherichia coli K-12 the ability to synthesize 2-hydroxymyristate modified lipid A ( J. Biol. Chem. (2000) 275, 32940-32949). We now demonstrate that inactivation of lpxO, which encodes a putative Fe (2+)/O 2/alpha-ketoglutarate-dependent dioxygenase, abolishes S-2-hydroxymyristate formation in S. typhimurium. Membranes of E. coli strains expressing lpxO are able to hydroxylate Kdo 2-[4'- (32)P]-lipid A in vitro in the presence of Fe (2+), O 2, alpha-ketoglutarate, ascorbate, and Triton X-100. The Fe (2+) chelator 2,2'-bipyridyl inhibits the reaction. The product generated in vitro is a monohydroxylated Kdo 2-lipid A derivative. The [4'- (32)P]-lipid A released by mild acid hydrolysis from the in vitro product migrates with authentic S-2-hydroxlyated lipid A isolated from (32)P-labeled S. typhimurium cells. Electrospray ionization mass spectrometry and gas chromatography/mass spectrometry of the in vitro product are consistent with the 2-hydroxylation of the 3'-secondary myristoyl chain of Kdo 2-lipid A. LpxO contains two predicted trans-membrane helices (one at each end of the protein), and its active site likely faces the cytoplasm. LpxO is an unusual example of an integral membrane protein that is a member of the Fe (2+)/O 2/alpha-ketoglutarate-dependent dioxygenase family.

Immunological responses against Salmonella enterica serovar Typhimurium Braun lipoprotein and lipid A mutant strains in Swiss-Webster mice: potential use as live-attenuated vaccines

We generated and characterized Salmonella enterica serovar Typhimurium mutants that were deleted for the genes encoding Braun lipoprotein (lpp) alone or in conjunction with the msbB gene, which codes for an enzyme required for the acylation of the lipid A moiety of lipopolysaccharide. Two copies of the lpp gene, designated as lppA and lppB, exist on the chromosome of S. Typhimurium. These mutants were highly attenuated in a mouse infection model and induced minimal histopathological changes in mouse organs compared to those seen in infection with wild-type (WT) S. Typhimurium. The lppB/msbB and the lppAB/msbB mutants were maximally attenuated, and hence further examined in this study for their ability to induce humoral and cellular immune responses. Importantly, infection of out-bred Swiss-Webster mice with the mutant S. Typhimurium generated superior T helper cell type 2 (Th2) responses compared to WT S. Typhimurium, as determined by measuring IgG subclasses and cytokines. WT S. Typhimurium induced higher levels of IgG2a in sera of infected mice, while the lppB/msbB and lppAB/msbB mutants mounted higher levels of IgG1 as determined by an enzyme-linked immunosorbent assay. Mice immunized with lppB/msbB and lppAB/msbB mutants rapidly cleared WT S. Typhimurium upon subsequent rechallenge, and naïve mice passively immunized with sera from animals infected with S. Typhimurium mutants were protected against subsequent challenge with WT S. Typhimurium. Splenic T cells produced higher levels of interferon-gamma following ex vivo exposure to WT S. Typhimurium, while splenic T cells infected with the above-mentioned two mutants evoked higher levels of interleukin-6. Further, mice infected with lppB/msbB and lppAB/msbB mutants showed much higher levels of splenic T cell activation as measured by CD44(+) expression on CD4(+) T cells by flow cytometry and by incorporation of (3)H-thymidine compared to mice that were infected with WT S. Typhimurium. We expect the lppB/msbB and lppAB/msbB mutants to be excellent live-attenuated vaccine candidates, because they induced minimal inflammatory responses and evoked stronger and specific antibody and cellular immune responses.

Salmonellae interplay with host cells

Salmonellae are important causes of enteric diseases in all vertebrates. Characterization of the molecular mechanisms that underpin the interactions of salmonellae with their animal hosts has advanced greatly over the past decade, mainly through the study of Salmonella enterica serovar Typhimurium in tissue culture and animal models of infection. Knowledge of these bacterial processes and host responses has painted a dynamic and complex picture of the interaction between salmonellae and animal cells. This Review focuses on the molecular mechanisms of these host-pathogen interactions, in terms of their context, significance and future perspectives.

A p38 mitogen-activated protein kinase inhibitor arrests active alveolar bone loss in a rat periodontitis model

BACKGROUND

Gram-negative bacterial species, such as Actinobacillus actinomycetemcomitans, contain lipopolysaccharide (LPS) that initiates the innate immune system, resulting in inflammatory alveolar bone loss. LPS activates Toll-like receptors on membrane surfaces, stimulating many intracellular signaling cascades, including the p38 mitogen-activated protein kinase (MAPK). Activation of p38 signaling mediates inflammatory cytokine expression, contributing toward osteoclastogenesis and bone loss. The aim of this study was to determine whether the novel, orally active p38 MAPK inhibitor SD282 could arrest progression of LPS-induced alveolar bone destruction in rats.

