Involvement of alpha5beta1-integrin and TNF-alpha in Staphylococcus aureus alpha-toxin-induced death of epithelial cells

α-Hemolysin-mediated endothelial injury contributes to the development of Staphylococcus aureus-induced dermonecrosis

Staphylococcus aureus α-hemolysin (Hla) is a pore-forming toxin critical for the pathogenesis of skin and soft tissue infections, which causes the pathognomonic lesion of cutaneous necrosis (dermonecrosis) in mouse models. To determine the mechanism by which dermonecrosis develops during S. aureus skin infection, mice were given control serum, Hla-neutralizing antiserum, or an inhibitor of Hla receptor [A-disintegrin and metalloprotease 10 (ADAM10) inhibitor] followed by subcutaneous infection by S. aureus, and the lesions were evaluated using immunohistochemistry and immunofluorescence. Hla induced apoptosis in the vascular endothelium at 6 hours post-infection (hpi), followed by apoptosis in keratinocytes at 24 hpi. The loss of vascular endothelial (VE)-cadherin expression preceded the loss of epithelial-cadherin expression. Hla also induced hypoxia in the keratinocytes at 24 hpi following vascular injury. Treatment with Hla-neutralizing antibody or ADAM10 inhibitor attenuated early cleavage of VE-cadherin, cutaneous hypoxia, and dermonecrosis. These findings suggest that Hla-mediated vascular injury with cutaneous hypoxia underlies the pathogenesis of S. aureus-induced dermonecrosis.

Integrins as a bridge between bacteria and cells: key targets for therapeutic wound healing

Integrins are heterodimers composed of α and β subunits that are bonded through non-covalent interactions. Integrins mediate the dynamic connection between extracellular adhesion molecules and the intracellular actin cytoskeleton. Integrins are present in various tissues and organs where these heterodimers participate in diverse physiological and pathological responses at the molecular level in living organisms. Wound healing is a crucial process in the recovery from traumatic diseases and comprises three overlapping phases: inflammation, proliferation and remodeling. Integrins are regulated during the entire wound healing process to enhance processes such as inflammation, angiogenesis and re-epithelialization. Prolonged inflammation may result in failure of wound healing, leading to conditions such as chronic wounds. Bacterial colonization of a wound is one of the primary causes of chronic wounds. Integrins facilitate the infectious effects of bacteria on the host organism, leading to chronic inflammation, bacterial colonization, and ultimately, the failure of wound healing. The present study investigated the role of integrins as bridges for bacteria-cell interactions during wound healing, evaluated the role of integrins as nodes for bacterial inhibition during chronic wound formation, and discussed the challenges and prospects of using integrins as therapeutic targets in wound healing.

Integrins regulation of wound healing processes: insights for chronic skin wound therapeutics

Integrins are heterodimers composed of non-covalently associated alpha and beta subunits that mediate the dynamic linkage between extracellular adhesion molecules and the intracellular actin cytoskeleton. Integrins are present in various tissues and organs and are involved in different physiological and pathological molecular responses in vivo. Wound healing is an important process in the recovery from traumatic diseases and consists of three overlapping phases: inflammation, proliferation, and remodeling. Integrin regulation acts throughout the wound healing process to promote wound healing. Prolonged inflammation may lead to failure of wound healing, such as wound chronicity. One of the main causes of chronic wound formation is bacterial colonization of the wound. In this review, we review the role of integrins in the regulation of wound healing processes such as angiogenesis and re-epithelialization, as well as the role of integrins in mediating bacterial infections during wound chronicity, and the challenges and prospects of integrins as therapeutic targets for infected wound healing.

Targeting multidrug resistant Staphylococcus infections with bacterial histidine kinase inhibitors

The emergence of drug-resistant bacteria, such as methicillin-resistant Staphylococcus aureus (MRSA), which are not susceptible to current antibiotics has necessitated the development of novel approaches and targets to tackle this growing challenge. Bacterial two-component systems (TCSs) play a central role in the adaptative response of bacteria to their ever-changing environment. They are linked to antibiotic resistance and bacterial virulence making the proteins of the TCSs, histidine kinases and response regulators, attractive for the development of novel antibacterial drugs. Here, we developed a suite of maleimide-based compounds that we evaluated against a model histidine kinase, HK853, in vitro and in silico. The most potent leads were then assessed for their ability to decrease the pathogenicity and virulence of MRSA, resulting in the identification of a molecule that decreased the lesion size caused by a methicillin-resistant S. aureus skin infection by 65% in a murine model.

Identification and Validation of a Novel Antibacterial Compound MZ-01 against Methicillin-Resistant Staphylococcus aureus

The discovery of new classes of antibiotics is slow, and it is being greatly outpaced by the development of bacterial resistance. This disparity places us in an increasingly vulnerable position because we are running out of safe and effective therapeutic options to treat antibiotic-resistant infections. This is exemplified by the emergence and persistence of hospital-acquired and community-associated methicillin-resistant S. aureus (MRSA), which has markedly narrowed our options for treating life-threatening staph infections. Thus, there is an urgent need to develop novel, potent, preventive, and therapeutic agents. In our current study, we performed a whole-cell screening assay of synthetic libraries for antibacterial activity and identified a novel molecule, MZ-01. MZ-01 exhibited potent bactericidal activity against Gram-positive bacterial pathogens, including MRSA, Streptococcus pyogenes, and Streptococcus pneumoniae, at low concentrations. MZ-01 killed and lysed both the late exponential phase of an S. aureus population and bacteria inside mammalian cells. Furthermore, MZ-01 exhibited low cytotoxicity. These results indicate that MZ-01 is a promising scaffold to guide the development of novel, potent antibacterial agents against multidrug-resistant Gram-positive bacterial pathogens such as MRSA.

Contribution of Coagulase and Its Regulator SaeRS to Lethality of CA-MRSA 923 Bacteremia

Coagulase is a critical factor for distinguishing Staphylococcus aureus and coagulase-negative Staphylococcus. Our previous studies demonstrated that the null mutation of coagulase (coa) or its direct regulator, SaeRS, significantly enhanced the ability of S. aureus (CA-MRSA 923) to survive in human blood in vitro. This led us to further investigate the role of coagulase and its direct regulator, SaeRS, in the pathogenicity of CA-MRSA 923 in bacteremia during infection. In this study, we found that the null mutation of coa significantly decreased the mortality of CA-MRSA 923; moreover, the single null mutation of saeRS and the double deletion of coa/saeRS abolished the virulence of CA-MRSA 923. Moreover, the mice infected with either the saeRS knockout or the coa/saeRS double knockout mutant exhibited fewer histological lesions and less neutrophils infiltration in the infected kidneys compared to those infected with the coa knockout mutant or their parental control. Furthermore, we examined the impact of coa and saeRS on bacterial survival in vitro. The null mutation of coa had no impact on bacterial survival in mice blood, whereas the deletion mutation of saeRS or coa/saeRS significantly enhanced bacterial survival in mice blood. These data indicate that SaeRS plays a key role in the lethality of CA-MRSA 923 bacteremia, and that coagulase is one of the important virulence factors that is regulated by SaeRS and contributes to the pathogenicity of CA-MRSA 923.

