Staphylococcus aureus agr and sarA functions are required for invasive infection but not inflammatory responses in the lung

Quorum sensing in bacterial virulence

Bacteria communicate through the production of diffusible signal molecules termed autoinducers. The molecules are produced at basal levels and accumulate during growth. Once a critical concentration has been reached, autoinducers can activate or repress a number of target genes. Because the control of gene expression by autoinducers is cell-density-dependent, this phenomenon has been called quorum sensing. Quorum sensing controls virulence gene expression in numerous micro-organisms. In some cases, this phenomenon has proven relevant for bacterial virulence in vivo. In this article, we provide a few examples to illustrate how quorum sensing can act to control bacterial virulence in a multitude of ways. Several classes of autoinducers have been described to date and we present examples of how each of the major types of autoinducer can be involved in bacterial virulence. As quorum sensing controls virulence, it has been considered an attractive target for the development of new therapeutic strategies. We discuss some of the new strategies to combat bacterial virulence based on the inhibition of bacterial quorum sensing systems.

Changes in the agr locus affect enteritis caused by methicillin-resistant Staphylococcus aureus

We studied the characteristics of methicillin (meticillin)-resistant Staphylococcus aureus (MRSA) strains that caused enteritis. In a previous report, we demonstrated that both phenotypic and genotypic changes were associated with MRSA enteritis; and we hypothesized that the accessory gene regulator (agr), which is a global regulator of staphylococcal virulence and upregulates several exoproteins, is the key factor associated with the development of MRSA enteritis. In this study, we examined 12 MRSA isolates associated with enteritis from stool samples and 17 MRSA isolates not associated with enteritis that had the following characteristics: the strains associated with enteritis had the same genotype (genotype A), as detected by pulsed-field gel electrophoresis, or the strains were isolated from stools. The differences between strains that caused enteritis and those that did not cause enteritis strains were examined by quantitative reverse transcription-PCR to assess RNAII, agrA, RNAIII, and tst expression and by sequencing of the agr locus. The levels of expression of agrA, RNAIII, and tst were higher by the MRSA isolates associated with enteritis than by the MRSA isolates not associated with enteritis, whether or not they were of the same genotype. The levels of expression of RNAII by almost all the clinical isolates were similar. Sequencing of the agr locus showed that all MRSA isolates that caused enteritis have agr mutations, whereas the MRSA isolates that did not cause enteritis, with three exceptions, did not. Many of the isolates associated with enteritis had the same mutation, especially at the C-terminal end of agrA. These results suggest a trend in which mutations in the agr locus modify the expression of agrA and RNAIII and the production of toxin, all of which may increase the virulence and influence the occurrence of MRSA enteritis.

Neutrophil depletion causes a fatal defect in murine pulmonary Staphylococcus aureus clearance

BACKGROUND

Staphylococcus aureus is the most common cause of healthcare-associated pneumonia. Despite the significant morbidity and mortality associated with the disease, animal models of S. aureus pneumonia are rare.

MATERIALS AND METHODS

We examined the pathogenicity of four different strains of S. aureus (both methicillin-sensitive and -resistant as well as Panton-Valentine leukocidin-positive and -negative) in four strains of immunocompetent inbred and outbred mice (FVB/N, C57Bl/6, BALB/c, ND4; n = 148). The immunological basis for the development of murine S. aureus pneumonia was then determined by selectively depleting neutrophils, lymphocytes, or pulmonary macrophages prior to the onset of infection. An additional cohort of animals was rendered immunosuppressed by induction of abdominal sepsis via cecal ligation and puncture 2, 4, or 7 d prior to the onset of pneumonia.

RESULTS

Nearly all immunocompetent mice survived, regardless of which strain of S. aureus was used or which strain of mouse was infected. Among animals with immune depletion or prior immunosuppression, survival was decreased only following neutrophil depletion (26% versus 90% alive at 7 d, P < 0.0001). Compared to immunocompetent animals, neutrophil-depleted mice with S. aureus pneumonia had delayed pulmonary bacterial clearance at 16 and 40 h but had no difference in levels of bacteremia. Neutrophil-depleted mice also had elevated levels of pulmonary monocyte chemotactic protein-1 (822 pg/mL versus 150 pg/mL, P < 0.05). In contrast, pulmonary histological appearance was similar in both groups as was dry/wet lung weight.

CONCLUSIONS

These results suggest that neutrophils play a critical role in the host response to S. aureus pneumonia, and the survival differences observed in neutrophil-depleted mice are associated with alterations in bacterial clearance and pulmonary cytokine response.

