Resistance to Acute Macrophage Killing Promotes Airway Fitness of Prevalent Community-Acquired Staphylococcus aureus Strains

Bacterial single-cell RNA sequencing captures biofilm transcriptional heterogeneity and differential responses to immune pressure

Biofilm formation is an important mechanism of survival and persistence for many bacterial pathogens. These multicellular communities contain subpopulations of cells that display vast metabolic and transcriptional diversity along with high recalcitrance to antibiotics and host immune defenses. Investigating the complex heterogeneity within biofilm has been hindered by the lack of a sensitive and high-throughput method to assess stochastic transcriptional activity and regulation between bacterial subpopulations, which requires single-cell resolution. We have developed an optimized bacterial single-cell RNA sequencing method, BaSSSh-seq, to study Staphylococcus aureus diversity during biofilm growth and transcriptional adaptations following immune cell exposure. We validated the ability of BaSSSh-seq to capture extensive transcriptional heterogeneity during biofilm compared to planktonic growth. Application of new computational tools revealed transcriptional regulatory networks across the heterogeneous biofilm subpopulations and identification of gene sets that were associated with a trajectory from planktonic to biofilm growth. BaSSSh-seq also detected alterations in biofilm metabolism, stress response, and virulence that were tailored to distinct immune cell populations. This work provides an innovative platform to explore biofilm dynamics at single-cell resolution, unlocking the potential for identifying biofilm adaptations to environmental signals and immune pressure.

HDAC6 inhibitor promotes reactive oxygen species-meditated clearance of Staphylococcus aureus in macrophage

Staphylococcus aureus (S. aureus) pneumonia has become an increasingly important public health problem. Recent evidence suggests that epigenetic modifications are critical in the host immune defence against pathogen infection. In this study, we found that S. aureus infection induces the expression of histone deacetylase 6 (HDAC6) in a dose-dependent manner. Furthermore, by using a S. aureus pneumonia mouse model, we showed that the HDAC6 inhibitor, tubastatin A, demonstrates a protective effect in S. aureus pneumonia, decreasing the mortality and destruction of lung architecture, reducing the bacterial burden in the lungs and inhibiting inflammatory responses. Mechanistic studies in primary bone marrow-derived macrophages demonstrated that the HDAC6 inhibitors, tubastatin A and tubacin, reduced the intracellular bacterial load by promoting bacterial clearance rather than regulating phagocytosis. Finally, N-acetyl-L- cysteine, a widely used reactive oxygen species (ROS) scavenger, antagonized ROS production and significantly inhibited tubastatin A-induced S. aureus clearance. These findings demonstrate that HDAC6 inhibitors promote the bactericidal activity of macrophages by inducing ROS, an important host factor for S. aureus clearance and production. Our study identified HDAC6 as a suitable epigenetic modification target for preventing S. aureus infection, and tubastatin A as a useful compound in treating S. aureus pneumonia.

The Spx stress regulator confers high-level β-lactam resistance and decreases susceptibility to last-line antibiotics in methicillin-resistant Staphylococcus aureus

Infections caused by methicillin-resistant Staphylococcus aureus (MRSA) are a leading cause of mortality worldwide. MRSA has acquired resistance to next-generation β-lactam antibiotics through the horizontal acquisition of the mecA resistance gene. Development of high resistance is, however, often associated with additional mutations in a set of chromosomal core genes, known as potentiators, which, through poorly described mechanisms, enhance resistance. The yjbH gene was recently identified as a hot spot for adaptive mutations during severe infections. Here, we show that inactivation of yjbH increased β-lactam MICs up to 16-fold and transformed MRSA cells with low levels of resistance to being homogenously highly resistant to β-lactams. The yjbH gene encodes an adaptor protein that targets the transcriptional stress regulator Spx for degradation by the ClpXP protease. Using CRISPR interference (CRISPRi) to knock down spx transcription, we unambiguously linked hyper-resistance to the accumulation of Spx. Spx was previously proposed to be essential; however, our data suggest that Spx is dispensable for growth at 37°C but becomes essential in the presence of antibiotics with various targets. On the other hand, high Spx levels bypassed the role of PBP4 in β-lactam resistance and broadly decreased MRSA susceptibility to compounds targeting the cell wall or the cell membrane, including vancomycin, daptomycin, and nisin. Strikingly, Spx potentiated resistance independently of its redox-sensing switch. Collectively, our study identifies a general stress pathway that, in addition to promoting the development of high-level, broad-spectrum β-lactam resistance, also decreases MRSA susceptibility to critical antibiotics of last resort.