Application of impedance measurement to investigate in vitro inhalation toxicity of bacteria

BACKGROUND

Workers of agriculture and intensive life stock farming are exposed to highly contaminated workplaces. Bioaerosol exposures are suspected to trigger respiratory health effects of the workers. So far, risk evaluation of bioaerosols has been assessed through the infectivity of comprising biological agents that is classified in Europe by four risk groups according to the criteria of Directive 2000/54EC of the European Parliament. However, this directive additionally requires the risk assessment of allergenic and toxigenic effects without further elaboration. The aim of our study was to establish an in vitro screening system that is able to measure inhalative toxic effects of bacteria and their metabolites.

METHODS

In this study, we analyzed three bacterial toxins and five culture supernatants of selected bacteria with known toxicity as model agents exposed to the lung epithelial cell line NuLi-1. We used electrical cell-substrate impedance sensing (ECIS) method to monitor real-time cell changes and the viability test Prestoblue™.

RESULTS

We confirmed concentration dependent cytotoxic effects of the selected toxins in NuLi-1 cells over a period of up to 48 h. Each toxin resulted in a different but specific impedance profile over time according to their mode of action, whereas viability assay showed the metabolic activity of the cells at a chosen time point without revealing any information on their mode of action. Furthermore, dose-response-relationships were monitored. Tested model bacteria (Streptoccous pneumoniae, Acinetobacter radioresistens, Aerococcus viridans, Aeromonas hydrophila) reacted according to their expected toxicity except one bacterium (Enterococcus faecalis). The established assays revealed the concentration dependent onset and intensity of bacterial cytotoxicity and the viability of the cells at 24 h and 48 h exposure.

CONCLUSION

Impedance measurement and the viability assay Prestoblue™ in combination are suitable as sensitive screening methods to analyze toxic potential of bacteria and can therefor support the risk assessment of workplaces in terms of the directive 2000/54/EC.

Intracellular Staphylococcus aureus and host cell death pathways

Staphylococcus aureus is a major opportunistic human pathogen that is globally prevalent. Although S. aureus and humans may have co-evolved to the point of commensalism, the bacterium is equipped with virulence factors causing devastating infections. The adoption of an intracellular lifestyle by S. aureus is an important facet of its pathogenesis. Occupying a privileged intracellular compartment permits evasion from the bactericidal actions of host immunity and antibiotics. However, this localization exposes S. aureus to cell-intrinsic processes comprising autophagy, metabolic challenges and clearance mechanisms orchestrated by host programmed cell death pathways (PCDs), including apoptosis, pyroptosis and necroptosis. Mounting evidence suggests that S. aureus deploys pathoadaptive mechanisms that modulate the expression of its virulence factors to prevent elimination through PCD pathways. In this review, we critically analyse the current literature on the interplay between S. aureus virulence factors with the key, intertwined nodes of PCD. We discuss how S. aureus adaptation to the human host plays an essential role in the evasion of PCD, and we consider future directions to study S. aureus-PCD interactions.

Selective Host Cell Death by Staphylococcus aureus: A Strategy for Bacterial Persistence

Host cell death programs are fundamental processes that shape cellular homeostasis, embryonic development, and tissue regeneration. Death signaling and downstream host cell responses are not only critical to guide mammalian development, they often act as terminal responses to invading pathogens. Here, we briefly review and contrast how invading pathogens and specifically Staphylococcus aureus manipulate apoptotic, necroptotic, and pyroptotic cell death modes to establish infection. Rather than invading host cells, S. aureus subverts these cells to produce diffusible molecules that cause death of neighboring hematopoietic cells and thus shapes an immune environment conducive to persistence. The exploitation of cell death pathways by S. aureus is yet another virulence strategy that must be juxtaposed to mechanisms of immune evasion, autophagy escape, and tolerance to intracellular killing, and brings us closer to the true portrait of this pathogen for the design of effective therapeutics and intervention strategies.

Calcium Dynamics Regulate Protective Responses and Growth of Staphylococcus aureus in Macrophages

Staphylococcus aureus (S. aureus) is a gram-positive bacteria, which causes various fatal respiratory infections including pneumonia. The emergence of Methicillin-Resistance Staphylococcus aureus (MRSA) demands a thorough understanding of host-pathogen interactions. Here we report the role of calcium in regulating defence responses of S. aureus in macrophages. Regulating calcium fluxes in cells by different routes differentially governs the expression of T cell costimulatory molecule CD80 and Th1 promoting IL-12 receptor. Inhibiting calcium influx from extracellular medium increased expression of IFN-γ and IL-10 while blocking calcium release from the intracellular stores inhibited TGF-β levels. Blocking voltage-gated calcium channels (VGCC) inhibited the expression of multiple cytokines. While VGCC regulated the expression of apoptosis protein Bax, extracellular calcium-regulated the expression of Cytochrome-C. Similarly, VGCC regulated the expression of autophagy initiator Beclin-1. Blocking VGCC or calcium release from intracellular stores promoted phagosome-lysosome fusion, while activating VGCC inhibited phagosomelysosome fusion. Finally, calcium homeostasis regulated intracellular growth of Staphylococcus, although using different mechanisms. While blocking extracellular calcium influx seems to rely on IFN-γ and IL-12Rβ receptor mediated reduction in bacterial survival, blocking either intracellular calcium release or via VGCC route seem to rely on enhanced autophagy mediated reduction of intracellular bacterial survival. These results point to fine-tuning of defence responses by routes of calcium homeostasis.

α-Hemolysin suppresses osteogenesis by inducing lipid rafts accumulation in bone marrow stromal cells

α-hemolysin (Hla) is considered an essential virulent factor for Staphylococcus aureus (S. aureus) toxicity, the mechanism by which Hla affect bone metabolism is poorly understood. In this study, 2-month-old C57BL/6 mice were treated with Hla (40 μg/kg, i.p.) or S. aureus (1 × 10⁶ CFU/ml, 100 μl, i.v.) with the presence or absence of methyl-β-cyclodextrin (MβCD) (300 mg/kg, i.p.). MicroCT analysis showed progressive bone loss from week 2 to week 4 after Hla treatment, accompanied by a decreased osteoblasts and increased osteoclasts in femoral metaphysis in mice. Further, Hla stimulated the expression of Caveolin-1 in vivo and in vitro, activated lipid rafts accumulation in cell membrane of bone marrow stromal cells (BMSCs), and suppressed osteogenesis of BMSCs. Destruction of lipid rafts with MβCD or inhibition of Caveolin-1 with Daidzein blocked the detrimental effect of Hla on osteogenesis of BMSCs. Importantly, treating mice with MβCD rescued the loss of osteoblasts and increased osteoclastogenesis induced by Hla as well as the bone loss induced by S. aureus infection. Together, we demonstrate that Hla induces bone destruction directly by suppressing osteogenesis and indirectly by stimulating osteoclastogenesis, and that lipid rafts may mediate the detrimental effect of Hla and S. aureus on osteogenesis and bone formation.