METHODS

Three groups of female Sprague-Dawley rats received LPS injections to the palatal molar gingiva three times per week for 4 weeks to establish periodontitis. From weeks 5 through 8, two groups received the drug SD282 (N = 14) or 1% polyethylene glycol drug vehicle (N = 14) via oral gavage in addition to LPS injections. The third group continued to receive only LPS injections (N = 8). Microcomputed tomography was used to measure volumetric alveolar bone loss, expressed as bone volume fraction (BVF). Expression of interleukin (IL)-1 and -6 and tumor necrosis factor-alpha (TNF-alpha) was assessed by immunohistochemistry, and osteoclasts were enumerated by tartrate-resistant acid phosphatase staining.

RESULTS

By 4 weeks, severe alveolar bone resorption was seen in LPS-injected animals. Administration of SD282 significantly blocked additional volumetric bone loss in the LPS-only versus LPS + SD282 groups (0.37 +/- 0.01 BVF versus 0.43 +/- 0.01 BVF; P < 0.01). Significant reductions in IL-1beta (P < 0.01 ), TNF-alpha (P < 0.05), and osteoclast formation (P < 0.01) occurred in the presence of SD282.

CONCLUSIONS

An orally active p38 MAPK inhibitor reduced LPS-induced inflammatory cytokine expression, osteoclastogenesis, and alveolar bone loss in rats. Within the limits of the current study, SD282 arrested periodontal disease progression, thus highlighting the therapeutic potential of this novel class of inhibitors.

Salmonella sensing of anti-microbial mechanisms to promote survival within macrophages

Salmonella enterica is a facultative intracellular pathogen that replicates within macrophages. The interaction of this pathogen with mammalian cells is a complex process involving hundreds of bacterial products that are sensed by and alter mammalian hosts. Numerous bacterial genes and their protein products have been identified that are required for Salmonella to resist killing by host innate immunity and to modify host processes. Many of these genes are regulated by a specific bacterial sensor, the PhoQ protein, which responds to the acidified phagosome environment. PhoQ is a sensor histidine kinase, which when activated in vivo within acidified macrophage phagosomes, regulates cell surface modifications that promote resistance to antimicrobial peptides and oxidative stress, alter the phagosome to promote intracellular survival, and reduce innate immune recognition. In this review, we discuss mechanisms by which Salmonella interacts with macrophages and focus in detail on recent reports describing the role of antimicrobial peptides and pH in PhoQ activation.

An undecaprenyl phosphate-aminoarabinose flippase required for polymyxin resistance in Escherichia coli

Modification of lipid A with the 4-amino-4-deoxy-L-arabinose (L-Ara4N) moiety is required for resistance to polymyxin and cationic antimicrobial peptides in Escherichia coli and Salmonella typhimurium. An operon of seven genes (designated pmrHFIJKLM in S. typhimurium), which is regulated by the PmrA transcription factor and is also present in E. coli, is necessary for the maintenance of polymyxin resistance. We previously elucidated the roles of pmrHFIJK in the biosynthesis and attachment of L-Ara4N to lipid A and renamed these genes arn-BCADT, respectively. We now propose functions for the last two genes of the operon, pmrL and pmrM. Chromosomal inactivation of each of these genes in an E. coli pmrA(c) parent switched its phenotype from polymyxin-resistant to polymyxin-sensitive. Lipid A was no longer modified with L-Ara4N, even though the levels of the lipid-linked donor of the L-Ara4N moiety, undecaprenyl phosphate-alpha-L-Ara4N, were not reduced in the mutants. However, the undecaprenyl phosphate-alpha-L-Ara4N present in the mutants was less concentrated on the periplasmic surface of the inner membrane, as judged by 4-5-fold reduced labeling with the inner membrane-impermeable amine reagent N-hydroxysulfosuccin-imidobiotin. In an arnT mutant of the same pmrA(c) parent, which lacks the enzyme that transfers the L-Ara4N unit to lipid A but retains the same high levels of undecaprenyl phosphate-alpha-L-Ara4N as the parent, N-hydroxysulfosuccinimidobiotin labeling was not reduced. These results implicate pmrL and pmrM, but not arnT, in transporting undecaprenyl phosphate-alpha-L-Ara4N across the inner membrane. PmrM and PmrL, now renamed ArnE and ArnF because of their involvement in L-Ara4N modification of lipid A, may be subunits of an undecaprenyl phosphate-alpha-L-Ara4N flippase.