STAT3 Deficiency Alters the Macrophage Activation Pattern and Enhances Matrix Metalloproteinase 9 Expression during Staphylococcal Pneumonia

Staphylococcus aureus is a significant cause of morbidity and mortality in pulmonary infections. Patients with autosomal-dominant hyper-IgE syndrome due to STAT3 deficiency are particularly susceptible to acquiring staphylococcal pneumonia associated with lung tissue destruction. Because macrophages are involved in both pathogen defense and inflammation, we investigated the impact of murine myeloid STAT3 deficiency on the macrophage phenotype in vitro and on pathogen clearance and inflammation during murine staphylococcal pneumonia. Murine bone marrow-derived macrophages (BMDM) from STAT3 LysMCre+ knockout or Cre- wild-type littermate controls were challenged with S. aureus, LPS, IL-4, or vehicle control in vitro. Pro- and anti-inflammatory responses as well as polarization and activation markers were analyzed. Mice were infected intratracheally with S. aureus, bronchoalveolar lavage and lungs were harvested, and immunohistofluorescence was performed on lung sections. S. aureus infection of STAT3-deficient BMDM led to an increased proinflammatory cytokine release and to enhanced upregulation of costimulatory MHC class II and CD86. Murine myeloid STAT3 deficiency did not affect pathogen clearance in vitro or in vivo. Matrix metalloproteinase 9 was upregulated in Staphylococcus-treated STAT3-deficient BMDM and in lung tissues of STAT3 knockout mice infected with S. aureus. Moreover, the expression of miR-155 was increased. The enhanced inflammatory responses and upregulation of matrix metalloproteinase 9 and miR-155 expression in murine STAT3-deficient as compared with wild-type macrophages during S. aureus infections may contribute to tissue damage as observed in STAT3-deficient patients during staphylococcal pneumonia.

Antibacterial and anti-inflammatory activities of Nothopanax scutellarium, Moringa oleifera and Piper betle extracts on staphylococcal mastitis animal model

Inappropriate and prolonged administration of antibiotics in mastitis could cause resistance and herbal treatment might could be one alternative treatment. Nothopanax scutellarium, Moringa oleifera, and Piper betle are medicinal plants that contain various active compounds, including antibacterial and anti-inflammatory agents, but their potential in treating mastitis is minimum. The aim of this study was to assess the effectiveness of those plants against mastitis in rabbit model induced by Staphylococcus aureus. A total of 25 lactating rabbits (Oryctolagus cuniculus) weighing 3.0±0.4 kg were grouped into five groups: healthy control; mastitis control, and three treatment groups (Nothopanax scutellarium, Moringa oleifera, and Piper betle). Except the negative control, all animals were inoculated with 0.15 mL of S. aureus containing 1.5x107 colony forming unit (CFU)/mL on eight days after giving birth. The extract was administered orally after four hours Staphylococcus aureus inoculation at a dose of 50 mg/kg body weight, twice a day for five consecutive days. The number of bacteria in the milk and the level of serum interleukin 6 (IL-6) were measured and histopathological examination of mammary gland tissues were analyzed. The log number of total plate count of Staphylococcus aureus indicated that all extract groups had significant lower of bacterial logs compared to mastitis control (all comparisons had p<0.05) with the lowest was found in Piper betle group, followed by Nothopanax scutellarium and Moringa oleifera groups. The enzyme-linked immunosorbent assay (ELISA) results showed that all ethanolic extract groups had significantly lower levels of IL-6 compared to the mastitis control (all comparisons had p<0.05). The histopathology assessment suggested that extract groups had lower infiltration of inflammatory cells such as lymphocytes and macrophages in alveoli compared to the mastitis control group. In conclusion, all three extracts contained antibacterial and anti-inflammatory activities and Piper betle had the most effective in reducing bacterial growth and IL-6 level compared to others.