Cell surface O-glycans limit Staphylococcus aureus adherence to corneal epithelial cells

The mucin-rich environment of the intact corneal epithelium is thought to contribute to the prevention of Staphylococcus aureus infection. This study examined whether O-glycans, which constitute the majority of the mucin mass of epithelial cell glycocalyces, prevented bacterial adhesion and growth. Abrogation of mucin O glycosylation using the chemical primer benzyl-alpha-GalNAc resulted in increased adherence of parental strain RN6390 to apical human corneal-limbal epithelial (HCLE) cells and to biotinylated cell surface protein in static and liquid phase adhesion assays, consistent with a role of mucin O-glycans in preventing bacterial adhesion. Comparable results were found with ALC135, an isogenic mutant strain defective in the accessory gene regulators agr and sar, indicating that the agr- and/or sar-regulated virulence factors did not play a major role in mediating adhesion to the corneal cell surface after mucin O-glycan truncation. In exoglycosidase digestion studies, treatment with sialidase from Arthrobacter ureafaciens--which hydrolyzed mucin-associated O-acetyl sialic acid--but not from Clostridium perfringens resulted in an increase in RN6390 and ALC135 adhesion. Abrogation of mucin O glycosylation in HCLE cell cultures did not affect bacterial growth. Overall, these data indicate that mucin O-glycans contribute to the prevention of bacterial adherence to the apical surface of corneal epithelial cells and suggest that alteration of cell surface glycosylation from disease or trauma, including that stemming from contact lens wear, could contribute to a higher risk of infection.

Staphylococcus aureus elicits marked alterations in the airway proteome during early pneumonia

Pneumonia caused by Staphylococcus aureus is a growing concern in the health care community. We hypothesized that characterization of the early innate immune response to bacteria in the lungs would provide insight into the mechanisms used by the host to protect itself from infection. An adult mouse model of Staphylococcus aureus pneumonia was utilized to define the early events in the innate immune response and to assess the changes in the airway proteome during the first 6 h of pneumonia. S. aureus actively replicated in the lungs of mice inoculated intranasally under anesthesia to cause significant morbidity and mortality. By 6 h postinoculation, the release of proinflammatory cytokines caused effective recruitment of neutrophils to the airway. Neutrophil influx, loss of alveolar architecture, and consolidated pneumonia were observed histologically 6 h postinoculation. Bronchoalveolar lavage fluids from mice inoculated with phosphate-buffered saline (PBS) or S. aureus were depleted of overabundant proteins and subjected to strong cation exchange fractionation followed by liquid chromatography and tandem mass spectrometry to identify the proteins present in the airway. No significant changes in response to PBS inoculation or 30 min following S. aureus inoculation were observed. However, a dramatic increase in extracellular proteins was observed 6 h postinoculation with S. aureus, with the increase dominated by inflammatory and coagulation proteins. The data presented here provide a comprehensive evaluation of the rapid and vigorous innate immune response mounted in the host airway during the earliest stages of S. aureus pneumonia.

Discovery of a quorum-sensing inhibitor of drug-resistant staphylococcal infections by structure-based virtual screening

Staphylococci are a major health threat because of increasing resistance to antibiotics. An alternative to antibiotic treatment is preventing virulence by inhibition of bacterial cell-to-cell communication using the quorum-sensing inhibitor RNAIII-inhibiting peptide (RIP). In this work, we identified 2',5-di-O-galloyl-d-hamamelose (hamamelitannin) as a nonpeptide analog of RIP by virtual screening of a RIP-based pharmacophore against a database of commercially available small-molecule compounds. Hamamelitannin is a natural product found in the bark of Hamamelis virginiana (witch hazel), and it has no effect on staphylococcal growth in vitro; but like RIP, it does inhibit the quorum-sensing regulator RNAIII. In a rat graft model, hamamelitannin prevented device-associated infections in vivo, including infections caused by methicillin-resistant Staphylococcus aureus and Staphylococcus epidermidis strains. These findings suggest that hamamelitannin may be used as a suppressor to staphylococcal infections.

Host airway proteins interact with Staphylococcus aureus during early pneumonia

Staphylococcus aureus is a major cause of hospital-acquired pneumonia and is emerging as an important etiological agent of community-acquired pneumonia. Little is known about the specific host-pathogen interactions that occur when S. aureus first enters the airway. A shotgun proteomics approach was utilized to identify the airway proteins associated with S. aureus during the first 6 h of infection. Host proteins eluted from bacteria recovered from the airways of mice 30 min or 6 h following intranasal inoculation under anesthesia were subjected to liquid chromatography and tandem mass spectrometry. A total of 513 host proteins were associated with S. aureus 30 min and/or 6 h postinoculation. A majority of the identified proteins were host cytosolic proteins, suggesting that S. aureus was rapidly internalized by phagocytes in the airway and that significant host cell lysis occurred during early infection. In addition, extracellular matrix and secreted proteins, including fibronectin, antimicrobial peptides, and complement components, were associated with S. aureus at both time points. The interaction of 12 host proteins shown to bind to S. aureus in vitro was demonstrated in vivo for the first time. The association of hemoglobin, which is thought to be the primary staphylococcal iron source during infection, with S. aureus in the airway was validated by immunoblotting. Thus, we used our recently developed S. aureus pneumonia model and shotgun proteomics to validate previous in vitro findings and to identify nearly 500 other proteins that interact with S. aureus in vivo. The data presented here provide novel insights into the host-pathogen interactions that occur when S. aureus enters the airway.