Periplasmic phosphorylation of lipid A is linked to the synthesis of undecaprenyl phosphate

One-third of the lipid A found in the Escherichia coli outer membrane contains an unsubstituted diphosphate unit at position 1 (lipid A 1-diphosphate). We now report an inner membrane enzyme, LpxT (YeiU), which specifically transfers a phosphate group to lipid A, forming the 1-diphosphate species. (32)P-labelled lipid A obtained from lpxT mutants do not produce lipid A 1-diphosphate. In vitro assays with Kdo(2)-[4'-(32)P]lipid A as the acceptor shows that LpxT uses undecaprenyl pyrophosphate as the substrate donor. Inhibition of lipid A 1-diphosphate formation in wild-type bacteria was demonstrated by sequestering undecaprenyl pyrophosphate with the cyclic polypeptide antibiotic bacitracin, providing evidence that undecaprenyl pyrophosphate serves as the donor substrate within whole bacteria. LpxT-catalysed phosphorylation is dependent upon transport of lipid A across the inner membrane by MsbA, a lipid A flippase, indicating a periplasmic active site. In conclusion, we demonstrate a novel pathway in the periplasmic modification of lipid A that is directly linked to the synthesis of undecaprenyl phosphate, an essential carrier lipid required for the synthesis of various bacterial polymers, such as peptidoglycan.

The PhoQ-activating potential of antimicrobial peptides contributes to antimicrobial efficacy and is predictive of the induction of bacterial resistance

Antimicrobial peptides (AMPs) are among the leading candidates to replace antibiotics which have been rendered ineffective by the evolution of resistant bacterial strains. Concerns do exist, however, that the therapeutic administration of AMPs may also select for resistant strains but with much more dire consequences, as these peptides represent an endogenous and essential component of host immune defense. The recent demonstration that AMPs function as ligands for the bacterial sensory kinase PhoQ for the initiation of virulence and adaptive responses lends credence to these concerns. While the ability to serve as PhoQ ligands suggests that the therapeutic administration of AMPs could (i) exacerbate infections by promoting bacterial virulence and (ii) select resistant mutants by encouraging adaptive behaviors, it also provides a rational basis for AMP selection and optimization. Here, we demonstrate that derivatives of a representative AMP have differential abilities to serve as PhoQ ligands and that this correlates with the ability to induce bacterial adaptive responses. We propose that PhoQ-activating potential is a logical parameter for AMP optimization and introduce a novel strategy for the treatment of minimal bactericidal concentration data that permits the discrimination and quantification of the contributions of PhoQ-activating potential and direct antimicrobial activity to net antimicrobial efficiency.

PhoPQ-mediated regulation produces a more robust permeability barrier in the outer membrane of Salmonella enterica serovar typhimurium

The PhoPQ two-component system of Salmonella enterica serovar Typhimurium produces a remodeling of the lipid A domain of the lipopolysaccharide, including the PagP-catalyzed addition of palmitoyl residue, the PmrAB-regulated addition of the cationic sugar 4-aminoarabinose and phosphoethanolamine, and the LpxO-catalyzed addition of a 2-OH group onto one of the fatty acids. By using the diffusion rates of the dyes ethidium, Nile red, and eosin Y across the outer membrane, as well as the susceptibility of cells to large, lipophilic agents, we evaluated the function of this membrane as a permeability barrier. We found that the remodeling process in PhoP-constitutive strains produces an outer membrane that serves as a very effective permeability barrier in an environment that is poor in divalent cations or that contains cationic peptides, whereas its absence in phoP null mutants produces an outer membrane severely compromised in its barrier function under these conditions. Removing combinations of the lipid A-remodeling functions from a PhoP-constitutive strain showed that the known modification reactions explain a major part of the PhoPQ-regulated changes in permeability. We believe that the increased barrier property of the remodeled bilayer is important in making the pathogen more resistant to the stresses that it encounters in the host, including attack by the cationic antimicrobial peptides. On the other hand, drug-induced killing assays suggest that the outer membrane containing unmodified lipid A may serve as a better barrier in the presence of high concentrations (e.g., 5 mM) of Mg(2+).

Crystal structures of the receiver domain of the response regulator PhoP from Escherichia coli in the absence and presence of the phosphoryl analog beryllofluoride

The response regulator PhoP is part of the PhoQ/PhoP two-component system involved in responses to depletion of extracellular Mg(2+). Here, we report the crystal structures of the receiver domain of Escherichia coli PhoP determined in the absence and presence of the phosphoryl analog beryllofluoride. In the presence of beryllofluoride, the active receiver domain forms a twofold symmetric dimer similar to that seen in structures of other regulatory domains from the OmpR/PhoB family, providing further evidence that members of this family utilize a common mode of dimerization in the active state. In the absence of activating agents, the PhoP receiver domain crystallizes with a similar structure, consistent with the previous observation that high concentrations can promote an active state of PhoP independent of phosphorylation.