Copper ions inhibit pentose phosphate pathway function in Staphylococcus aureus

To gain a better insight of how Copper (Cu) ions toxify cells, metabolomic analyses were performed in S. aureus strains that lacks the described Cu ion detoxification systems (ΔcopBL ΔcopAZ; cop-). Exposure of the cop- strain to Cu(II) resulted in an increase in the concentrations of metabolites utilized to synthesize phosphoribosyl diphosphate (PRPP). PRPP is created using the enzyme phosphoribosylpyrophosphate synthetase (Prs) which catalyzes the interconversion of ATP and ribose 5-phosphate to PRPP and AMP. Supplementing growth medium with metabolites requiring PRPP for synthesis improved growth in the presence of Cu(II). A suppressor screen revealed that a strain with a lesion in the gene coding adenine phosphoribosyltransferase (apt) was more resistant to Cu. Apt catalyzes the conversion of adenine with PRPP to AMP. The apt mutant had an increased pool of adenine suggesting that the PRPP pool was being redirected. Over-production of apt, or alternate enzymes that utilize PRPP, increased sensitivity to Cu(II). Increasing or decreasing expression of prs resulted in decreased and increased sensitivity to growth in the presence of Cu(II), respectively. We demonstrate that Prs is inhibited by Cu ions in vivo and in vitro and that treatment of cells with Cu(II) results in decreased PRPP levels. Lastly, we establish that S. aureus that lacks the ability to remove Cu ions from the cytosol is defective in colonizing the airway in a murine model of acute pneumonia, as well as the skin. The data presented are consistent with a model wherein Cu ions inhibits pentose phosphate pathway function and are used by the immune system to prevent S. aureus infections.

Time-Course Transcriptome Analysis of the Lungs of Mice Challenged with Aerosols of Methicillin-Resistant Staphylococcus aureus USA300 Clone Reveals Inflammatory Balance

USA300, a dominant clone of community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA), is circulating globally and can cause necrotizing pneumonia with high morbidity and mortality. To further reveal the host anti-MRSA infection immune response, we established a mouse model of acute primary MRSA pneumonia challenged with aerosols of the USA300 clone. A time-course transcriptome analysis of the lungs collected at 0, 12, 24, 48 and 96 h post-infection (hpi) was conducted using RNA sequencing (RNA-seq) and multiple bioinformatic analysis methods. The change trend of histopathology and five innate immune cell (neutrophils, mononuclear cells, eosinophils, macrophages, DC cells) proportions in the lungs after infection was also examined. We observed a distinct acute pulmonary recovery process. A rapid initiation period of inflammation was present at 12 hpi, during which the IL-17 pathway dominantly mediated inflammation and immune defense. The main stages of host inflammatory response occurred at 24 and 48 hpi, and the regulation of interferon activation and macrophage polarization played an important role in the control of inflammatory balance at this stage. At 96 hpi, cellular proliferation processes associated with host repair were observed, as well as adaptive immunity and complement system responses involving C1q molecules. More importantly, the data provide new insight into and identify potential functional genes involved in the checks and balances occurring between host anti-inflammatory and proinflammatory responses. To the best of our knowledge, this is the first study to investigate transcriptional responses throughout the inflammatory recovery process in the lungs after MRSA infection. Our study uncovers valuable research targets for key regulatory mechanisms underlying the pathogenesis of MRSA lung infections, which may help to develop novel treatment strategies for MRSA pneumonia.

Transcriptional Profiling of Phagocytic Leukocytes and Microglia Reveals a Critical Role for Reactive Oxygen Species in Biofilm Containment during Staphylococcus aureus Craniotomy Infection