S. aureus alpha-toxin monomer binding and heptamer formation in host cell membranes - Do they determine sensitivity of airway epithelial cells toward the toxin?

Alpha-toxin (Hla) is a major virulence factor of Staphylococcus aureus (S. aureus) and plays an important role in S. aureus-induced pneumonia. It binds as a monomer to the cell surface of eukaryotic host cells and forms heptameric transmembrane pores. Sensitivities toward the toxin of various types of potential host cells have been shown to vary substantially, and the reasons for these differences are unclear. We used three human model airway epithelial cell lines (16HBE14o-, S9, A549) to correlate cell sensitivity (measured as rate of paracellular gap formation in the cell layers) with Hla monomer binding, presence of the potential Hla receptors ADAM10 or α5β1 integrin, presence of the toxin-stabilizing factor caveolin-1 as well as plasma membrane lipid composition (phosphatidylserine/choline, sphingomyelin). The abundance of ADAM10 correlated best with gap formation or cell sensitivities, respectively, when the three cell types were compared. Caveolin-1 or α5β1 integrin did not correlate with toxin sensitivity. The relative abundance of sphingomyelin in plasma membranes may also be used as a proxi for cellular sensitivity against alpha-toxin as sphingomyelin abundances correlated well with the intensities of alpha-toxin mediated gap formation in the cell layers.

Impact of Bacterial Toxins in the Lungs

Bacterial toxins play a key role in the pathogenesis of lung disease. Based on their structural and functional properties, they employ various strategies to modulate lung barrier function and to impair host defense in order to promote infection. Although in general, these toxins target common cellular signaling pathways and host compartments, toxin- and cell-specific effects have also been reported. Toxins can affect resident pulmonary cells involved in alveolar fluid clearance (AFC) and barrier function through impairing vectorial Na⁺ transport and through cytoskeletal collapse, as such, destroying cell-cell adhesions. The resulting loss of alveolar-capillary barrier integrity and fluid clearance capacity will induce capillary leak and foster edema formation, which will in turn impair gas exchange and endanger the survival of the host. Toxins modulate or neutralize protective host cell mechanisms of both the innate and adaptive immunity response during chronic infection. In particular, toxins can either recruit or kill central players of the lung's innate immune responses to pathogenic attacks, i.e., alveolar macrophages (AMs) and neutrophils. Pulmonary disorders resulting from these toxin actions include, e.g., acute lung injury (ALI), the acute respiratory syndrome (ARDS), and severe pneumonia. When acute infection converts to persistence, i.e., colonization and chronic infection, lung diseases, such as bronchitis, chronic obstructive pulmonary disease (COPD), and cystic fibrosis (CF) can arise. The aim of this review is to discuss the impact of bacterial toxins in the lungs and the resulting outcomes for pathogenesis, their roles in promoting bacterial dissemination, and bacterial survival in disease progression.

Therapeutic applications of nucleic acid aptamers in microbial infections

Today, the treatment of bacterial infections is a major challenge, due to growing rate of multidrug-resistant bacteria, complication of treatment and increased healthcare costs. Moreover, new treatments for bacterial infections are limited. Oligonucleotide aptamers are single stranded DNAs or RNAs with target-selective high-affinity feature, which considered as nucleic acid-based affinity ligands, replacing monoclonal antibodies. The aptamer-based systems have been found to be talented tools in the treatment of microbial infections, regarding their promising anti-biofilm and antimicrobial activities; they can reduce or inhibit the effects of bacterial toxins, and inhibit pathogen invasion to immune cell, as well as they can be used in drug delivery systems. The focus of this review is on the therapeutic applications of aptamers in infections. In this regard, an introduction of infections and related challenges were presented, first. Then, aptamer definition and selection, with a brief history of aptamers development against various pathogens and toxins were reviewed. Diverse strategies of aptamer application in drug delivery, as well as, the effect of aptamers on the immune system, as the main natural agents of human defense against pathogens, were also discussed. Finally, the future trends in clinical applications of this technology were discussed.

Sphingomyelin Depletion from Plasma Membranes of Human Airway Epithelial Cells Completely Abrogates the Deleterious Actions of S. aureus Alpha-Toxin

Interaction of Staphylococcus aureus alpha-toxin (hemolysin A, Hla) with eukaryotic cell membranes is mediated by proteinaceous receptors and certain lipid domains in host cell plasma membranes. Hla is secreted as a 33 kDa monomer that forms heptameric transmembrane pores whose action compromises maintenance of cell shape and epithelial tightness. It is not exactly known whether certain membrane lipid domains of host cells facilitate adhesion of Ha monomers, oligomerization, or pore formation. We used sphingomyelinase (hemolysin B, Hlb) expressed by some strains of staphylococci to pre-treat airway epithelial model cells in order to specifically decrease the sphingomyelin (SM) abundance in their plasma membranes. Such a pre-incubation exclusively removed SM from the plasma membrane lipid fraction. It abrogated the formation of heptamers and prevented the formation of functional transmembrane pores. Hla exposure of rHlb pre-treated cells did not result in increases in [Ca²⁺]_i, did not induce any microscopically visible changes in cell shape or formation of paracellular gaps, and did not induce hypo-phosphorylation of the actin depolymerizing factor cofilin as usual. Removal of sphingomyelin from the plasma membranes of human airway epithelial cells completely abrogates the deleterious actions of Staphylococcus aureus alpha-toxin.

Inflammatory immune response in rabbits with Staphylococcus aureus biofilm-associated sinusitis

Background

Staphylococcus aureus is the most commonly isolated bacterium from patients with surgically recalcitrant chronic rhinosinusitis (CRS). Understanding the immune responses to S aureus biofilms will provide insights into how the host response may be manipulated by therapeutic agents to improve the chances of successfully preventing and treating these infections. In this study, we investigated the inflammatory immune response in a rabbit model of S aureus biofilm–related sinusitis by analyzing the levels of some major inflammatory cytokines.

Methods

Eighteen New Zealand white rabbits were randomly divided into 3 groups: a blank‐control group; a negative‐control group; and a model group. Four weeks after the biofilm‐associated sinusitis models were established, the sinus mucosa was harvested and examined using hematoxylin‐eosin (H&E) staining, scanning electron microscopy (SEM), reverse transcription polymerase chain reaction (RT‐PCR), and western blotting. The expression levels of inflammatory cytokines were analyzed statistically.

