Host-derived protease promotes aggregation of Staphylococcus aureus by cleaving the surface protein SasG

Staphylococcus aureus is one of the leading causes of hospital-acquired infections, many of which begin following attachment and accumulation on indwelling medical devices or diseased tissue. These infections are often linked to the establishment of biofilms, but another often overlooked key characteristic allowing S. aureus to establish persistent infection is the formation of planktonic aggregates. Such aggregates are physiologically similar to biofilms and protect pathogens from innate immune clearance and increase antibiotic tolerance. The cell-wall-associated protein SasG has been implicated in biofilm formation via mechanisms of intercellular aggregation but the mechanism in the context of disease is largely unknown. We have previously shown that the expression of cell-wall-anchored proteins involved in biofilm formation is controlled by the ArlRS-MgrA regulatory cascade. In this work, we demonstrate that the ArlRS two-component system controls aggregation, by repressing the expression of sasG by activation of the global regulator MgrA. We also demonstrate that SasG must be proteolytically processed by a non-staphylococcal protease to induce aggregation and that strains expressing functional full-length sasG aggregate significantly upon proteolysis by a mucosal-derived host protease found in human saliva. We used fractionation and N-terminal sequencing to demonstrate that human trypsin within saliva cleaves within the A domain of SasG to expose the B domain and induce aggregation. Finally, we demonstrated that SasG is involved in virulence during mouse lung infection. Together, our data point to SasG, its processing by host proteases, and SasG-driven aggregation as important elements of S. aureus adaptation to the host environment.IMPORTANCEHere, we demonstrate that the Staphylococcus aureus surface protein SasG is important for cell-cell aggregation in the presence of host proteases. We show that the ArlRS two-component regulatory system controls SasG levels through the cytoplasmic regulator MgrA. We identified human trypsin as the dominant protease triggering SasG-dependent aggregation and demonstrated that SasG is important for S. aureus lung infection. The discovery that host proteases can induce S. aureus aggregation contributes to our understanding of how this pathogen establishes persistent infections. The observations in this study demonstrate the need to strengthen our knowledge of S. aureus surface adhesin function and processing, regulation of adhesin expression, and the mechanisms that promote biofilm formation to develop strategies for preventing chronic infections.

Bacterial pneumonia-induced shedding of epithelial heparan sulfate inhibits the bactericidal activity of cathelicidin in a murine model

Bacterial pneumonia is a common clinical syndrome leading to significant morbidity and mortality worldwide. In the current study, we investigate a novel, multidirectional relationship between the pulmonary epithelial glycocalyx and antimicrobial peptides in the setting of methicillin-resistant Staphylococcus aureus (MRSA) pneumonia. Using an in vivo pneumonia model, we demonstrate that highly sulfated heparan sulfate (HS) oligosaccharides are shed into the airspaces in response to MRSA pneumonia. In vitro, these HS oligosaccharides do not directly alter MRSA growth or gene transcription. However, in the presence of an antimicrobial peptide (cathelicidin), increasing concentrations of HS inhibit the bactericidal activity of cathelicidin against MRSA as well as other nosocomial pneumonia pathogens (Klebsiella pneumoniae and Pseudomonas aeruginosa) in a dose-dependent manner. Surface plasmon resonance shows avid binding between HS and cathelicidin with a dissociation constant of 0.13 μM. These findings highlight a complex relationship in which shedding of airspace HS may hamper host defenses against nosocomial infection via neutralization of antimicrobial peptides. These findings may inform future investigation into novel therapeutic targets designed to restore local innate immune function in patients suffering from primary bacterial pneumonia.NEW & NOTEWORTHY Primary Staphylococcus aureus pneumonia causes pulmonary epithelial heparan sulfate (HS) shedding into the airspace. These highly sulfated HS fragments do not alter bacterial growth or transcription, but directly bind with host antimicrobial peptides and inhibit the bactericidal activity of these cationic polypeptides. These findings highlight a complex local interaction between the pulmonary epithelial glycocalyx and antimicrobial peptides in the setting of bacterial pneumonia.