Peptidoglycan-mediated IL-8 expression in human alveolar type II epithelial cells requires lipid raft formation and MAPK activation

Staphylococcus aureus, a major sepsis-causing Gram-positive bacterium, invades pulmonary epithelial cells and causes lung diseases. In the lung, alveolar type II epithelial cells play an important role in innate immunity by secreting chemokines and antimicrobial peptides upon bacterial infection whereas type I cells mainly function in gas-exchange. In this study, we investigated the ability of S. aureus peptidoglycan (PGN) to induce expression of a chemokine, IL-8, in a human alveolar type II epithelial cell line, A549. PGN induces IL-8 mRNA and protein expression in a dose- and time-dependent manner. Supplementation of soluble CD14 further enhanced the PGN-induced IL-8 expression. Interestingly, PGN-induced IL-8 expression was inhibited by nystatin, a specific inhibitor for lipid rafts, but not by chlorpromazine, a specific inhibitor for clathrin-coated pits. Furthermore, PGN-induced IL-8 expression was attenuated by inhibitors for MAP kinases such as ERK, p38 kinase, and JNK/SAPK, whereas no inhibitory effect was observed by inhibitors for reactive oxygen species or protein kinase C. Electrophoretic mobility shift assay demonstrates that PGN increased the DNA binding of the transcription factors, AP-1 and NF-kappaB while minimally, NF-IL6, all of which are involved in the transcription of IL-8. Taken together, these results suggest that PGN induces IL-8 expression in a CD14-enhanced manner in human alveolar type II epithelial cells, through the formation of lipid rafts and the activation of MAP kinases, which ultimately leads to activation of AP-1, NF-kappaB, and NF-IL6.

Toxicogenomic response to chlorination includes induction of major virulence genes in Staphylococcus aureus

Despite the widespread use of chlorination for microbial control in aqueous environments, cellular response mechanisms of human pathogens, such as Staphylococcus aureus, against chlorination remain unknown. In this work, genome-wide transcriptional analysis was performed to elucidate cellular response of S. aureusto hypochlorous acid, an active antimicrobial product of chlorination in aqueous solution. Our results suggest that hypochlorous acid repressed transcription of genes involved in cell wall synthesis, membrane transport, protein synthesis, and primary metabolism, while amino acid synthesis genes were induced. Furthermore, hypochlorous acid induced transcription of genes encoding major virulence factors of S. aureus, such as exotoxins, hemolysins, leukocidins, coagulases, and surface adhesion proteins, which all play essential roles in staphylococcal virulence. This work implies that chlorination may stimulate production of virulence factors, which provides new insight into host-pathogen interactions and effects of chlorine application for microbial control.

Erosion from Staphylococcus aureus biofilms grown under physiologically relevant fluid shear forces yields bacterial cells with reduced avidity to collagen

An estimated 65% of infective diseases are associated with the presence of bacterial biofilms. Biofilm-issued planktonic cells promote blood-borne, secondary sites of infection by the inoculation of the infected sites with bacteria from the intravascular space. To investigate the potential role of early detachment events in initiating secondary infections, we studied the phenotypic attributes of Staphylococcus aureus planktonic cells eroding from biofilms with respect to expression of the collagen adhesin, CNA. The collagen-binding abilities of S. aureus have been correlated to the development of osteomyelitis and septic arthritis. In this study, we focused on the impact of CNA expression on S. aureus adhesion to immobilized collagen in vitro under physiologically relevant shear forces. In contrast to the growth phase-dependent adhesion properties characteristic of S. aureus cells grown in suspension, eroding planktonic cells expressed invariant and lower effective adhesion rates regardless of the age of the biofilm from which they originated. These results correlated directly with the surface expression level of CNA. However, subsequent analysis revealed no qualitative differences between biofilms initiated with suspension cells and secondary biofilms initiated with biofilm-shed planktonic cells. Taken together, our findings suggest that, despite their low levels of CNA expression, S. aureus planktonic cells shed from biofilms retain the capacity for metastatic spread and the initiation of secondary infection. These findings demonstrate the need for a better understanding of the phenotypic properties of eroding planktonic cells, which could lead to new therapeutic strategies to target secondary infections.

Staphylococcal alpha-toxin causes increased tracheal epithelial permeability

Staphylococcus aureus is an important cause of pulmonary infections. The role of S. aureus alpha-toxin as a virulence factor is unclear. We hypothesized that airway epithelium is a target of S. aureus alpha-toxin and that exposure of airway epithelium to alpha-toxin results in damage to the airway epithelium. To examine the hypothesis that alpha-toxin is capable of independently producing airway epithelium damage as measured by permeability and morphometry, an isolated whole mouse trachea test apparatus was developed. In vitro epithelial permeability (P) was calculated and digital micrographs were analyzed morphometrically. Purified S. aureus alpha-toxin produced a significant increase in tracheal epithelial P (P < 0.05). Morphometric analysis revealed the ratio of adherent tracheal epithelium attached to the basement membrane divided by the total length of the basement membrane decreased in a dose-dependent manner with 1 microg/ml alpha-toxin and 10 microg/ml alpha-toxin (P < 0.05). We developed a novel isolated whole mouse trachea test apparatus for the measurement of tracheal epithelium damage. Increased P and separation of the tracheal epithelium from the basement membrane occurred after S. aureus alpha-toxin exposure. We conclude that mammalian airway epithelium is a target of S. aureus alpha-toxin.