Identification of an orphan response regulator required for the virulence of Francisella spp. and transcription of pathogenicity island genes

Francisella tularensis is a category A agent of biowarfare/biodefense. Little is known about the regulation of virulence gene expression in Francisella spp. Comparatively few regulatory factors exist in Francisella, including those belonging to two-component systems (TCS). However, orphan members of typical TCS can be identified. To determine if orphan TCS members affect Francisella gene expression, a gene encoding a product with high similarity to the Salmonella PmrA response regulator (FTT1557c/FNU0663.2) was deleted in Francisella novicida (a model organism for F. tularensis). The F. novicida pmrA mutant was defective in survival/growth within human and murine macrophage cell lines and was 100% defective in virulence in mice at a dose of up to 10(8) CFU. In addition, the mutant strain demonstrated increased susceptibility to antimicrobial peptide killing, but no differences were observed between the lipid A of the mutant and the parental strain, as has been observed with pmrA mutants of other microbes. The F. novicida pmrA mutant was 100% protective as a single-dose vaccine when challenge was with 10(6) CFU of F. novicida but did not protect against type A Schu S4 wild-type challenge. DNA microarray analysis identified 65 genes regulated by PmrA. The majority of these genes were located in the region surrounding pmrA or within the Francisella pathogenicity island (FPI). These FPI genes are also regulated by MglA, but MglA does not regulate pmrA, nor does PmrA regulate MglA. Thus, the orphan response regulator PmrA is an important factor in controlling virulence in F. novicida, and a pmrA mutant strain is an effective vaccine against homologous challenge.

T cell immunity evasion by virulent Salmonella enterica

Salmonella enterica are Gram-negative bacteria that cause systemic disease in their specific hosts. One of the recently appreciated features of Salmonella pathogenicity is the capacity of the bacteria to impair host adaptive immunity by interfering with DC function and T cell activation. It is likely that this feature of virulent Salmonella is needed to promote systemic dissemination in the host. Recent studies have suggested explanations for some of the molecular mechanisms developed by virulent Salmonella to impair DC and T cell function. Several of these mechanisms require the expression of virulence genes encoded within Salmonella pathogenicity islands. Targeted deletion of these genes diminishes Salmonella pathogenicity and leads to efficient activation of T cells by Salmonella-infected DCs. In this review, recent data that support the subversion of DC function by Salmonella as a means to evade host adaptive immunity and cause systemic infection are discussed. These new findings suggest a new pathogenesis model with DCs as key targets for Salmonella virulence factors.

Structural heterogeneity and environmentally regulated remodeling of Francisella tularensis subspecies novicida lipid A characterized by tandem mass spectrometry

The structural characterization of environmentally-regulated lipid A derived from Francisella tularensis subspecies novicida (Fn) U112 is described using negative electrospray ionization with a linear ion trap Fourier transform ion cyclotron resonance (IT-FT-ICR) hybrid mass spectrometer. The results indicate that a unique profile of lipid A molecular structures are synthesized in response to Fn growth at 25 degrees C versus 37 degrees C. Molecular species were found to be tetra-acylated, sharing a conserved glucosamine disaccharide backbone, a galactosamine-1-phosphate linked to the reducing glucosamine, and multiple O- and N-linked fatty acyl groups. Deprotonated molecules were interrogated by MS(n) scanning techniques at both high and nominal mass resolution and were found to be complex heterogeneous mixtures where structures differed based on the positions and identities of the O- and N-linked fatty acyl substituents. For the dominant ion series, which consisted of five peaks, 30 unique lipid A structures were identified. Estimates for the relative abundance of each structure were derived from MS relative abundance ratios and fragment ion ratios from comparable dissociation pathways from MS(2) through MS(4) experiments. The results suggest a remodeling pathway in which the amide linked fatty acid of the reducing glucosamine favors a 3-hydroxyhexadecanoic acid substituent for growth conditions at 25 degrees C versus a 3-hydroxyoctadecanoic acid substituent for growth conditions at 37 degrees C.

Cytosolic expression of SecA2 is a prerequisite for long-term protective immunity

Induction of efficient adaptive T cell-mediated immunity against the intracellular bacterium Listeria monocytogenes requires its successful invasion of host cell cytosol. However, it is not clear whether its cytosolic escape and growth are sufficient to induce T cell-mediated clearance and protection upon secondary infection. To investigate this issue, we have searched for mutants that do not induce long-term protective immunity yet invade the cytosol of infected cells. We found that mice immunized with L. monocytogenes lacking the SecA2 ATPase, an auxiliary protein secretion system present in several Gram-positive pathogenic bacteria, mounted a robust cytolytic IFN-gamma-secreting CD8+ T cell response but were not protected against a secondary challenge with wild-type (wt) bacteria. Furthermore, CD8+ T cells from mice immunized with secA2- bacteria failed to transfer protection when injected into recipient mice demonstrating that they were unable to confer protection. Also, secA2- and wt L. monocytogenes spread to the same myeloid-derived cell types in vivo and SecA2 deficiency does not interfere with intracytosolic bacteria multiplication. Therefore, cytosol invasion is not sufficient for inducing secondary protective responses and induction of memory CD8+ T cells mediating long-term antibacterial protective immunity is dependent upon SecA2 expression inside the cytosol of host cells in vivo.