Results

Interleukin (IL)‐1β, IL‐8, and tumor necrosis factor (TNF)‐α expression levels were significantly higher in the model group than in the blank‐control group (p < 0.05); mRNA levels were increased by 1600%, 230%, and 130%, respectively, and the protein levels were increased by 180%, 100%, and 100%, respectively. In contrast, IL‐4 and IL‐5 mRNA levels were reduced by 44% and 70%, respectively, compared with the blank‐control group (p < 0.05).

Conclusion

S aureus biofilms in the rabbit maxillary sinus mucosa were associated with increased IL‐1β, IL‐8, and TNF‐α expression, and decreased IL‐4 and IL‐5 expression.

The SaeRS Two-Component System Controls Survival of Staphylococcus aureus in Human Blood through Regulation of Coagulase

The SaeRS two-component system plays important roles in regulation of key virulence factors and pathogenicity. In this study, however, we found that the deletion mutation of saeRS enhanced bacterial survival in human blood, whereas complementation of the mutant with SaeRS returned survival to wild-type levels. Moreover, these phenomena were observed in different MRSA genetic background isolates, including HA-MRSA WCUH29, CA-MRSA 923, and MW2. To elucidate which gene(s) regulated by SaeRS contribute to the effect, we conducted a series of complementation studies with selected known SaeRS target genes in trans. We found coagulase complementation abolished the enhanced survival of the SaeRS mutant in human blood. The coa and saeRS deletion mutants exhibited a similar survival phenotype in blood. Intriguingly, heterologous expression of coagulase decreased survival of S. epidermidis in human blood. Further, the addition of recombinant coagulase to blood significantly decreased the survival of S. aureus. Further, analysis revealed staphylococcal resistance to killing by hydrogen peroxide was partially dependent on the presence or absence of coagulase. Furthermore, complementation with coagulase, but not SaeRS, returned saeRS/coa double mutant survival in blood to wild-type levels. These data indicate SaeRS modulates bacterial survival in blood in coagulase-dependent manner. Our results provide new insights into the role of staphylococcal SaeRS and coagulase on bacterial survival in human blood.

Sodium Octanoate Modulates the Innate Immune Response of Bovine Mammary Epithelial Cells through the TLR2/P38/JNK/ERK1/2 Pathway: Implications during Staphylococcus aureus Internalization

Bovine mammary epithelial cells (bMECs) contribute to mammary gland defense against invading pathogens, such as Staphylococcus aureus (intracellular facultative), which is recognized by TLR2. In a previous report, we showed that sodium octanoate [NaO, a medium chain fatty acid (C8)] induces (0.25 mM) or inhibits (1 mM) S. aureus internalization into bMECs and differentially regulates the innate immune response (IIR). However, the molecular mechanisms have not been described, which was the aim of this study. The results showed that α5β1 integrin membrane abundance (MA) was increased in 0.25 mM NaO-treated cells, but TLR2 or CD36 MA was not modified. When these receptors were blocked individually, 0.25 mM NaO-increased S. aureus internalization was notably reduced. Interestingly, in this condition, the IIR of the bMECs was impaired because MAPK (p38, JNK, and ERK1/2) phosphorylation and the activation of transcription factors related to these pathways were decreased. In addition, the 1 mM NaO treatment induced TLR2 MA, but neither the integrin nor CD36 MA was modified. The reduction in S. aureus internalization induced by 1 mM NaO was increased further when TLR2 was blocked. In addition, the phosphorylation levels of the MAPKs increased, and 13 transcriptional factors related to the IIR were slightly activated (CBF, CDP, c-Myb, AP-1, Ets-1/Pea-3, FAST-1, GAS/ISRE, AP-2, NFAT-1, OCT-1, RAR/DR-5, RXR/DR-1, and Stat-3). Moreover, the 1 mM NaO treatment up-regulated gene expression of IL-8 and RANTES and secretion of IL-1β. Notably, when 1 mM NaO-treated bMECs were challenged with S. aureus, the gene expression of IL-8 and IL-10 increased, while IL-1β secretion was reduced. In conclusion, our results showed that α5β1 integrin, TLR2 and CD36 are involved in 0.25 mM NaO-increased S. aureus internalization in bMECs. In addition, 1 mM NaO activates bMECs via the TLR2 signaling pathways (p38, JNK, and ERK1/2), which improves IIR before S. aureus invasion. Additionally, NaO (1 mM) might exert anti-inflammatory effects after bacterial internalization.

Interferon-γ enhances both the anti-bacterial and the pro-inflammatory response of human mast cells to Staphylococcus aureus

Human mast cells (huMCs) are involved in both innate and adaptive immune responses where they release mediators including amines, reactive oxygen species (ROS), eicosanoids and cytokines. We have reported that interferon-γ (IFN-γ) enhances FcγR-dependent ROS production. The aim of this study was to extend these observations by investigating the effect of IFN-γ on the biological responses of huMCs to Staphylococcus aureus. We found that exposure of huMCs to S. aureus generated intracellular and extracellular ROS, which were enhanced in the presence of IFN-γ. IFN-γ also promoted bacteria killing, β-hexosaminidase release and eicosanoid production. Interferon-γ similarly increased expression of mRNAs encoding CCL1 to CCL4, granulocyte-macrophage colony-stimulating factor (GM-CSF), tumour necrosis factor-α and CXCL8 in S. aureus-stimulated huMCs. The ability of IFN-γ to increase CXCL8 and GM-CSF protein levels was confirmed by ELISA. Fibronectin or a β1 integrin blocking antibody completely abrogated IFN-γ-dependent S. aureus binding and reduced S. aureus-dependent CXCL8 secretion. These data demonstrate that IFN-γ primes huMCs for enhanced anti-bacterial and pro-inflammatory responses to S. aureus, partially mediated by β1 integrin.

The novel transcriptional regulator SA1804 Is involved in mediating the invasion and cytotoxicity of Staphylococcus aureus

The two-component regulatory system, SaeRS, controls expression of important virulence factors, including toxins and invasins, which contribute to the pathogenicity of Staphylococcus aureus. Previously, we conducted a transcriptomics study for identification of SaeRS regulon and found that inactivation of SaeRS dramatically enhances the transcription of a novel transcriptional regulator (SA1804). This led us to question whether SA1804 is involved in bacterial pathogenicity by regulating the expression of virulence factors. To address this question, we created sa1804, saeRS, and sa1804/saeRS double deletion mutants in a USA300 community-acquired MRSA strain, 923, and determined their impact on the pathogenicity. The deletion of sa1804 dramatically increased the cytotoxicity and enhanced the capacity of bacteria to invade into the epithelial cells (A549), whereas the deletion of saeRS eliminated the cytotoxicity and abolished the bacterial ability to invade into the epithelial cells. Moreover, the double deletions of sa1804 and saeRS appeared a similar phenotype with the saeRS null mutation. Furthermore, we determined the regulatory mechanism of SA1804 using qPCR and gel-shift approaches. Our data indicate that the novel virulence repressor SA1804 is dependent on the regulation of SaeRS. This study sheds light on the regulatory mechanism of virulence factors and allows for us further elucidate the molecular pathogenesis of S. aureus.