Surface proteins and exotoxins are required for the pathogenesis of Staphylococcus aureus pneumonia

A model of Staphylococcus aureus-induced pneumonia in adult, immunocompetent C57BL/6J mice is described. This model closely mimics the clinical and pathological features of pneumonia in human patients. Using this system, we defined a role for S. aureus strain Newman surface proteins and secreted exotoxins in pneumonia-related mortality.

Use of Silkworm Larvae to Study Pathogenic Bacterial Toxins

Injection of stationary phase culture-supernatants of Staphylococcus aureus and Pseudomonas aeruginosa into the hemolymph of silkworm larvae caused their death, whereas a culture-supernatant of a non-pathogenic strain of Escherichia coli did not. A culture-supernatant of a mutant of agr, a global virulence regulator of S. aureus that is required for exotoxin production, was much less toxic to silkworm larvae. A culture-supernatant of a disruption mutant of the S. aureus beta-toxin gene did not kill larvae, whereas one of a deletion mutant of alpha-toxin, gamma-toxin, or aureolysin killed larvae, indicating that the beta-toxin gene is required for staphylococcal supernatant-mediated killing of silkworm larvae. The 50% lethal doses (LD50) of staphylococcal alpha-toxin and beta-toxin, Pseudomonas exotoxin A and diphtheria toxin were 12 microg/g, 9 microg/g, 0.14 microg/g and 1.1 microg/g, respectively. As the purified toxins killed the larvae, silkworm larvae could be used as a model to study the actions of pathogenic bacterial toxins in animal bodies.

Staphylococcus quorum sensing in biofilm formation and infection

Cell population density-dependent regulation of gene expression is an important determinant of bacterial pathogenesis. Staphylococci have two quorum-sensing (QS) systems. The accessory gene regulator (agr) is genus specific and uses a post-translationally modified peptide as an autoinducing signal. In the pathogens Staphylococcus aureus and Staphylococcus epidermidis, agr controls the expression of a series of toxins and virulence factors and the interaction with the innate immune system. However, the role of agr during infection is controversial. A possible second QS system of staphylococci, luxS, is found in a variety of Gram-positive and Gram-negative bacteria. Importantly, unlike many QS systems described in Gram-negative bacteria, agr and luxS of staphylococci reduce rather than induce biofilm formation and virulence during biofilm-associated infection. agr enhances biofilm detachment by up-regulation of the expression of detergent-like peptides, whereas luxS reduces cell-to-cell adhesion by down-regulating expression of biofilm exopolysaccharide. Significant QS activity in staphylococci is observed for actively growing cells at a high cell density, such as during the initial stages of an infection and under optimal environmental conditions. In contrast, the metabolically quiescent biofilm mode of growth appears to be characterized by an overall low activity of the staphylococcal QS systems. It remains to be shown whether QS control in staphylococci represents a promising target for the development of novel antibacterial agents.

Staphylococcus aureus: the search for novel targets

In the UK, 20,000 cases of Staphylococcus aureus bacteraemia are reported each year, half of which are antibiotic resistant and approximately 4% are fatal, exemplifying a worldwide phenomenon of tremendous economic and human impact. Novel treatments and prophylaxis are urgently required to combat such a serious threat. A common goal in the postgenomic era is to identify new targets for drug intervention (using small molecules) and immunologicals. Several promising cellular targets are now being developed in the quest to control such a life-threatening pathogen.

Virulence determinants in Staphylococcus aureus and their involvement in clinical syndromes

Staphylococcus aureus is a major human pathogen responsible for a variety of toxin-mediated and suppurative diseases. About 50 staphylococcal virulence factors have been described to date. In this review, we examine the clinical implications of key staphylococcal virulence factors in toxin-mediated diseases, septic shock, and severe focal infections such as arthritis, infective endocarditis, pneumonia acquired during mechanical ventilation, and necrotizing pneumonia. Staphylococcal pathogenicity is sometimes due principally to a single virulence factor, as in toxic shock syndrome and necrotizing pneumonia. In contrast, several virulence factors are involved in other staphylococcal disease, such as septic shock. A better knowledge of the mechanism of action of each virulence factor involved in the different staphylococcal diseases could open the way to the use of specific inhibitors in the clinical setting.