Release of the lipopolysaccharide deacylase PagL from latency compensates for a lack of lipopolysaccharide aminoarabinose modification-dependent resistance to the antimicrobial peptide polymyxin B in Salmonella enterica

Salmonella enterica modifies its lipopolysaccharide (LPS), including the lipid A portion, to adapt to its environments. The lipid A 3-O-deacylase PagL exhibits latency; deacylation of lipid A is not usually observed in vivo despite the expression of PagL, which is under the control of a two-component regulatory system, PhoP-PhoQ. In contrast, PagL is released from latency in pmrA and pmrE mutants, both of which are deficient in aminoarabinose-modified lipid A, although the biological significance of this is not clear. The attachment of aminoarabinose to lipid A decreases the net anionic charge at the membrane's surface and reduces electrostatic repulsion between neighboring LPS molecules, leading to increases in bacterial resistance to cationic antimicrobial peptides, including polymyxin B. Here we examined the effects of the release of PagL from latency on resistance to polymyxin B. The pmrA pagL and pmrE pagL double mutants were more susceptible to polymyxin B than were the parental pmrA and pmrE mutants, respectively. Furthermore, introduction of the PagL expression plasmid into the pmrA pagL double mutant increased the resistance to polymyxin B. In addition, PagL-dependent deacylation of lipid A was observed in a mutant in which lipid A could not be modified with phosphoethanolamine, which partly contributes to the PmrA-dependent resistance to polymyxin B. These results, taken together, suggest that the release of PagL from latency compensates for the loss of resistance to polymyxin B that is due to a lack of other modifications to LPS.

Modulation of innate immune responses with synthetic lipid A derivatives

The lipid A moiety of lipopolysaccharides (LPS) initiates innate immune responses by interacting with Toll-like receptor 4 (TLR4), which results in the production of a wide range of cytokines. Derivatives of lipid A show potential for use as immuno-modulators for the treatment of a wide range of diseases and as adjuvants for vaccinations. Development to these ends requires a detailed knowledge of patterns of cytokines induced by a wide range of derivatives. This information is difficult to obtain by using isolated compounds due to structural heterogeneity and possible contaminations with other inflammatory components. To address this problem, we have developed a synthetic approach that provides easy access to a wide range of lipid A's by employing a common disaccharide building block functionalized with a versatile set of protecting groups. The strategy was employed for the preparation of lipid A's derived from E. coli and S. typhimurium. Mouse macrophages were exposed to the synthetic compounds and E. coli 055:B5 LPS, and the resulting supernatants were examined for tumor necrosis factor alpha (TNF-alpha), interferon beta (IFN-beta), interleukin 6 (IL-6), interferon-inducible protein 10 (IP-10), RANTES, and IL-1beta. It was found that for each compound, the potencies (EC50 values) for the various cytokines differed by as much as 100-fold. These differences did not follow a bias toward a MyD88- or TRIF-dependent response. Instead, it was established that the observed differences in potencies of secreted TNF-alpha and IL-1beta were due to differences in the processing of respective pro-proteins. Examination of the efficacies (maximum responses) of the various cytokines showed that each synthetic compound and E. coli 055:B5 LPS induced similar efficacies for the production of IFN-beta and IP-10. However, lipid A's 1-4 gave lower efficacies for the production of RANTES and IL-6 as compared to LPS. Collectively, the presented results demonstrate that cytokine secretion induced by LPS and lipid A is complex, which can be exploited for the development of immuno-modulating therapies.

pmrA(Con) confers pmrHFIJKL-dependent EGTA and polymyxin resistance on msbB Salmonella by decorating lipid A with phosphoethanolamine

Mutations in pmrA were recombined into Salmonella strain ATCC 14028 msbB to determine if pmrA-regulated modifications of lipopolysaccharide could suppress msbB growth defects. A mutation that functions to constitutively activate pmrA [pmrA(Con)] suppresses msbB growth defects on EGTA-containing media. Lipid A structural analysis showed that Salmonella msbB pmrA(Con) strains, compared to Salmonella msbB strains, have increased amounts of palmitate and phosphoethanolamine but no aminoarabinose addition, suggesting that aminoarabinose is not incorporated into msbB lipid A. Surprisingly, loss-of-function mutations in the aminoarabinose biosynthetic genes restored EGTA and polymyxin sensitivity to Salmonella msbB pmrA(Con) strains. These blocks in aminoarabinose biosynthesis also prevented lipid A phosphoethanolamine incorporation and reduced the levels of palmitate addition, indicating previously unknown roles for the aminoarabinose biosynthetic enzymes. Lipid A structural analysis of the EGTA- and polymyxin-resistant triple mutant msbB pmrA(Con) pagP::Tn10, which contains phosphoethanolamine but no palmitoylated lipid A, suggests that phosphoethanolamine addition is sufficient to confer EGTA and polymyxin resistance on Salmonella msbB strains. Additionally, palmitoylated lipid A was observed only in wild-type Salmonella grown in the presence of salt in rich media. Thus, we correlate EGTA resistance and polymyxin resistance with phosphoethanolamine-decorated lipid A and demonstrate that the aminoarabinose biosynthetic proteins play an essential role in lipid A phosphoethanolamine addition and affect lipid A palmitate addition in Salmonella msbB strains.