Protective or deleterious role of scavenger receptors SR-A and CD36 on host resistance to Staphylococcus aureus depends on the site of infection

Staphylococcus aureus is a major human opportunistic pathogen responsible for a broad spectrum of infections ranging from benign skin infection to more severe life threatening disorders (e.g. pneumonia, sepsis), particularly in intensive care patients. Scavenger receptors (SR-A and CD36) are known to be involved in S. aureus recognition by immune cells in addition to MARCO, TLR2, NOD2 and α5β1 integrin. In the present study, we further deciphered the contribution of SR-A and CD36 scavenger receptors in the control of infection of mice by S. aureus. Using double SR-A/CD36 knockout mice (S/C-KO) and S. aureus strain HG001, a clinically relevant non-mutagenized strain, we showed that the absence of these two scavenger receptors was protective in peritoneal infection. In contrast, the deletion of these two receptors was detrimental in pulmonary infection following intranasal instillation. For pulmonary infection, susceptible mice (S/C-KO) had more colony-forming units (CFU) in their broncho-alveolar lavages fluids, associated with increased recruitment of macrophages and neutrophils. For peritoneal infection, susceptible mice (wild-type) had more CFU in their blood, but recruited less macrophages and neutrophils in the peritoneal cavity than resistant mice. Exacerbated cytokine levels were often observed in the susceptible mice in the infected compartment as well as in the plasma. The exception was the enhanced compartmentalized expression of IL-1β for the resistant mice (S/C-KO) after peritoneal infection. A similar mirrored susceptibility to S. aureus infection was also observed for MARCO and TLR2. Marco and tlr2 -/- mice were more resistant to peritoneal infection but more susceptible to pulmonary infection than wild type mice. In conclusion, our results show that innate immune receptors can play distinct and opposite roles depending on the site of infection. Their presence is protective for local pulmonary infection, whereas it becomes detrimental in the peritoneal infection.

DNA aptamers as a novel approach to neutralize Staphylococcus aureus α-toxin

Staphylococcus aureus is a versatile pathogen capable of causing a broad spectrum of diseases ranging from superficial skin infections to life threatening conditions such as endocarditis, septicemia, pneumonia and toxic shock syndrome. In vitro and in vivo studies identified an exotoxin, α-toxin, as a major cause of S. aureus toxicity. Because S. aureus has rapidly evolved resistance to a number of antibiotics, including methicillin, it is important to identify new therapeutic strategies, other than antibiotics, for inhibiting the harmful effects of this pathogen. Aptamers are single-stranded DNA or RNA oligonucleotides with three-dimensional folded conformations that bind with high affinity and selectivity to targets and modulate their biological functions. The goal of this study was to isolate DNA aptamers that specifically inhibit the cytotoxic activity of α-toxin. After 10 rounds of Systematic Evolution of Ligands by EXponential Enrichment (SELEX), 49 potential anti-α-toxin aptamers were identified. In vitro neutralization assays demonstrated that 4 of these 49 aptamers, AT-27, AT-33, AT-36, and AT-49, significantly inhibited α-toxin-mediated cell death in Jurkat T cells. Furthermore, RT-PCR analysis revealed that α-toxin increased the transcription of the inflammatory cytokines TNF-α and IL-17 and that anti-α-toxin aptamers AT-33 and AT-36 inhibited the upregulation of these genes. Collectively, the data suggest the feasibility of generating functionally effective aptamers against α-toxin for treatment of S. aureus infections.

Puerarin protects against Staphylococcus aureus-induced injury of human alveolar epithelial A549 cells via downregulating alpha-hemolysin secretion

Alpha-hemolysin, a secreted pore-forming toxin, plays an indispensable role in the pathogenicity of Staphylococcus aureus. In this study, the antimicrobial activity of puerarin against S. aureus was investigated; as a result, puerarin showed no influence on the growth of this organism. However, hemolysis and western blotting assays showed that puerarin concentration dependently inhibited the secretion of alpha-hemolysin at low concentrations. Real-time RT-PCR assay was further employed to evaluate the transcriptional level of hla, the gene encoding alpha-hemolysin, and RNAIII, an effector molecule of the agr system. The results indicated that the RNAIII expression and subsequent hla transcription were also inhibited by puerarin in a dose-dependent manner. Furthermore, puerarin significantly prevented human alveolar epithelial A549 cells from S. aureus-induced injury. Thereby, puerarin may be considered as a potential candidate for the development of antivirulence drugs in the treatment of S. aureus-mediated infections.

Glucose-6-phosphate dehydrogenase (G6PD)-deficient epithelial cells are less tolerant to infection by Staphylococcus aureus

Glucose-6-phosphate dehydrogenase (G6PD) is a key enzyme in the pentose phosphate pathway and provides reducing energy to all cells by maintaining redox balance. The most common clinical manifestations in patients with G6PD deficiency are neonatal jaundice and acute hemolytic anemia. The effects of microbial infection in patients with G6PD deficiency primarily relate to the hemolytic anemia caused by Plasmodium or viral infections and the subsequent medication that is required. We are interested in studying the impact of bacterial infection in G6PD-deficient cells. G6PD knock down A549 lung carcinoma cells, together with the common pathogen Staphylococcus aureus, were employed in our cell infection model. Here, we demonstrate that a lower cell viability was observed among G6PD-deficient cells when compared to scramble controls upon bacterial infection using the MTT assay. A significant increase in the intracellular ROS was detected among S. aureus-infected G6PD-deficient cells by observing dichlorofluorescein (DCF) intensity within cells under a fluorescence microscope and quantifying this signal using flow cytometry. The impairment of ROS removal is predicted to enhance apoptotic activity in G6PD-deficient cells, and this enhanced apoptosis was observed by annexin V/PI staining under a confocal fluorescence microscope and quantified by flow cytometry. A higher expression level of the intrinsic apoptotic initiator caspase-9, as well as the downstream effector caspase-3, was detected by Western blotting analysis of G6PD-deficient cells following bacterial infection. In conclusion, we propose that bacterial infection, perhaps the secreted S. aureus α-hemolysin in this case, promotes the accumulation of intracellular ROS in G6PD-deficient cells. This would trigger a stronger apoptotic activity through the intrinsic pathway thereby reducing cell viability when compared to wild type cells.