Craniotomies are performed to treat a variety of intracranial pathology. Surgical site infection remains a complication of craniotomy despite the use of prophylactic antibiotics and universal sterile precautions. Infections occur in 1-3% of procedures, with approximately half caused by Staphylococcus aureus that forms a biofilm on the bone flap and is recalcitrant to systemic antibiotic therapy. We used an S. aureus-dsRed construct to compare the phagocytic capacity of leukocytes and microglia in vitro and in vivo using a mouse model of craniotomy infection. In addition, single-cell RNA sequencing (scRNA-seq) was applied to determine whether a transcriptional signature could be identified for phagocytic versus nonphagocytic cells in vivo. S. aureus was phagocytosed to equivalent extents in microglia, macrophages, neutrophils, and granulocytic myeloid-derived suppressor cells in vitro; however, microglial uptake of S. aureus was limited in vivo, whereas the other leukocyte populations exhibited phagocytic activity. scRNA-seq comparing the transcriptional signatures of phagocytic (S. aureus-dsRed+) versus nonphagocytic (S. aureus-dsRed-) leukocytes identified classical pathways enriched in phagocytic cells (i.e., reactive oxygen species [ROS]/reactive nitrogen species, lysosome, iron uptake, and transport), whereas nonphagocytic populations had increased ribosomal, IFN, and hypoxia signatures. scRNA-seq also revealed a robust ROS profile, which led to the exploration of craniotomy infection in NADPH oxidase 2 knockout mice. S. aureus burden, leukocyte recruitment, and intracellular bacterial load were significantly increased in NADPH oxidase 2 KO compared with wild-type animals. Collectively, these results highlight the importance of ROS generation in phagocytes for S. aureus biofilm containment, but not clearance, during craniotomy infection.

Age-induced prostaglandin E2 impairs mitochondrial fitness and increases mortality to influenza infection

Aging impairs the immune responses to influenza A virus (IAV), resulting in increased mortality to IAV infections in older adults. However, the factors within the aged lung that compromise host defense to IAV remain unknown. Using a murine model and human samples, we identified prostaglandin E₂ (PGE₂), as such a factor. Senescent type II alveolar epithelial cells (AECs) are overproducers of PGE₂ within the aged lung. PGE₂ impairs the proliferation of alveolar macrophages (AMs), critical cells for defense against respiratory pathogens, via reduction of oxidative phosphorylation and mitophagy. Importantly, blockade of the PGE₂ receptor EP2 in aged mice improves AM mitochondrial function, increases AM numbers and enhances survival to IAV infection. In conclusion, our study reveals a key mechanism that compromises host defense to IAV, and possibly other respiratory infections, with aging and suggests potential new therapeutic or preventative avenues to protect against viral respiratory disease in older adults.

Staphylococcus aureus and Acinetobacter sp. inhibit osseointegration of orthopaedic implants

Bacterial infections routinely cause inflammation and thereby impair osseointegration of orthopaedic implants. Acinetobacter spp., which causes osteomyelitis following trauma, on or off the battlefield, was however reported to cause neither osteomyelitis nor osteolysis in rodents. We therefore compared the effects of Acinetobacter strain M2 to those of Staphylococcus aureus in a murine implant infection model. Sterile implants and implants with adherent bacteria were inserted in the femur of mice. Bacterial burden, levels of pro-inflammatory cytokines, and osseointegration were measured. All infections were localized to the implant site. Infection with either S. aureus or Acinetobacter strain M2 increased the levels of pro-inflammatory cytokines and the chemokine CCL2 in the surrounding femurs, inhibited bone formation around the implant, and caused loss of the surrounding cortical bone leading to decreases in both histomorphometric and biomechanical measures of osseointegration. Genetic deletion of TLR2 and TLR4 from the mice partially reduced the effects of Acinetobacter strain M2 on osseointegration but did not alter the effects of S. aureus. This is the first report that Acinetobacter spp. impair osseointegration of orthopaedic implants in mice and the murine model developed for this study will be useful for future efforts to clarify the mechanism of implant failure due to Acinetobacter spp. and to assess novel diagnostic tools or therapeutic agents.

Copper Resistance Promotes Fitness of Methicillin-Resistant Staphylococcus aureus during Urinary Tract Infection