An agr-like two-component regulatory system in Lactobacillus plantarum is involved in production of a novel cyclic peptide and regulation of adherence

We have analyzed a locus on the annotated Lactobacillus plantarum WCFS1 genome that showed homology to the staphylococcal agr quorum-sensing system and designated it lam for Lactobacillus agr-like module. Production of the lamBDCA transcript was shown to be growth phase dependent. Analysis of a response regulator-defective mutant (Delta)lamA) in an adherence assay showed that lam regulates adherence of L. plantarum to a glass surface. Global transcription analysis of the wild-type and (Delta)lamA strains in early, mid-, and late log phase of growth was performed using a clone-based microarray. Remarkably, only a small set of genes showed significant differences in transcription profiles between the wild-type and lamA mutant strains. The microarray analysis confirmed that lamBDCA is autoregulatory and showed that lamA is involved in regulation of expression of genes encoding surface polysaccharides, cell membrane proteins, and sugar utilization proteins. The lamBD genes encoding the putative autoinducing peptide precursor (LamD) and its processing protein (LamB) were overexpressed using the nisin-controlled expression system, and culture supernatants were analyzed by liquid chromatography/mass spectrometry (LC/MS) to identify overproduced LamD-derived peptides. In this way, a cyclic thiolactone pentapeptide that possesses a ring structure similar to those of autoinducing peptides of the staphylococcal agr system was identified. The peptide was designated LamD558, and its sequence (CVGIW) matched the annotated precursor peptide sequence. Time course analysis of wild-type culture supernatants by LC/MS indicated that LamD558 production was increased markedly from mid-log to late log growth phase. This is the first example of an agr-like system in nonpathogenic bacteria that encodes a cyclic thiolactone autoinducing peptide and is involved in regulation of adherence.

Silkworm pathogenic bacteria infection model for identification of novel virulence genes

Silkworms are killed by injection of pathogenic bacteria, such as Staphylococcus aureus and Streptococcus pyogenes, into the haemolymph. Gene disruption mutants of S. aureus whose open reading frames were previously uncharacterized and that are conserved among bacteria were examined for their virulence in silkworms. Of these 100 genes, three genes named cvfA, cvfB, and cvfC were required for full virulence of S. aureus in silkworms. Haemolysin production was decreased in these mutants. The cvfA and cvfC mutants also had attenuated virulence in mice. S. pyogenes cvfA-disrupted mutants produced less exotoxin and had attenuated virulence in both silkworms and mice. These results indicate that the silkworm-infection model is useful for identifying bacterial virulence genes.

Clinical Isolates of Staphylococcus aureus Vary in Ability to Stimulate Cytokine Expression in Human Endothelial Cells

Human umbilical vein endothelial cells (HUVEC) were infected for 24 h with 18 well-characterized Staphylococcus aureus isolates, and the supernatants from infected HUVEC were analysed for interleukin (IL)-1beta, tumour necrosis factor-alpha, IL-6, IL-8, IL-10, IL-12p70, growth-related oncogene (GRO)-alpha, granulocyte macrophage colony-stimulating factor (GM-CSF) and regulated upon activation, normal T cell expressed and secreted (RANTES) by immunoassay. All staphylococcal isolates induced the expression of IL-6, IL-8, GRO-alpha, GM-CSF and RANTES. The magnitude of cytokine expression varied between isolates. Staphylococcus aureus inducing high expression of one of these cytokines also showed simultaneous high expression of the other four, indicating a common mechanism for the ability of individual S. aureus to induce expression of these cytokines. No direct correlation between cytokine expression and adhesion of S. aureus to HUVEC was observed, indicating that bacterial properties besides adhesion contribute to the activation of HUVEC.

Staphylococcus aureus AgrA binding to the RNAIII-agr regulatory region

The control of virulence gene expression in the human pathogen Staphylococcus aureus is under the partial control of the two-component quorum-sensing system encoded by genes of the agr locus. The product of the agrA gene has been shown by amino acid sequence similarity to be the putative response regulator; however, binding of AgrA to promoters under its control has not yet been demonstrated. In this study, we isolated and purified soluble AgrA by expression under osmotic shock conditions and ion-exchange chromatography. Purified AgrA showed high-affinity binding to the RNAIII-agr intergenic region by electrophoretic mobility shift assays. Binding was localized by DNase I protection assays to a pair of direct repeats in the P2 and P3 promoter regions of the agr locus. We found that this binding was enhanced by the addition of the small phosphoryl donor, acetyl phosphate. The difference in binding affinity between these two promoters was found to result from a 2-bp difference between the downstream direct repeats of the P2 and P3 sites. Mutation of these base pairs in the P3 site to match those found in the P2 site increased the affinity of AgrA for the P3 site relative to that for the P2 site. These results are consistent with the function of AgrA as a response regulator with recognition sites in the promoter regions of RNAIII and the agr locus.

Innate immune response of corneal epithelial cells to Staphylococcus aureus infection: role of peptidoglycan in stimulating proinflammatory cytokine secretion

PURPOSE

This study sought to elucidate the innate immune responses of cultured human corneal epithelial cells (HCECs) to infection by the Gram-positive bacterium Staphylococcus aureus and to determine the underlying mechanisms.

METHODS

HUCL, a telomerase-immortalized HCEC line, and primary cultures of HCECs were challenged with live or heat-killed S. aureus, its exoproducts, or cell wall components lipoteichoic acid (LTA) and peptidoglycan (PGN). IkappaB-alpha phosphorylation and degradation as well as phosphorylation of MAPKs, p38, and JNK-1/2, were assessed by Western blot analysis. The expression of interleukin (IL)-6, IL-8, TNF-alpha, and beta-defensin-2 were determined using RT-PCR and secretion of IL-6, IL-8, TNF-alpha, and beta-defensin were measured using enzyme-linked immunosorbent assay and immunoblot analysis of culture medium.