Bacillus cereus immune escape: a journey within macrophages

During bacterial infection, professional phagocytes are attracted to the site of infection, where they constitute a first line of host cell defense. Their function is to engulf and destroy the pathogens. Thus, bacteria must withstand the bactericidal activity of professional phagocytes, including macrophages to counteract the host immune system. Bacillus cereus infections are characterized by bacteremia despite the accumulation of inflammatory cells at the site of infection. This implies that the bacteria have developed means of resisting the host immune system. Bacillus cereus spores survive, germinate, and multiply in contact with macrophages, eventually producing toxins that kill these cells. However, the exact mechanism by which B. cereus evades immune attack remains unclear. This review addresses the interaction between B. cereus and macrophages, highlighting, in particular, the ways in which the bacteria escape the microbicidal activities of professional phagocytes.

The mutated staphylococcal H35A α-toxin inhibits adhesion and invasion of Staphylococcus aureus and group A streptococci

In previous studies we demonstrated that the staphylococcal α-toxin inhibits adhesion and invasion of S. aureus by epithelial cells through binding to α5β1 integrin, a receptor of fibronectin. Moreover, we revealed that a H35A mutation abolishes the cytotoxicity of α-toxin completely. These findings led us to hypothesize that the H35A mutated α-toxin may be explored as a potential inhibitor for bacterial adhesion and invasion of epithelial cells. In this study, we examined the impact of the H35A α-toxin on staphylococcal capacity of adhering to and invading into epithelial cells and found that the addition of H35A α-toxin in the culture medium dramatically inhibited S. aureus' ability to adhere to and internalize into epithelial cells. Importantly, we demonstrated that both the staphylococcal α-toxin and H35A mutated α-toxin are capable of retarding the adhesion and invasion of epithelial cells by Streptococcus pyogenes. These findings suggest that the H35A toxoid has the potential to be utilized as an inhibitor of S. aureus and S. pyogenes ability to adhere to and invade epithelial cells.

The pore-forming haemolysins of bacillus cereus: a review

The Bacillus cereus sensu lato group contains diverse Gram-positive spore-forming bacteria that can cause gastrointestinal diseases and severe eye infections in humans. They have also been incriminated in a multitude of other severe, and frequently fatal, clinical infections, such as osteomyelitis, septicaemia, pneumonia, liver abscess and meningitis, particularly in immuno-compromised patients and preterm neonates. The pathogenic properties of this organism are mediated by the synergistic effects of a number of virulence products that promote intestinal cell destruction and/or resistance to the host immune system. This review focuses on the pore-forming haemolysins produced by B. cereus: haemolysin I (cereolysin O), haemolysin II, haemolysin III and haemolysin IV (CytK). Haemolysin I belongs to the cholesterol-dependent cytolysin (CDC) family whose best known members are listeriolysin O and perfringolysin O, produced by L. monocytogenes and C. perfringens respectively. HlyII and CytK are oligomeric ß-barrel pore-forming toxins related to the α-toxin of S. aureus or the ß-toxin of C. perfringens. The structure of haemolysin III, the least characterized haemolytic toxin from the B. cereus, group has not yet been determined.

Antisense RNA regulation and application in the development of novel antibiotics to combat multidrug resistant bacteria

Despite the availability of antibiotics and vaccines, infectious diseases remain one of most dangerous threats to humans and animals. The overuse and misuse of antibacterial agents have led to the emergence of multidrug resistant bacterial pathogens. Bacterial cells are often resilient enough to survive in even the most extreme environments. To do so, the organisms have evolved different mechanisms, including a variety of two-component signal transduction systems, which allow the bacteria to sense the surrounding environment and regulate gene expression in order to adapt and respond to environmental stimuli. In addition, some bacteria evolve resistance to antibacterial agents while many bacterial cells are able to acquire resistance genes from other bacterial species to enable them to survive in the presence of toxic antimicrobial agents. The crisis of antimicrobial resistance is an unremitting menace to human health and a burden on public health. The rapid increase in antimicrobial resistant organisms and limited options for development of new classes of antibiotics heighten the urgent need to develop novel potent antibacterial therapeutics in order to combat multidrug resistant infections. In this review, we introduce the regulatory mechanisms of antisense RNA and significant applications of regulated antisense RNA interference technology in early drug discovery. This includes the identification and evaluation of drug targets in vitro and in vivo, the determination of mode of action for antibiotics and new antibacterial agents, as well as the development of peptide-nucleic acid conjugates as novel antibacterials.

Immunopathogenesis of Staphylococcus aureus pulmonary infection

Staphylococcus aureus is a common human pathogen highly evolved as both a component of the commensal flora and as a major cause of invasive infection. Severe respiratory infection due to staphylococci has been increasing due to the prevalence of more virulent USA300 CA-MRSA strains in the general population. The ability of S. aureus to adapt to the milieu of the respiratory tract has facilitated its emergence as a respiratory pathogen. Its metabolic versatility, the ability to scavenge iron, coordinate gene expression, and the horizontal acquisition of useful genetic elements have all contributed to its success as a component of the respiratory flora, in hospitalized patients, as a complication of influenza and in normal hosts. The expression of surface adhesins facilitates its persistence in the airways. In addition, the highly sophisticated interactions of the multiple S. aureus virulence factors, particularly the α-hemolysin and protein A, with diverse immune effectors in the lung such as ADAM10, TNFR1, EGFR, immunoglobulin, and complement all contribute to the pathogenesis of staphylococcal pneumonia.

Identification of single nucleotide polymorphisms associated with hyperproduction of alpha-toxin in Staphylococcus aureus

The virulence factor α-toxin (hla) is needed by Staphylococcus aureus in order to cause infections in both animals and humans. Although the complicated regulation of hla expression has been well studied in human S. aureus isolates, the mechanisms of of hla regulation in bovine S. aureus isolates remain undefined. In this study, we found that many bovine S. aureus isolates, including the RF122 strain, generate dramatic amounts of α-toxin in vitro compared with human clinical S. aureus isolates, including MRSA WCUH29 and MRSA USA300. To elucidate potential regulatory mechanisms, we analyzed the hla promoter regions and identified predominant single nucleotide polymorphisms (SNPs) at positions −376, −483, and −484 from the start codon in α-toxin hyper-producing isolates. Using site-directed mutagenesis and hla promoter-gfp-luxABCDE dual reporter approaches, we demonstrated that the SNPs contribute to the differential control of hla expression among bovine and human S. aureus isolates. Using a DNA affinity assay, gel-shift assays and a null mutant, we identified and revealed that an hla positive regulator, SarZ, contributes to the involvement of the SNPs in mediating hla expression. In addition, we found that the bovine S. aureus isolate RF122 exhibits higher transcription levels of hla positive regulators, including agrA, saeR, arlR and sarZ, but a lower expression level of hla repressor rot compared to the human S. aureus isolate WCUH29. Our results indicate α-toxin hyperproduction in bovine S. aureus is a multifactorial process, influenced at both the genomic and transcriptional levels. Moreover, the identification of predominant SNPs in the hla promoter region may provide a novel method for genotyping the S. aureus isolates.