Urinary tract infection (UTI) is one of the most common infectious conditions affecting people in the United States and around the world. Our knowledge of the host-pathogen interaction during UTI caused by Gram-positive bacterial uropathogens is limited compared to that for Gram-negative pathogens. Here, we investigated whether copper and the primary copper-containing protein, ceruloplasmin, are mobilized to urine during naturally occurring UTI caused by Gram-positive uropathogens in patients. Next, we probed the role of copper resistance in the fitness of methicillin-resistant Staphylococcus aureus (MRSA) during experimental UTI in a murine model. Our findings demonstrate that urinary copper and ceruloplasmin content are elevated during UTI caused by Enterococcus faecalis, S. aureus, S. epidermidis, and S. saprophyticus. MRSA strains successfully colonize the urinary tract of female CBA mice with selective induction of inflammation in the kidneys but not the bladder. MRSA mutants lacking CopL, a copper-binding cell surface lipoprotein, and the ACME genomic region containing copL, exhibit decreased fitness in the mouse urinary tract compared to parental strains. Copper sensitivity assays, cell-associated copper and iron content, and bioavailability of iron during copper stress demonstrate that homeostasis of copper and iron is interlinked in S. aureus. Importantly, relative fitness of the MRSA mutant lacking the ACME region is further decreased in mice that receive supplemental copper compared to the parental strain. In summary, copper is mobilized to the urinary tract during UTI caused by Gram-positive pathogens, and copper resistance is a fitness factor for MRSA during UTI. IMPORTANCE Urinary tract infection (UTI) is an extremely common infectious condition affecting people throughout the world. Increasing antibiotic resistance in pathogens causing UTI threatens our ability to continue to treat patients in the clinics. Better understanding of the host-pathogen interface is critical for development of novel interventional strategies. Here, we sought to elucidate the role of copper in host-Staphylococcus aureus interaction during UTI. Our results reveal that copper is mobilized to the urine as a host response in patients with UTI. Our findings from the murine model of UTI demonstrate that copper resistance is involved in the fitness of methicillin-resistant S. aureus (MRSA) during interaction with the host. We also establish a critical link between adaptation to copper stress and iron homeostasis in S. aureus.

The Role of Macrophages in Staphylococcus aureus Infection

Staphylococcus aureus is a member of the human commensal microflora that exists, apparently benignly, at multiple sites on the host. However, as an opportunist pathogen it can also cause a range of serious diseases. This requires an ability to circumvent the innate immune system to establish an infection. Professional phagocytes, primarily macrophages and neutrophils, are key innate immune cells which interact with S. aureus, acting as gatekeepers to contain and resolve infection. Recent studies have highlighted the important roles of macrophages during S. aureus infections, using a wide array of killing mechanisms. In defense, S. aureus has evolved multiple strategies to survive within, manipulate and escape from macrophages, allowing them to not only subvert but also exploit this key element of our immune system. Macrophage-S. aureus interactions are multifaceted and have direct roles in infection outcome. In depth understanding of these host-pathogen interactions may be useful for future therapeutic developments. This review examines macrophage interactions with S. aureus throughout all stages of infection, with special emphasis on mechanisms that determine infection outcome.

Combating Implant Infections: Shifting Focus from Bacteria to Host

The widespread use of biomaterials to support or replace body parts is increasingly threatened by the risk of implant-associated infections. In the quest for finding novel anti-infective biomaterials, there generally has been a one-sided focus on biomaterials with direct antibacterial properties, which leads to excessive use of antibacterial agents, compromised host responses, and unpredictable effectiveness in vivo. This review sheds light on how host immunomodulation, rather than only targeting bacteria, can endow biomaterials with improved anti-infective properties. How antibacterial surface treatments are at risk to be undermined by biomaterial features that dysregulate the protection normally provided by critical immune cell subsets, namely, neutrophils and macrophages, is discussed. Accordingly, how the precise modification of biomaterial surface biophysical cues, or the incorporation of immunomodulatory drug delivery systems, can render biomaterials with the necessary immune-compatible and immune-protective properties to potentiate the host defense mechanisms is reviewed. Within this context, the protective role of host defense peptides, metallic particles, quorum sensing inhibitors, and therapeutic adjuvants is discussed. The highlighted immunomodulatory strategies may lay a foundation to develop anti-infective biomaterials, while mitigating the increasing threat of antibacterial drug resistance.

Linezolid Attenuates Lethal Lung Damage during Postinfluenza Methicillin-Resistant Staphylococcus aureus Pneumonia