Mutualism and pathogenesis in Xenorhabdus and Photorhabdus: two roads to the same destination

Photorhabdus and Xenorhabdus bacteria colonize the intestines of the infective soil-dwelling stage of entomophagous nematodes, Heterorhabditis and Steinernema, respectively. These nematodes infect susceptible insect larvae and release the bacteria into the insect blood. The bacteria kill the insect larvae and convert the cadaver into a food source suitable for nematode growth and development. After several rounds of reproduction the nematodes are recolonized by the bacteria before emerging from the insect cadaver into the soil to search for a new host. Photorhabdus and Xenorhabdus bacteria therefore engage in both pathogenic and mutualistic interactions with different invertebrate hosts as obligate components of their life cycle. In this review we aim to describe current knowledge of the molecular mechanisms utilized by Photorhabdus and Xenorhabdus to control their host-dependent interactions. Recent work has established that there is a trade-off between pathogenicity and mutualism in both these species of bacteria suggesting that the transition between these interactions must be under regulatory control. Despite the superficial similarity between the life cycles of these bacteria, it is now apparent that the molecular components of the regulatory networks controlling pathogenicity and mutualism in Photorhabdus and Xenorhabdus are very different.

A putative gene cluster for aminoarabinose biosynthesis is essential for Burkholderia cenocepacia viability

Using a conditional mutagenesis strategy we demonstrate here that a gene cluster encoding putative aminoarabinose (Ara4N) biosynthesis enzymes is essential for the viability of Burkholderia cenocepacia. Loss of viability is associated with dramatic changes in bacterial cell morphology and ultrastructure, increased permeability to propidium iodide, and sensitivity to sodium dodecyl sulfate, suggesting a general cell envelope defect caused by the lack of Ara4N.

Actinobacillus actinomycetemcomitans lipopolysaccharide-mediated experimental bone loss model for aggressive periodontitis

BACKGROUND

Bacterial constituents, such as Gram-negative derived lipopolysaccharide (LPS), can initiate inflammatory bone loss through induction of host-derived inflammatory cytokines. The aim of this study was to establish a model of aggressive inflammatory alveolar bone loss in rats using LPS derived from the periodontal pathogen Actinobacillus actinomycetemcomitans.

METHODS

Eighteen female Sprague-Dawley rats were divided into LPS test (N = 12) and saline control (N = 6) groups. All animals received injections to the palatal molar gingiva three times per week for 8 weeks. At 8 weeks, linear and volumetric alveolar bone loss was measured by micro-computed tomography (microCT). The prevalence of inflammatory infiltrate, proinflammatory cytokines, and osteoclasts was assessed from hematoxylin and eosin, immunohistochemical, or tartrate-resistant acid phosphatase (TRAP)-stained sections. Statistical analysis was performed.

RESULTS

A. actinomycetemcomitans LPS induced severe bone loss over 8 weeks, whereas control groups were unchanged. Linear and volumetric analysis of maxillae by microCT indicated significant loss of bone with LPS administration. Histologic examination revealed increased inflammatory infiltrate, significantly increased immunostaining for interleukin IL-6 and -1beta and tumor necrosis factor-alpha, and more TRAP-positive osteoclasts in the LPS group compared to controls.

CONCLUSION

Oral injections of LPS derived from the periodontal pathogen A. actinomycetemcomitans can induce severe alveolar bone loss and proinflammatory cytokine production in rats by 8 weeks.

Stimulus perception in bacterial signal-transducing histidine kinases

Two-component signal-transducing systems are ubiquitously distributed communication interfaces in bacteria. They consist of a histidine kinase that senses a specific environmental stimulus and a cognate response regulator that mediates the cellular response, mostly through differential expression of target genes. Histidine kinases are typically transmembrane proteins harboring at least two domains: an input (or sensor) domain and a cytoplasmic transmitter (or kinase) domain. They can be identified and classified by virtue of their conserved cytoplasmic kinase domains. In contrast, the sensor domains are highly variable, reflecting the plethora of different signals and modes of sensing. In order to gain insight into the mechanisms of stimulus perception by bacterial histidine kinases, we here survey sensor domain architecture and topology within the bacterial membrane, functional aspects related to this topology, and sequence and phylogenetic conservation. Based on these criteria, three groups of histidine kinases can be differentiated. (i) Periplasmic-sensing histidine kinases detect their stimuli (often small solutes) through an extracellular input domain. (ii) Histidine kinases with sensing mechanisms linked to the transmembrane regions detect stimuli (usually membrane-associated stimuli, such as ionic strength, osmolarity, turgor, or functional state of the cell envelope) via their membrane-spanning segments and sometimes via additional short extracellular loops. (iii) Cytoplasmic-sensing histidine kinases (either membrane anchored or soluble) detect cellular or diffusible signals reporting the metabolic or developmental state of the cell. This review provides an overview of mechanisms of stimulus perception for members of all three groups of bacterial signal-transducing histidine kinases.