A structure activity relationship study of 3,4'-dimethoxyflavone for ArlRS inhibition in Staphylococcus aureus

Infections caused by methicillin-resistant Staphylococcus aureus (MRSA) are difficult to treat due to their resistance to many β-lactam antibiotics, and their highly coordinated excretion of virulence factors. One way in which MRSA accomplishes this is by responding to environmental stimuli using two-component systems (TCS). The ArlRS TCS has been identified as having a key role in regulating virulence in both systemic and local infections caused by S. aureus. We recently disclosed 3,4'-dimethoxyflavone as a selective ArlRS inhibitor. In this study we explore the structure-activity relationship (SAR) of the flavone scaffold for ArlRS inhibition and identify several compounds with increased activity compared to the parent. Additionally, we identify a compound that suppresses oxacillin resistance in MRSA, and begin to probe the mechanism of action behind this activity.

The influenza-injured lung microenvironment promotes MRSA virulence, contributing to severe secondary bacterial pneumonia

Influenza infection is substantially worsened by the onset of secondary pneumonia caused by bacteria, such as methicillin-resistant Staphylococcus aureus (MRSA). The bidirectional interaction between the influenza-injured lung microenvironment and MRSA is poorly understood. By conditioning MRSA ex vivo in bronchoalveolar lavage fluid collected from mice at various time points of influenza infection, we found that the influenza-injured lung microenvironment dynamically induces MRSA to increase cytotoxin expression while decreasing metabolic pathways. LukAB, a SaeRS two-component system-dependent cytotoxin, is particularly important to the severity of post-influenza MRSA pneumonia. LukAB's activity is likely shaped by the post-influenza lung microenvironment, as LukAB binds to (and is activated by) heparan sulfate (HS) oligosaccharide sequences shed from the epithelial glycocalyx after influenza. Our findings indicate that post-influenza MRSA pneumonia is shaped by bidirectional host-pathogen interactions: host injury triggers changes in bacterial expression of toxins, the activity of which may be shaped by host-derived HS fragments.

Human monoclonal antibodies against Staphylococcus aureus surface antigens recognize in vitro and in vivo biofilm

Implant-associated Staphylococcus aureus infections are difficult to treat because of biofilm formation. Bacteria in a biofilm are often insensitive to antibiotics and host immunity. Monoclonal antibodies (mAbs) could provide an alternative approach to improve the diagnosis and potential treatment of biofilm-related infections. Here, we show that mAbs targeting common surface components of S. aureus can recognize clinically relevant biofilm types. The mAbs were also shown to bind a collection of clinical isolates derived from different biofilm-associated infections (endocarditis, prosthetic joint, catheter). We identify two groups of antibodies: one group that uniquely binds S. aureus in biofilm state and one that recognizes S. aureus in both biofilm and planktonic state. Furthermore, we show that a mAb recognizing wall teichoic acid (clone 4497) specifically localizes to a subcutaneously implanted pre-colonized catheter in mice. In conclusion, we demonstrate the capacity of several human mAbs to detect S. aureus biofilms in vitro and in vivo.

Staphylococcus aureus uses the ArlRS and MgrA cascade to regulate immune evasion during skin infection

Skin is one of the most common sites of host immune response against Staphylococcus aureus infection. Here, through a combination of in vitro assays, mouse models, and intravital imaging, we find that S. aureus immune evasion in skin is controlled by a cascade composed of the ArlRS two-component regulatory system and its downstream effector, MgrA. S. aureus lacking either ArlRS or MgrA is less virulent and unable to form correct abscess structure due to de-repression of a giant surface protein, Ebh. These S. aureus mutants also have decreased expression of immune evasion factors (leukocidins, chemotaxis-inhibitory protein of S. aureus [CHIPS], staphylococcal complement inhibitor [SCIN], and nuclease) and are unable to kill neutrophils, block their chemotaxis, degrade neutrophil extracellular traps, and survive direct neutrophil attack. The combination of disrupted abscess structure and reduced immune evasion factors makes S. aureus susceptible to host defenses. ArlRS and MgrA are therefore the main regulators of S. aureus immune evasion and promising treatment targets.