Postinfluenza methicillin-resistant Staphylococcus aureus (MRSA) infection can quickly develop into severe, necrotizing pneumonia, causing over 50% mortality despite antibiotic treatments. In this study, we investigated the efficacy of antibiotic therapies and the impact of S. aureus alpha-toxin in a model of lethal influenza virus and MRSA coinfection. We demonstrate that antibiotics primarily attenuate alpha-toxin-induced acute lethality, even though both alpha-toxin-dependent and -independent mechanisms significantly contribute to animal mortality after coinfection. Furthermore, we found that the protein synthesis-suppressing antibiotic linezolid has an advantageous therapeutic effect on alpha-toxin-induced lung damage, as measured by protein leak and lactate dehydrogenase (LDH) activity. Importantly, using a Panton-Valentine leucocidin (PVL)-negative MRSA isolate from patient sputum, we show that linezolid therapy significantly improves animal survival from postinfluenza MRSA pneumonia compared with vancomycin treatment. Rather than improved viral or bacterial control, this advantageous therapeutic effect is associated with a significantly attenuated proinflammatory cytokine response and acute lung damage in linezolid-treated mice. Together, our findings not only establish a critical role of alpha-toxin in the extreme mortality of secondary MRSA pneumonia after influenza but also provide support for the possibility that linezolid could be a more effective treatment than vancomycin to improve disease outcomes.

Infection of Primary Human Alveolar Macrophages Alters Staphylococcus aureus Toxin Production and Activity

Pulmonary pathogens encounter numerous insults, including phagocytic cells designed to degrade bacteria, while establishing infection in the human lung. Staphylococcus aureus is a versatile, opportunistic pathogen that can cause severe pneumonia, and methicillin-resistant isolates are of particular concern. Recent reports present conflicting data regarding the ability of S. aureus to survive and replicate within macrophages. However, due to use of multiple strains and macrophage sources, making comparisons between reports remains difficult. Here, we established a disease-relevant platform to study innate interactions between S. aureus and human lungs. Human precision-cut lung slices (hPCLS) were subjected to infection by S. aureus LAC (methicillin-resistant) or UAMS-1 (methicillin-sensitive) isolates. Additionally, primary human alveolar macrophages (hAMs) were infected with S. aureus, and antibacterial activity was assessed. Although both S. aureus isolates survived within hAM phagosomes, neither strain replicated efficiently in these cells. S. aureus was prevalent within the epithelial and interstitial regions of hPCLS, with limited numbers present in a subset of hAMs, suggesting that the pathogen may not target phagocytic cells for intracellular growth during natural pulmonary infection. S. aureus-infected hAMs mounted a robust inflammatory response that reflected natural human disease. S. aureus LAC was significantly more cytotoxic to hAMs than UAMS-1, potentially due to isolate-specific virulence factors. The bicomponent toxin Panton-Valentine leukocidin was not produced during intracellular infection, while alpha-hemolysin was produced but was not hemolytic, suggesting that hAMs alter toxin activity. Overall, this study defined a new disease-relevant infection platform to study S. aureus interaction with human lungs and to define virulence factors that incapacitate pulmonary cells.

Monocytes Represent One Source of Bacterial Shielding from Antibiotics following Influenza Virus Infection

Methicillin-resistant Staphylococcus aureus has emerged as a significant contributor to morbidity and mortality associated with influenza infection. In this study, we show in a mouse model that preceding influenza infection promotes S. aureus resistance to killing by antibiotics. This resistance coincides with influenza-induced accumulation of inflammatory monocytes in the lung. CCR type 2 (CCR2) is responsible for pulmonary monocyte recruitment after influenza infection. We found that antibiotic-treated Ccr2-deficient (Ccr2^-/-) mice exhibit significantly improved bacterial control and survival from influenza and methicillin-resistant S. aureus coinfection, despite a delay in viral clearance. Mechanistically, our results from in vivo studies indicate that influenza-induced monocytes serve as reservoirs for intracellular S. aureus survival, thereby promoting bacterial resistance to antibiotic treatment. Blocking CCR2 with a small molecular inhibitor (PF-04178903), in conjunction with antibiotic treatment, enhanced lung bacterial clearance and significantly improved animal survival. Collectively, our study demonstrates that inflammatory monocytes constitute an important and hitherto underappreciated mechanism of the conflicting immune requirements for viral and bacterial clearance by hosts, which subsequently leads to exacerbated outcomes of influenza and S. aureus coinfection.