[Outer membrane remodeling of Salmonella typhimurium and host innate immunity]

Pathogenic gram-negative bacteria, including Salmonella typhimurium, remodel their outer membrane to survive within host tissues and phagosomes. The remodeling includes modifications of lipid A, a membrane anchor portion of lipopolysaccharide. Lipid A modifications, such as palmitoylation, deacylation, addition of aminoarabinose, and addition of phosphoethanolamine, are beneficial for salmonellae to resist host innate immunity. Aminoarabinose attachment, phosphoethanolamine attachment, and palmitoylation of lipid A increase salmonellae resistance to cationic antimicrobial peptides. Lipid A deacylation and palmitoylation reduce its ability to activate the Toll-like receptor 4-MD-2 complex, suggesting that these modifications are beneficial for salmonellae to evade host innate immune recognition. These modifications are regulated transcriptionally by the two-component regulatory system PhoP-PhoQ, which is essential for S. typhimurium virulence. Lipid A modifications are also regulated posttranslationally. Aminoarabinose modification of lipid A represses deacylation of lipid A by PagL. The posttranslational regulation may be involved in S. typhimurium pathogenesis.

What is the role of Toll-like receptors in bacterial infections?

Innate immunity relies on signalling by Toll-like receptors (TLRs) to alert the immune system of the presence of invading bacteria. TLR activation leads to the release of cytokines that allow for effective innate and adaptive immune responses. However, the contribution of different TLRs depends on the site of the infection and the pathogen. This review will describe the involvement of TLRs in the development of three different bacterial infections as well as our current understanding of the role of TLRs during microbial pathogenesis.

Lipid A modification systems in gram-negative bacteria

The lipid A moiety of lipopolysaccharide forms the outer monolayer of the outer membrane of most gram-negative bacteria. Escherichia coli lipid A is synthesized on the cytoplasmic surface of the inner membrane by a conserved pathway of nine constitutive enzymes. Following attachment of the core oligosaccharide, nascent core-lipid A is flipped to the outer surface of the inner membrane by the ABC transporter MsbA, where the O-antigen polymer is attached. Diverse covalent modifications of the lipid A moiety may occur during its transit from the outer surface of the inner membrane to the outer membrane. Lipid A modification enzymes are reporters for lipopolysaccharide trafficking within the bacterial envelope. Modification systems are variable and often regulated by environmental conditions. Although not required for growth, the modification enzymes modulate virulence of some gram-negative pathogens. Heterologous expression of lipid A modification enzymes may enable the development of new vaccines.

Hierarchical gene expression profiles of HUVEC stimulated by different lipid A structures obtained from Porphyromonas gingivalis and Escherichia coli

The ability of lipid A structural variants to elicit unique endothelial cell gene expression was examined by measuring global gene expression profiles in human umbilical cord vein endothelial cells (HUVEC) using Affymetrix full genome chips. Two lipid A structural variants obtained from Porphyromonas gingivalis designated PgLPS(1435/1449) and PgLPS(1690) as well as LPS obtained from Escherichia coli wild type and an E. coli msbB mutant (missing myristic acid in the lipid A) were examined. Each of these lipid A structures has been shown to interact with TLR4; however, PgLPS(1435/1449) and E. coli msbB LPS have been shown to be TLR4 antagonists while PgLPS(1690) and wild-type E. coli LPS are TLR4 agonists. It was found that PgLPS(1435/1449) and PgLPS(1690) as well as E. coli msbB LPS activated a subset of those genes significantly transcribed in response to E. coli wild-type LPS. Furthermore, the subset of genes expressed in response to the different lipid A structural forms were those most significantly activated by wild-type E. coli LPS demonstrating a hierarchy in TLR4-dependent endothelial cell gene activation. A unique gene expression profile for the weak TLR4 agonist PgLPS(1690) was observed and represents a TLR4 hierarchy in endothelial cell gene activation.

The non-typeable Haemophilus influenzae Sap transporter provides a mechanism of antimicrobial peptide resistance and SapD-dependent potassium acquisition

We have shown that non-typeable Haemophilus influenzae (NTHI) resists killing by antimicrobial peptides (APs). A mutant defective in expression of the sap (sensitivity to antimicrobial peptides) gene cluster product SapA is sensitive to killing by APs and is significantly attenuated in its ability to survive in a chinchilla model of otitis media compared with the parent strain. In NTHI, SapA is believed to function as the periplasmic solute binding protein of an ABC transporter. Here, we demonstrated that recombinant chinchilla beta defensin-1 specifically interacted with recombinant SapA and that AP exposure increased expression of the sap operon. We further demonstrated that the putative Sap transporter ATPase protein, SapD, was required for AP resistance as well as potassium uptake in NTHI strain 86-028NP. Loss of SapD additionally abrogated NTHI survival in vivo. Complementation of the sapD mutation restored the ability to grow in potassium-limited medium, resistance to AP-mediated killing and survival in vivo. Collectively, these data support a mechanism of Sap system-mediated resistance to APs that depends on Sap-dependent transport of APs and a Sap-dependent restoration of potassium homeostasis. Thus, NTHI required a functional Sap system to mediate bacterial survival and pathogenesis in vivo.