Haemolysin II is a Bacillus cereus virulence factor that induces apoptosis of macrophages

Bacillus cereus is a Gram-positive spore-forming bacterium causing food poisoning and serious opportunistic infections. These infections are characterized by bacterial accumulation despite the recruitment of phagocytic cells. The precise mechanisms and the bacterial factors allowing B. cereus to circumvent host immune responses remain to be elucidated. We have previously shown that B. cereus induces macrophage cell death by an unknown mechanism. Here we identified the toxic component from the B. cereus supernatant. We report that Haemolysin II (HlyII) provokes macrophage cell death by apoptosis through its pore-forming activity. The HlyII-induced apoptotic pathway is caspase 3 and 8 dependent, thus most likely mediated by the death receptor pathway. Using insects and mice as in vivo models, we show that deletion of hlyII strongly reduces virulence. In addition, we show that after infection of Bombyx mori larvae, the immune cells are apoptotic, demonstrating that HlyII induces apoptosis of phagocytic cells in vivo. Altogether, our results clearly unravel HlyII as a novel virulence protein that induces apoptosis in phagocytic cells in vitro and in vivo.

Heparan Sulfate Proteoglycans in Infection

To cause infections, microbial pathogens elaborate a multitude of factors that interact with host components. Using these host–pathogen interactions to their advantage, pathogens attach, invade, disseminate, and evade host defense mechanisms to promote their survival in the hostile host environment. Many viruses, bacteria, and parasites express adhesins that bind to cell surface heparan sulfate proteoglycans (HSPGs) to facilitate their initial attachment and subsequent cellular entry. Some pathogens also secrete virulence factors that modify HSPG expression. HSPGs are ubiquitously expressed on the cell surface of adherent cells and in the extracellular matrix. HSPGs are composed of one or several heparan sulfate (HS) glycosaminoglycan chains attached covalently to specific core proteins. For most intracellular pathogens, cell surface HSPGs serve as a scaffold that facilitates the interaction of microbes with secondary receptors that mediate host cell entry. Consistent with this mechanism, addition of HS or its pharmaceutical functional mimic, heparin, inhibits microbial attachment and entry into cultured host cells, and HS-binding pathogens can no longer attach or enter cultured host cells whose HS expression has been reduced by enzymatic treatment or chemical mutagenesis. In pathogens where the specific HS adhesin has been identified, mutant strains lacking HS adhesins are viable and show normal growth rates, suggesting that the capacity to interact with HSPGs is strictly a virulence activity. The goal of this chapter is to provide a mechanistic overview of our current understanding of how certain microbial pathogens subvert HSPGs to promote their infection, using specific HSPG–pathogen interactions as representative examples.

Integrins as therapeutic targets: lessons and opportunities

The integrins are a large family of cell adhesion molecules that are essential for the regulation of cell growth and function. The identification of key roles for integrins in a diverse range of diseases, including cancer, infection, thrombosis and autoimmune disorders, has revealed their substantial potential as therapeutic targets. However, so far, pharmacological inhibitors for only three integrins have received marketing approval. This article discusses the structure and function of integrins, their roles in disease and the chequered history of the approved integrin antagonists. Recent advances in the understanding of integrin function, ligand interaction and signalling pathways suggest novel strategies for inhibiting integrin function that could help harness their full potential as therapeutic targets.

Heterodimeric integrin complexes containing beta1-integrin promote internalization and lethality of anthrax toxin

To kill macrophages, the lethal factor component of Bacillus anthracis toxin binds to a carrier protein (PA), which then interacts with the CMG2 receptor protein on the cell surface and is endocytosed into the cytoplasm. CMG2, as well as TEM8, a second PA receptor not present on macrophages, contain a von Willebrand A domain that is crucial for toxin binding. Here we report that integrin beta1, another cell surface von Willebrand A domain protein, can mediate and potentiate anthrax toxin endocytosis. By using microarray-based analysis to globally correlate gene expression profiles with toxin sensitivity, we associated toxin effects with the integrin-activating proteins osteopontin and CD44. Further study showed that PA binds to alpha4beta1- and alpha5beta1-integrin complexes, leading to their conjoint endocytosis, and also interacts-weakly relative to CMG2 but comparably to TEM8--with purified alpha5beta1 complex in vitro. Monoclonal antibody directed against beta1-integrin or its alpha integrin partners reduced PA/integrin endocytosis and anthrax toxin lethality, and hyaluronic acid--which interferes with CD44-mediated integrin activation--had similar effects. Remarkably, whereas deficiency of CMG2 protected macrophages from rapid killing by large toxin doses (>50 ng/mL), by 24 h the toxin-treated cells were dead. Such late killing of CMG2-deficient cells by high dose toxin as well as the late death observed during exposure of CMG2-producing macrophages to low-dose toxin (<1 ng/mL), was dependent on integrin function. Effects of inactivating both CMG2 and integrin were synergistic. Collectively, our findings argue strongly that beta1-integrin can both potentiate CMG2-mediated endocytosis and serve independently as a low-affinity PA receptor.

Role of a disintegrin and metalloprotease 10 in Staphylococcus aureus alpha-hemolysin-mediated cellular injury

Staphylococcus aureus alpha-hemolysin (Hla), a potent cytotoxin, plays an important role in the pathogenesis of staphylococcal diseases, including those caused by methicillin-resistant epidemic strains. Hla is secreted as a water-soluble monomer that undergoes a series of conformational changes to generate a heptameric, beta-barrel structure in host membranes. Structural maturation of Hla depends on its interaction with a previously unknown proteinaceous receptor in the context of the cell membrane. It is reported here that a disintegrin and metalloprotease 10 (ADAM10) interacts with Hla and is required to initiate the sequence of events whereby the toxin is transformed into a cytolytic pore. Hla binding to the eukaryotic cell requires ADAM10 expression. Further, ADAM10 is required for Hla-mediated cytotoxicity, most notably when the toxin is present at low concentrations. These data thus implicate ADAM10 as the probable high-affinity toxin receptor. Upon Hla binding, ADAM10 relocalizes to caveolin 1-enriched lipid rafts that serve as a platform for the clustering of signaling molecules. It is demonstrated that the Hla-ADAM10 complex initiates intracellular signaling events that culminate in the disruption of focal adhesions.