Staphylococcus aureus toxin suppresses antigen-specific T cell responses

Staphylococcus aureus remains a leading cause of human infection. These infections frequently recur when the skin is a primary site of infection, especially in infants and children. In contrast, invasive staphylococcal disease is less commonly associated with reinfection, suggesting that tissue-specific mechanisms govern the development of immunity. Knowledge of how S. aureus manipulates protective immunity has been hampered by a lack of antigen-specific models to interrogate the T cell response. Using a chicken egg OVA-expressing S. aureus strain to analyze OVA-specific T cell responses, we demonstrated that primary skin infection was associated with impaired development of T cell memory. Conversely, invasive infection induced antigen-specific memory and protected against reinfection. This defect in adaptive immunity following skin infection was associated with a loss of DCs, attributable to S. aureus α-toxin (Hla) expression. Gene- and immunization-based approaches to protect against Hla during skin infection restored the T cell response. Within the human population, exposure to α-toxin through skin infection may modulate the establishment of T cell-mediated immunity, adversely affecting long-term protection. These studies prompt consideration that vaccination targeting S. aureus may be most effective if delivered prior to initial contact with the organism.

Staphylococcus aureus bloodstream infections: pathogenesis and regulatory mechanisms

Staphylococcus aureus is an opportunistic pathogen that normally colonizes the human anterior nares. At the same time, this pathogen is one of the leading causes of life-threatening bloodstream infections, such as sepsis and endocarditis. In this review we will present the current understanding of the pathogenesis of these invasive infections, focusing on the mechanisms of S. aureus clearance from the bloodstream by the immune system, and how this pathogen hijacks the host defense and coagulation systems and further interacts with the blood vessel endothelium. Additionally, we will delve into the regulatory mechanisms S. aureus employs during an invasive infection. These new insights into host-pathogen interactions show promising avenues for the development of novel therapies for treating bloodstream infections.

Merit Ptah, "The First Woman Physician": Crafting of a Feminist History with an Ancient Egyptian Setting

Merit Ptah is widely described as "the first woman physician and scientist" on the Internet and in popular history books. This essay explores the origins of this figure, showing that Merit Ptah came into being in the 1930s when Kate Campbell Hurd-Mead misinterpreted a report about an authentic ancient Egyptian healer. Merit Ptah gradually became a prominent figure in popular historical accounts during second-wave of feminism, and, in the twenty-first century she appeared in Wikipedia and subsequently spread throughout the Internet as a female (sometimes black African) founding figure. The history of Merit Ptah reveals powerful mechanisms of knowledge creation in the network of amateur historians, independently from the scholarly community. The case of Merit Ptah also pinpoints factors enabling the spread of erroneous historical accounts: the absence of professional audience, the development of echo chambers due to an obscured chain of knowledge transmission, the wide reach of the Internet, the coherence with existing preconceptions, the emotional charge of heritage, and even - in the case of ancient Egypt - the tendency to perceive certain pasts through a legendary lens. At the same time, the story of Merit Ptah reveals how important role models have been for women entering science and medicine.

The Staphylococcus aureus ArlRS two-component system regulates virulence factor expression through MgrA

The Gram-positive bacterium, Staphylococcus aureus, is a versatile pathogen that can sense and adapt to a wide variety of environments within the human host, in part through its 16 two-component regulatory systems. The ArlRS two-component system has been shown to affect many cellular processes in S. aureus, including autolysis, biofilm formation, capsule synthesis and virulence. Yet the molecular details of this regulation remained largely unknown. We used RNA sequencing to identify the ArlRS regulon, and found 70% overlap with that of the global regulator MgrA. These genes included cell wall-anchored adhesins (ebh, sdrD), polysaccharide and capsule synthesis genes, cell wall remodeling genes (lytN, ddh), the urease operon, genes involved in metal transport (feoA, mntH, sirA), anaerobic metabolism genes (adhE, pflA, nrdDG) and a large number of virulence factors (lukSF, lukAB, nuc, gehB, norB, chs, scn and esxA). We show that ArlR directly activates expression of mgrA and identify a probable ArlR-binding site (TTTTCTCAT-N4 -TTTTAATAA). A highly similar sequence is also found in the spx P2 promoter, which was recently shown to be regulated by ArlRS. We also demonstrate that ArlS has kinase activity toward ArlR in vitro, although it has slower kinetics than other similar histidine kinases.

Biofilm formation by Staphylococcus aureus clinical isolates correlates with the infection type

BACKGROUND

Biofilms are involved in many Staphylococcus aureus infections, but relation of biofilm formation and the infection types or the clinical outcomes remain unclear.

METHODS

We measured biofilm formation, with a microtiter plate assay, of a collection of methicillin-sensitive clinical isolates from 159 invasive S. aureus infections, encompassing all cases occurring within a hospital catchment area during two years, and from additional 49 non-invasive skin infections from the same region. Results were related to available clinical and microbiological documentation.