RESULTS

Exposure of HUCL cells to live, but not heat-killed, S. aureus resulted in NF-kappaB activation in a time-dependent manner, as assessed by the increase in IkappaB-alpha phosphorylation and degradation. Live bacteria also activated the p38 and JNK pathways. The effects of live bacteria on HUCL cells may be attributable to bacterial exoproducts, since the conditioned medium of S. aureus also effectively stimulated these signaling pathways. PGN, but not LTA, activated the NF-kappaB and MAPK pathways in a dose- and time-dependent manner. Concomitant with activation of NF-kappaB and MAPKs, transcriptional expression of IL-6, IL-8, TNF-alpha, and beta-defensin-2 were induced in cells challenged with bacterial exoproducts and PGN. Secretion of IL-6, IL-8, TNF-alpha, and beta-defensin-2 were also significantly increased in HCECs in response to bacterial exoproducts and PGN challenge.

CONCLUSIONS

Corneal epithelial cells possess the ability to recognize the presence of Gram-positive bacteria and to initiate the innate immune responses by the expression and/or release of proinflammatory cytokines and beta-defensin-2 in the cornea.

Inactivation of a bacterial virulence pheromone by phagocyte-derived oxidants: new role for the NADPH oxidase in host defense

Quorum sensing triggers virulence factor expression in medically important bacterial pathogens in response to a density-dependent increase in one or more autoinducing pheromones. Here, we show that phagocyte-derived oxidants target these autoinducers for inactivation as an innate defense mechanism of the host. In a skin infection model, expression of phagocyte NADPH oxidase, myeloperoxidase, or inducible nitric oxide synthase was critical for defense against a quorum-sensing pathogen, Staphylococcus aureus, but not for defense against a quorum sensing-deficient mutant. A virulence-inducing peptide of S. aureus was inactivated in vitro and in vivo by reactive oxygen and nitrogen intermediates, including HOCl and ONOO(-). Inactivation of the autoinducer prevented both the up-regulation of virulence gene expression and the downstream sequelae. MS analysis of the inactivated peptide demonstrated that oxidation of the C-terminal methionine was primarily responsible for loss of activity. Treatment of WT but not NADPH oxidase-deficient mice with N-acetyl methionine to scavenge the inhibitory oxidants increased in vivo quorum sensing independently of the bacterial burden at the site of infection. Thus, oxidant-mediated inactivation of an autoinducing peptide from S. aureus is a critical innate defense mechanism against infection with this pathogen.

Dynamic interaction between airway epithelial cells andStaphylococcus aureus

Staphylococcus aureus is a major cause of pulmonary infection, particularly in cystic fibrosis (CF) patients. However, few aspects of the interplay between S. aureus and host airway epithelial cells have been investigated thus far. We investigated by videomicroscopy the time- and bacterial concentration-dependent (104, 106, and 108CFU/ml) effect of S. aureus on adherence, internalization, and the associated damage of the airway epithelial cells. The balance between the secretion by S. aureus of the α-toxin virulence factor and by the airway cells of the antibacterial secretory leukoproteinase inhibitor (SLPI) was also analyzed. After 1 h of interaction, whatever the initial bacterial concentration, a low percentage of S. aureus (<8%) adhered to airway cells, and no airway epithelial cell damage was observed. In contrast, after 24 h of incubation, more bacteria adhered to airway epithelial cells, internalized bacteria were observed, and a bacterial concentration-dependent effect on airway cell damage was observed. At 24 h, most airway cells incubated with bacteria at 108CFU/ml exhibited a necrotic phenotype. The necrosis was preceded by a transient apoptotic process. In parallel, we observed a time- and bacterial concentration-dependent decrease in SLPI and increase in α-toxin expression. These results suggest that airway cells can defend against S. aureus in the early stages of infection. However, in later phases, there is a marked imbalance between the bactericidal capacity of host cells and bacterial virulence. These findings reinforce the potential importance of S. aureus in the pathogenicity of airway infections, including those observed early in CF patients.

Staphylococcus aureus protein A induces airway epithelial inflammatory responses by activating TNFR1

Staphylococcus aureus is a major human pathogen that is associated with diverse types of local and systemic infection characterized by inflammation dominated by polymorphonuclear leukocytes. Staphylococci frequently cause pneumonia, and these clinical isolates often have increased expression of protein A, suggesting that this protein may have a role in virulence. Here we show that TNFR1, a receptor for tumor-necrosis factor-alpha (TNF-alpha) that is widely distributed on the airway epithelium, is a receptor for protein A. We also show that the protein A-TNFR1 signaling pathway has a central role in the pathogenesis of staphylococcal pneumonia.