Salmonella typhimurium coordinately regulates FliC location and reduces dendritic cell activation and antigen presentation to CD4+ T cells

During infection, Salmonella transitions from an extracellular-phase (STEX, growth outside host cells) to an intracellular-phase (STIN, growth inside host cells): changes in gene expression mediate survival in the phagosome and modifies LPS and outer membrane protein expression, including altered production of FliC, an Ag recognized by immune CD4+ T cells. Previously, we demonstrated that systemic STIN bacteria repress FliC below the activation threshold of FliC-specific T cells. In this study, we tested the hypothesis that changes in FliC compartmentalization and bacterial responses triggered during the transition from STEX to STIN combine to reduce the ability of APCs to present FliC to CD4+ T cells. Approximately 50% of the Salmonella-specific CD4+ T cells from Salmonella-immune mice were FliC specific and produced IFN-gamma, demonstrating the potent immunogenicity of FliC. FliC expressed by STEX bacteria was efficiently presented by splenic APCs to FliC-specific CD4+ T cells in vitro. However, STIN bacteria, except when lysed, expressed FliC within a protected intracellular compartment and evaded stimulation of FliC-specific T cells. The combination of STIN-mediated responses that reduced FliC bioavailability were overcome by dendritic cells (DCs), which presented intracellular FliC within heat-killed bacteria; however, this ability was abrogated by live bacterial infection. Furthermore, STIN bacteria, unlike STEX, limited DC activation as measured by increased MHC class II, CD86, TNF-alpha, and IL-12 expression. These data indicate that STIN bacteria restrict FliC bioavailability by Ag compartmentalization, and together with STIN bacterial responses, limit DC maturation and cytokine production. Together, these mechanisms may restrain DC-mediated activation of FliC-specific CD4+ T cells.

Structural characterization of the lipid A region of Aeromonas salmonicida subsp. salmonicida lipopolysaccharide

The lipid A components of Aeromonas salmonicida subsp. salmonicida from strains A449, 80204-1 and an in vivo rough isolate were isolated by mild acid hydrolysis of the lipopolysaccharide. Structural studies carried out by a combination of fatty acid, electrospray ionization-mass spectrometry and nuclear magnetic resonance analyses confirmed that the structure of lipid A was conserved among different isolates of A. salmonicida subsp. salmonicida. All analyzed strains contained three major lipid A molecules differing in acylation patterns corresponding to tetra-, penta- and hexaacylated lipid A species and comprising 4'-monophosphorylated beta-2-amino-2-deoxy-d-glucopyranose-(1-->6)-2-amino-2-deoxy-d-glucopyranose disaccharide, where the reducing end 2-amino-2-deoxy-d-glucose was present primarily in the alpha-pyranose form. Electrospray ionization-tandem mass spectrometry fragment pattern analysis, including investigation of the inner-ring fragmentation, allowed the localization of fatty acyl residues on the disaccharide backbone of lipid A. The tetraacylated lipid A structure containing 3-(dodecanoyloxy)tetradecanoic acid at N-2',3-hydroxytetradecanoic acid at N-2 and 3-hydroxytetradecanoic acid at O-3, respectively, was found. The pentaacyl lipid A molecule had a similar fatty acid distribution pattern and, additionally, carried 3-hydroxytetradecanoic acid at O-3'. In the hexaacylated lipid A structure, 3-hydroxytetradecanoic acid at O-3' was esterified with a secondary 9-hexadecenoic acid. Interestingly, lipid A of the in vivo rough isolate contained predominantly tetra- and pentaacylated lipid A species suggesting that the presence of the hexaacyl lipid A was associated with the smooth-form lipopolysaccharide.

Protective and destructive immunity in the periodontium: Part 1--innate and humoral immunity and the periodontium

Based on the results of recent research in the field, the present paper will discuss the protective and destructive aspects of the innate vs. adaptive (humoral and cell-mediated) immunity associated with the bacterial virulent factors or antigenic determinants during periodontal pathogenesis. Attention will be focused on: (i) the Toll-like receptors (TLR), the innate immune repertoire for recognizing the unique molecular patterns of microbial components that trigger innate and adaptive immunity for effective host defenses, in some general non-oral vs. periodontal microbial infections; (ii) T-cell-mediated immunity, Th-cytokines, and osteoclastogenesis in periodontal disease progression; and (iii) some molecular techniques developed and used to identify critical microbial virulence factors or antigens associated with host immunity (using Actinobacillus actinomycetemcomitans and Porphyromonas gingivalis as the model species). Therefore, further understanding of the molecular interactions and mechanisms associated with the host's innate and adaptive immune responses will facilitate the development of new and innovative therapeutics for future periodontal treatments.