Alveolar Macrophages in the Resolution of Inflammation, Tissue Repair, and Tolerance to Infection

Pathogen persistence in the respiratory tract is an important preoccupation, and of particular relevance to infectious diseases such as tuberculosis. The equilibrium between elimination of pathogens and the magnitude of the host response is a sword of Damocles for susceptible patients. The alveolar macrophage is the first sentinel of the respiratory tree and constitutes the dominant immune cell in the steady state. This immune cell is a key player in the balance between defense against pathogens and tolerance toward innocuous stimuli. This review focuses on the role of alveolar macrophages in limiting lung tissue damage from potentially innocuous stimuli and from infections, processes that are relevant to appropriate tolerance of potential causes of lung disease. Notably, the different anti-inflammatory strategies employed by alveolar macrophages and lung tissue damage control are explored. These two properties, in addition to macrophage manipulation by pathogens, are discussed to explain how alveolar macrophages may drive pathogen persistence in the airways.

Bacterial-Host Interactions: Physiology and Pathophysiology of Respiratory Infection

It has long been thought that respiratory infections are the direct result of acquisition of pathogenic viruses or bacteria, followed by their overgrowth, dissemination, and in some instances tissue invasion. In the last decades, it has become apparent that in contrast to this classical view, the majority of microorganisms associated with respiratory infections and inflammation are actually common members of the respiratory ecosystem and only in rare circumstances do they cause disease. This suggests that a complex interplay between host, environment, and properties of colonizing microorganisms together determines disease development and its severity. To understand the pathophysiological processes that underlie respiratory infectious diseases, it is therefore necessary to understand the host-bacterial interactions occurring at mucosal surfaces, along with the microbes inhabiting them, during symbiosis. Current knowledge regarding host-bacterial interactions during asymptomatic colonization will be discussed, including a plausible role for the human microbiome in maintaining a healthy state. With this as a starting point, we will discuss possible disruptive factors contributing to dysbiosis, which is likely to be a key trigger for pathobionts in the development and pathophysiology of respiratory diseases. Finally, from this renewed perspective, we will reflect on current and potential new approaches for treatment in the future.

A horizontally gene transferred copper resistance locus confers hyper-resistance to antibacterial copper toxicity and enables survival of community acquired methicillin resistant Staphylococcus aureus USA300 in macrophages

Excess copper is highly toxic and forms part of the host innate immune system's antibacterial arsenal, accumulating at sites of infection and acting within macrophages to kill engulfed pathogens. We show for the first time that a novel, horizontally gene transferred copper resistance locus (copXL), uniquely associated with the SCCmec elements of the highly virulent, epidemic, community acquired methicillin resistant Staphylococcus aureus (CA-MRSA) USA300, confers copper hyper-resistance. These genes are additional to existing core genome copper resistance mechanisms, and are not found in typical S. aureus lineages, but are increasingly identified in emerging pathogenic isolates. Our data show that CopX, a putative P_1B-3 -ATPase efflux transporter, and CopL, a novel lipoprotein, confer copper hyper-resistance compared to typical S. aureus strains. The copXL genes form an operon that is tightly repressed in low copper environments by the copper regulator CsoR. Significantly, CopX and CopL are important for S. aureus USA300 intracellular survival within macrophages. Therefore, the emergence of new S. aureus clones with the copXL locus has significant implications for public health because these genes confer increased resistance to antibacterial copper toxicity, enhancing bacterial fitness by altering S. aureus interaction with innate immunity.

Biofilm-Forming Methicillin-Resistant Staphylococcus aureus Survive in Kupffer Cells and Exhibit High Virulence in Mice

Although Staphylococcus aureus is part of the normal body flora, heavy usage of antibiotics has resulted in the emergence of methicillin-resistant strains (MRSA). MRSA can form biofilms and cause indwelling foreign body infections, bacteremia, soft tissue infections, endocarditis, and osteomyelitis. Using an in vitro assay, we screened 173 clinical blood isolates of MRSA and selected 20 high-biofilm formers (H-BF) and low-biofilm formers (L-BF). These were intravenously administered to mice and the general condition of mice, the distribution of bacteria, and biofilm in the liver, lung, spleen, and kidney were investigated. MRSA count was the highest in the liver, especially within Kupffer cells, which were positive for acid polysaccharides that are associated with intracellular biofilm. After 24 h, the general condition of the mice worsened significantly in the H-BF group. In the liver, bacterial deposition and aggregation and the biofilm-forming spot number were all significantly greater for H-BF group than for L-BF. CFU analysis revealed that bacteria in the H-BF group survived for long periods in the liver. These results indicate that the biofilm-forming ability of MRSA is a crucial factor for intracellular persistence, which could lead to chronic infections.