TLR2 is mobilized into an apical lipid raft receptor complex to signal infection in airway epithelial cells

Toll-like receptors (TLRs) mediate host responses to bacterial gene products. As the airway epithelium is potentially exposed to many diverse inhaled bacteria, TLRs involved in defense of the airways must be broadly responsive, available at the exposed apical surface of the cells, and highly regulated to prevent activation following trivial encounters with bacteria. We demonstrate that TLR2 is enriched in caveolin-1-associated lipid raft microdomains presented on the apical surface of airway epithelial cells after bacterial infection. These receptor complexes include myeloid differentiation protein (MyD88), interleukin-1 receptor-activated kinase-1, and TNF receptor-associated factor 6. The signaling capabilities of TLR2 are amplified through its association with the asialoganglioside gangliotetraosylceramide (Gal beta 1,2GalNAc beta 1,4Gal beta 1,4Glc beta 1,1Cer), which has receptor function itself for many pulmonary pathogens. Ligation of either TLR2 or asialoGM1 by ligands with specificity for either receptor, by Pseudomonas aeruginosa, or by Staphylococcus aureus stimulates IL-8 production through activation of NF-kappa B, as mediated by TLR2 and MyD88. Thus, TLR2 in association with asialo-glycolipids presented within the context of lipid rafts provides a broadly responsive signaling complex at the apical surfaces of airway cells to initiate the host response to potential bacterial infection.

Regulated expression of pathogen-associated molecular pattern molecules in Staphylococcus epidermidis: quorum-sensing determines pro-inflammatory capacity and production of phenol-soluble modulins

Phenol-soluble modulin (PSM) is a peptide complex produced by the nosocomial pathogen Staphylococcus epidermidis that has a strong capacity to activate the human innate immune response. We developed a novel method based on liquid chromatography-mass spectrometry (LC-MS) to quantify the production of the individual PSM components. Each PSM peptide was abundant in most of the 76 S epidermidis strains tested. Importantly, none of the PSM components were secreted by an agr mutant strain, indicating that PSM synthesis is regulated strictly by the agr quorum-sensing system. Furthermore, the agr mutant strain failed to elicit production of TNFalpha by human myeloid cells and induced significantly less neutrophil chemotaxis compared with the wild-type strain. Thus, quorum-sensing in S. epidermidis dramatically influenced activation of human host defence. We propose that an agr quorum-sensing mechanism facilitates growth and survival in infected hosts by adapting production of the pro-inflammatory PSMs to the stage of infection.

Cell density – dependent regulation: basic principles and effects on the virulence of Gram-positive cocci

PRINCIPLES

Quorum sensing (QS) regulation appears to be a consequence of interbacterial communication by which bacteria of one or even different species learn about their current population density and react in a defined way to that information. QS-regulation is a three step process: in the first step specific signaling molecules are produced and secreted to the exterior space. In the second step, the molecules accumulate e.g. with growing population density. In the last step, a supra-threshold concentration of the molecules is extra- or intra-cellularly sensed by the bacteria and leads to a cascade of regulatory activities. While Gram-negative bacteria can employ five or more different chemical classes of signaling molecules, Gram-positive cocci predominantly use special oligopeptides for specific signaling.

DESCRIPTIONS

Examples of QS-regulatory effects on virulence factor expression in Staphylococcus aureus, Streptococcus mutans, and Enterococcus faecalis are given. In these bacteria, QS-regulation appears to be crucial for displaying tissue invasiveness and/or biofilm formation.

APPLICATIONS

The high specificity of the initial signal sensing and the importance for expressing special virulence traits make this type of gene expression control a promising target for the development of novel therapeutics. The options for such therapies are critically discussed based on practical experiences with interference in S. aureus QS-regulation.

Experimental models of pulmonary infection

Experimental models of pulmonary infection are being discussed, focused on various aspects of good experimental design, such as choice of animal species and infecting strain, and route of infection/inoculation techniques (intranasal inoculation, aerosol inoculation, and direct instillation into the lower respiratory tract). In addition, parameters to monitor pulmonary infection are being reviewed such as general clinical signs, pulmonary-associated signs, complication of the pulmonary infection, mortality rate, and parameters after dissection of animals. Examples of pulmonary infection models caused by bacteria, fungi, viruses or parasites in experimental animals with intact or impaired host defense mechanisms are shortly summarized including key-references.

Characterization of RAP, a quorum sensing activator of Staphylococcus aureus

Staphylococcus aureus are Gram-positive bacteria and cause diverse serious diseases in humans and animals through the production of toxins. The production of toxins is regulated by quorum sensing mechanisms, where proteins such as RNAIII activating protein (RAP) are secreted by the bacteria and induce virulence. Antibodies to RAP have been shown to protect mice from infection, but the molecular structure of RAP was not known and hindered vaccine development. To characterize RAP, recombinant protein was made and tested for its ability to induce genes important for pathogenesis (agr). In addition, monoclonal antibodies were produced to identify its cellular localization. Results shown here indicate that RAP is a 277-aa protein that is an ortholog of the ribosomal protein L2. Like the native molecule, recombinant RAP activates the production of RNAIII (encoded by agr). Using RAP specific monoclonal antibodies we demonstrate that RAP is continuously secreted and while RAP is expressed also in other bacteria (like Staphylococcus epidermidis, Staphylococcus xylosus and Escherichia coli), it is secreted to the culture medium only by S. aureus. Our results show that the ribosomal protein L2 has an extraribosomal function and that when secreted RAP acts as an autoinducer of virulence to regulate S. aureus pathogenesis.