Activation of NF-κB pathway and TNF-α are involved in the cytotoxicity of anthrax lethal toxin in bovine BoMac macrophages

Anthrax lethal toxin (LeTx) is an important virulence factor of Bacillus anthracis and causes illness and lethality for both animals and humans. Because species demonstrate varied sensitivity to anthrax intoxication, we investigated signaling pathways involved in anthrax LeTx cytotoxicity using a bovine macrophage cell line (BoMac). We found that bovine macrophages are sensitive to LeTx as displayed by a concentration-dependent increase in cell death. LeTx induced the degradation of I-κB and increased the nuclear translocation of NF-κB in BoMac cells. Blocking NF-κB activation with either chemical inhibitors or a dominant negative super-repressor I-κBαm eliminated LeTx-induced cell death. LeTx-induced production of TNF-α that contributed dramatically to cellular cytotoxicity. Inhibiting NF-κB activation eliminated TNF-α release and decreased cytotoxicity. The caspase pathway was also important for cytotoxicity as specific inhibitors abrogated LeTx-induced cell death. Taken together, our results show that activation of the NF-κB pathway and TNF-α production contribute to the cytotoxicity of anthrax LeTx in bovine macrophages.

NOD2 contributes to cutaneous defense against Staphylococcus aureus through alpha-toxin-dependent innate immune activation

Staphylococcus aureus is a major cause of community-acquired and nosocomial infections including the life-threatening conditions endocarditis, necrotizing pneumonia, necrotizing fasciitis, and septicemia. Toll-like receptor (TLR)-2, a membrane-bound microbial sensor, detects staphylococcal components, but macrophages lacking TLR2 or both TLR2 and TLR4 remain S. aureus responsive, suggesting that an alternative microbial recognition receptor might be involved. The cytoplasmic sensor nucleotide-binding oligomerization domain containing (NOD) 2/caspase recruitment domain (CARD) 15 detects muramyl dipeptide from bacterial peptidoglycans and mediates cytokine responses to S. aureus in vitro, but the physiological significance of these observations is not well defined. Here we show that NOD2-deficient mice exhibit a delayed but ultimately exacerbated ulcerative response and impaired bacterial clearance after s.c. infection with S. aureus. NOD2-dependent recognition of S. aureus and muramyl dipeptide is facilitated by alpha-toxin (alpha-hemolysin), a pore-forming toxin and virulence factor of the pathogen. The action of NOD2 is dependent on IL-1beta-amplified production of IL-6, which promotes rapid bacterial killing by neutrophils. These results significantly broaden the physiological importance of NOD2 in innate immunity from the recognition of bacteria that primarily enter the cytoplasm to the detection of bacteria that typically reside extracellularly and demonstrate that this microbial sensor contributes to the discrimination between commensal bacteria and bacterial pathogens that elaborate pore-forming toxins.

Alpha-toxin facilitates the generation of CXC chemokine gradients and stimulates neutrophil homing in Staphylococcus aureus pneumonia

BACKGROUND

Staphylococcus aureus alpha-toxin is a major virulence factor, but its mechanism of action in vivo is incompletely understood.

METHODS

We examined the role of alpha-toxin in S. aureus pneumonia using the mouse model of intranasal lung infection with S. aureus strain 8325-4 (hla(+) S. aureus) and an alpha-toxin-deficient mutant strain made on the 8325-4 background (hla(-) S. aureus).

RESULTS

Intranasal infection of mice with hla(-) S. aureus resulted in substantially less lung injury and inflammation, pulmonary edema, and tissue bacterial burden than did infection with hla(+) S. aureus. Furthermore, fewer mice infected with hla(-) S. aureus died of the infection, compared with those infected with hla(+) S. aureus. Levels of the CXC chemokines keratinocyte-derived chemokine and macrophage inflammatory protein-2 were significantly lower in the airways of mice infected with hla(-) S. aureus, and this difference was the result of reduced secretion of newly synthesized chemokines into the airway. Consistent with these data, significantly fewer neutrophils were present in the airways and lungs of mice infected with hla(-) S. aureus, compared with those infected with hla(+) S. aureus.

CONCLUSIONS

These data suggest that alpha-toxin enhances virulence by facilitating the generation of CXC chemokine gradients and stimulating chemokine-induced neutrophil influx in S. aureus pneumonia.

The H35A mutated alpha-toxin interferes with cytotoxicity of staphylococcal alpha-toxin

Staphylococcal alpha-toxin is an important virulence factor for Staphylococcus aureus to cause severe infections. In this study, we explored whether the toxoid of alpha-toxin may be utilized to block the toxicity of wild-type alpha-toxin. We created a series of H35A mutated alpha-toxin expression strains and revealed that the H35A mutation eliminates the activity of alpha-toxin using a human lung epithelial cell line (A549). More importantly, we found that either the pretreatment or simultaneous treatment of the epithelial cells with alpha-toxin-H35A completely disrupted the cytotoxicity of alpha-toxin. Specifically, we demonstrated that alpha-toxin-H35A can effectively interfere with the pore formation and the internalization of alpha-toxin using cytotoxicity and immunofluorescence assays. In addition, we found that the removal of either the 30-amino-acid (aa) or 99-aa C-terminal region of alpha-toxin-H35A reactivated its cytotoxicity, indicating that interactions between the alanine residue at position 35 and these C-terminal regions may be associated with interrupting the toxic activity of alpha-toxin-H35A. Taken together, these results suggest that the alpha-toxin-H35A protein may be developed as a potential alternative therapeutic agent for treating early stages of S. aureus infections.

Effects of Staphylococcus aureus-hemolysin A on calcium signalling in immortalized human airway epithelial cells

Part of the innate defence of bronchial epithelia against bacterial colonization is secretion of salt and water which generally depends on coordinated actions of receptor-mediated cAMP- and calcium signalling. The hypothesis that Staphylococcus aureus-virulence factors interfere with endogenous signals in host cells was tested by measuring agonist-mediated changes in [Ca(2+)](i) in S9 cells upon pre-incubation with bacterial secretory products. S9 cells responded to mAChR-activation with calcium release from intracellular stores and capacitative calcium influx. Treatment of cells with culture supernatants of S. aureus (COL) or with recombinant alpha-hemolysin (Hla) resulted in time- and concentration-dependent changes in [Ca(2+)](i). High concentrations of Hla (2000 ng/ml) resulted in elevations in [Ca(2+)](i) elicited by accelerated calcium influx. A general Hla-mediated permeabilization of S9 cell membranes to small molecules, however, did not occur. Lower concentrations of Hla (200 ng/ml) induced a reduction in [Ca(2+)](i)-levels during the sustained plateau phase of receptor-mediated calcium signalling which was abolished by pre-incubation of cells with carboxyeosin, an inhibitor of the plasma membrane calcium-ATPase. This indicates that low concentrations of Hla change calcium signalling by accelerating pump-driven extrusion of Ca(2+) ions. In vivo, such a mechanism may result in attenuation of calcium-mediated cellular defence functions and facilitation of bacterial adherence to the bronchial epithelium.