RESULTS

Isolates from medical device infections (intravenous line-associated and prosthetic joint infections), as well as isolates from superficial skin infections, were particularly proficient in forming biofilms. No increased biofilm-forming capacity was seen in isolates from endocarditis, osteomyelitis, or other infections. There was also a correlation of biofilm formation with the agr type of the isolates. Thicker biofilms were more resistant to antibiotic treatment in vitro. No correlation between biofilm formation and clinical outcomes was noted.

CONCLUSIONS

S. aureus isolates from 'classical' biofilm-related infections, but also from superficial skin infections, are especially proficient in forming biofilms. There is, however, no obvious relation of biofilm-forming capacity of isolates and the clinical outcome of the infection, and more studies on this issue are needed.

Staphylococcus aureus adhesion in endovascular infections is controlled by the ArlRS-MgrA signaling cascade

Staphylococcus aureus is a leading cause of endovascular infections. This bacterial pathogen uses a diverse array of surface adhesins to clump in blood and adhere to vessel walls, leading to endothelial damage, development of intravascular vegetations and secondary infectious foci, and overall disease progression. In this work, we describe a novel strategy used by S. aureus to control adhesion and clumping through activity of the ArlRS two-component regulatory system, and its downstream effector MgrA. Utilizing a combination of in vitro cellular assays, and single-cell atomic force microscopy, we demonstrated that inactivation of this ArlRS—MgrA cascade inhibits S. aureus adhesion to a vast array of relevant host molecules (fibrinogen, fibronectin, von Willebrand factor, collagen), its clumping with fibrinogen, and its attachment to human endothelial cells and vascular structures. This impact on S. aureus adhesion was apparent in low shear environments, and in physiological levels of shear stress, as well as in vivo in mouse models. These effects were likely mediated by the de-repression of giant surface proteins Ebh, SraP, and SasG, caused by inactivation of the ArlRS—MgrA cascade. In our in vitro assays, these giant proteins collectively shielded the function of other surface adhesins and impaired their binding to cognate ligands. Finally, we demonstrated that the ArlRS—MgrA regulatory cascade is a druggable target through the identification of a small-molecule inhibitor of ArlRS signaling. Our findings suggest a novel approach for the pharmacological treatment and prevention of S. aureus endovascular infections through targeting the ArlRS—MgrA regulatory system.

Adhesion is central to the success of Staphylococcus aureus as a bacterial pathogen. We describe a novel mechanism through which S. aureus alters adhesion to ligands by regulating expression of giant inhibitory surface proteins. These giant proteins shield normal surface adhesins, preventing binding to ligands commonly found in the bloodstream and vessel walls. Using this unique regulatory scheme, S. aureus can bypass the need for individualized regulation of numerous adhesins to control overall adhesive properties. Our study establishes the importance of these giant proteins for S. aureus pathogenesis and demonstrates that a single regulatory cascade can be targeted for treating infections.

Biofilm formation by Staphylococcus aureus clinical isolates correlates with the infection type

BACKGROUND

Biofilms are involved in many Staphylococcus aureus infections, but relation of biofilm formation and the infection types or the clinical outcomes remain unclear.

METHODS

We measured biofilm formation, with a microtiter plate assay, of a collection of methicillin-sensitive clinical isolates from 159 invasive S. aureus infections, encompassing all cases occurring within a hospital catchment area during two years, and from additional 49 non-invasive skin infections from the same region. Results were related to available clinical and microbiological documentation.

RESULTS

Isolates from medical device infections (intravenous line-associated and prosthetic joint infections), as well as isolates from superficial skin infections, were particularly proficient in forming biofilms. No increased biofilm-forming capacity was seen in isolates from endocarditis, osteomyelitis, or other infections. There was also a correlation of biofilm formation with the agr type of the isolates. Thicker biofilms were more resistant to antibiotic treatment in vitro. No correlation between biofilm formation and clinical outcomes was noted.

CONCLUSIONS

S. aureus isolates from 'classical' biofilm-related infections, but also from superficial skin infections, are especially proficient in forming biofilms. There is, however, no obvious relation of biofilm-forming capacity of isolates and the clinical outcome of the infection, and more studies on this issue are needed.