The role of fibronectin binding proteins in the pathogenesis of Staphylococcus aureus infections

PURPOSE OF REVIEW

Staphylococcus aureus produces two closely-related fibronectin-binding proteins (FnBPs) that facilitate attachment by this versatile pathogen. Recent studies of staphylococcal FnBP have increased our understanding of the molecular mechanisms that are critical in bacterial-host cell interactions and in infection.

RECENT FINDINGS

This review will summarize current knowledge of the role of the FnBPs of Staphylococcus aureus in the pathogenesis of infection. The FnBPs, which facilitate attachment of this pathogen to host cells and to fibronectin-coated biomaterials, are important mediators of infection in experimental endocarditis. In addition, recent vaccine studies utilizing FnBP derivatives have shown partial protection in animals. FnBPs also act as invasins permitting uptake of the staphylococcus by cultured non-professional phagocytes using host fibronectin to bridge with integrins on the cell surface. However, the precise role of FnBP in tissue invasion and the relevance of intracellular invasion in disease remain to be elucidated.

SUMMARY

FnBP is one of many adhesins expressed by S. aureus that influence host tissue adherence by binding to host fibronectin. FnBP-based vaccine strategies and novel anti-adherence tools based upon FnBP derivatives are in the early stages of investigation but may show promise in preventing staphylococcal infections.

Relationship of plcR-regulated factors to Bacillus endophthalmitis virulence

The explosive, destructive course of Bacillus endophthalmitis has been attributed to the production of toxins during infection. In this study we analyzed the contribution of toxins controlled by the global regulator plcR to the pathogenesis of experimental Bacillus endophthalmitis. Isogenic plcR-deficient mutants of Bacillus cereus and Bacillus thuringiensis were constructed by insertional inactivation of plcR by the kanamycin resistance cassette, aphA3. Rabbit eyes were injected intravitreally with approximately 100 CFU of wild-type B. cereus or B. thuringiensis or a plcR-deficient mutant. The evolution of endophthalmitis resulting from each plcR-deficient mutant was considerably slower than that caused by each wild-type strain. Retinal function was not eliminated until 42 h postinfection in rabbits with endophthalmitis caused by the plcR-deficient mutants, whereas wild-type infections resulted in a complete loss of retinal function within 18 h. The intraocular inflammatory cell influx and retinal destruction in plcR-deficient endophthalmitis approached the severity observed in wild-ype infections, but not until 36 h postinfection. Gross and histological examinations of eyes infected with plcR mutants demonstrated that the anterior and posterior segment changes were muted compared to the changes observed in eyes infected with the wild types. The loss of plcR-regulated factors significantly attenuated the severity of Bacillus endophthalmitis. The results therefore suggest that plcR may represent a target for which adjunct therapies could be designed for the prevention of blindness during Bacillus endophthalmitis.

Quorum sensing and its relevance to infectious diseases

Quorum sensing allows bacteria to detect the density of their own species and alter their metabolism to take advantage of this density. Quorum sensing is used by a wide variety of bacteria including human pathogens. Quorum sensing genes are important for the pathogenic potential of Pseudomonas aeruginosa and Staphylococcus aureus, as well as other invasive bacteria. An understanding of quorum sensing may lead to new therapeutic strategies.

The prevalence and mechanisms of vancomycin resistance in Staphylococcus aureus

The emergence of Staphylococcus aureus resistant to vancomycin has caused considerable concern. Such strains are currently rare, although they have been isolated from many areas of the world. Considerable controversy surrounds strains of S. aureus displaying heterogeneous resistance to vancomycin regarding their definition and methods for detection. This has led to considerable variance in estimates of prevalence (0-1.3%-20% in Japan) and has hindered efforts to define the clinical relevance of these strains. The mechanism of resistance involves a complex reorganization of cell wall metabolism, leading to a grossly thickened cell wall with reduced peptidoglycan cross-linking. There may be many different ways in which strains achieve this endpoint. Current knowledge and theories are summarized.

The Genomic Aspect of Virulence, Sepsis, and Resistance to Killing Mechanisms in Staphylococcus aureus

Staphylococcus aureus is widely appreciated as a pathogen, despite the fact that this microorganism is usually a benign colonizer of the host, rarely if ever causing infection. However, this bacterium, in response to changing environments, will occasionally switch from a commensal to a lethal pathogen. S. aureus uses an array of two-component signal transduction systems, winged-helix transcription proteins, and alternate sigma factor to create an intricate network of regulation in response to environmental change/stimuli. The interactions between members of this large cast of regulatory elements are beginning to be appreciated. Predicated upon recent genomic data, this review focuses on how this regulatory apparatus functions to control the expression of the multitude of virulence factors this "Jekyl and Hyde" organism produces.