Staphylococcus aureus delta toxin modulates both extracellular membrane vesicle biogenesis and amyloid formation.

IMPORTANCE: Extracellular membrane vesicles (MVs) produced by Staphylococcus aureus in planktonic cultures encapsulate a diverse cargo of bacterial proteins, nucleic acids, and glycopolymers that are protected from destruction by external factors. δ-toxin, a member of the phenol soluble modulin family, was shown to be critical for MV biogenesis. Amyloid fibrils co-purified with MVs generated by virulent, community-acquired S. aureus strains, and fibril formation was dependent on expression of the S. aureus δ-toxin gene (hld). Mass spectrometry data confirmed that the amyloid fibrils were comprised of δ-toxin. Although S. aureus MVs were produced in vivo in a localized murine infection model, amyloid fibrils were not observed in the in vivo setting. Our findings provide critical insights into staphylococcal factors involved in MV biogenesis and amyloid formation.

Understanding mechanisms of virulence in MRSA: implications for antivirulence treatment strategies.

INTRODUCTION: Methicillin-resistant Staphylococcus aureus (MRSA) is a widespread pathogen, often causing recurrent and deadly infections in the hospital and community. Many S. aureus virulence factors have been suggested as potential targets for antivirulence therapy to decrease the threat of diminishing antibiotic availability. Antivirulence methods hold promise due to their adjunctive and prophylactic potential and decreased risk for selective pressure. AREAS COVERED: This review describes the dominant virulence mechanisms exerted by MRSA and antivirulence therapeutics that are currently undergoing testing in clinical or preclinical stages. We also discuss the advantages and downsides of several investigational antivirulence approaches, including the targeting of bacterial transporters, host-directed therapy, and quorum-sensing inhibitors. For this review, a systematic search of literature on PubMed, Google Scholar, and Web of Science for relevant search terms was performed in April and May 2023. EXPERT OPINION: Vaccine and antibody strategies have failed in clinical trials and could benefit from more basic science-informed approaches. Antivirulence-targeting approaches need to be set up better to meet the requirements of drug development, rather than only providing limited results to provide 'proof-of-principle' translational value of pathogenesis research. Nevertheless, there is great potential of such strategies and potential particular promise for novel probiotic approaches.

Staphylococcus aureus Delta Toxin Modulates both Extracellular Membrane Vesicle Biogenesis and Amyloid Formation.

Staphylococcus aureus secretes phenol-soluble modulins (PSMs), a family of small, amphipathic, secreted peptides with multiple biologic activities. Community-acquired S. aureus strains produce high levels of PSMs in planktonic cultures, and PSM alpha peptides have been shown to augment the release of extracellular membrane vesicles (MVs). We observed that amyloids, aggregates of proteins characterized by a fibrillar morphology and stained with specific dyes, co-purified with MVs harvested from cell-free culture supernatants of community-acquired S. aureus strains. δ-toxin was a major component of amyloid fibrils that co-purified with strain LAC MVs, and δ-toxin promoted the production of MVs and amyloid fibrils in a dose-dependent manner. To determine whether MVs and amyloid fibrils were generated under in vivo conditions, we inoculated mice with S. aureus harvested from planktonic cultures. Bacterial MVs could be isolated and purified from lavage fluids recovered from infected animals. Although δ-toxin was the most abundant PSM in lavage fluids, amyloid fibrils could not be detected in these samples. Our findings expand our understanding of amyloid fibril formation in S. aureus cultures, reveal important roles of δ-toxin in amyloid fibril formation and MV biogenesis, and demonstrate that MVs are generated in vivo in a staphylococcal infection model. Importance: Extracellular membrane vesicles (MVs) produced by Staphylococcus aureus in planktonic cultures encapsulate a diverse cargo of bacterial proteins, nucleic acids, and glycopolymers that are protected from destruction by external factors. δ-toxin, a member of the phenol soluble modulin family, was shown to be critical for MV biogenesis. Amyloid fibrils co-purified with MVs generated by virulent, community-acquired S. aureus strains, and fibril formation was dependent on expression of the S. aureus δ-toxin gene ( hld ). Mass spectrometry data confirmed that the amyloid fibrils were comprised of δ-toxin. Although S. aureus MVs were produced in vivo in a localized murine infection model, amyloid fibrils were not observed in the in vivo setting. Our findings provide critical insights into staphylococcal factors involved in MV biogenesis and amyloid formation.

Two transporters cooperate to secrete amphipathic peptides from the cytoplasmic and membranous milieus.

Diverse organisms secrete amphipathic biomolecules for competitive gains. However, how cells cope with producing these membrane-permeabilizing molecules is unclear. We focused on the PSM family of secreted amphipathic peptides in the pathogen Staphylococcus aureus that uses two ABC transporters, PmtCD and AbcA, to export peptides across the bacterial cell membrane. We found that increased peptide hydrophobicity favors PSM secretion through PmtCD over AbcA and that only PmtCD protected cells against amphipathic peptides. We propose a two-system model in which PmtCD and AbcA independently export PSMs from either membrane or cytosolic environments, respectively. Our model provides a rationale for the encoding of multiple transport systems on diverse biosynthetic gene clusters used to produce distinct amphipathic molecules. In addition, our data serve as a guide for selectively blocking PSM secretion to achieve antimicrobial or antivirulence approaches and to disrupt established roles of PSM-mediated virulence.

Investigational agents for the treatment of methicillin-resistant Staphylococcus aureus (MRSA) bacteremia: progress in clinical trials.

INTRODUCTION: Bacteremia caused by Staphylococcus aureus is common. Cases caused by methicillin-resistant S. aureus (MRSA) are particularly formidable and often lethal. The mortality associated with MRSA bacteremia has not significantly decreased over the past couple of decades and concerns regarding efficacy and toxicity of standard therapy highlight the need for novel agents and new therapeutic approaches. AREAS COVERED: This paper explores clinical trials investigating novel therapeutic approaches to S. aureus bacteremia. There is a special focus on MRSA bacteremia. Monotherapy and combination therapies and novel antimicrobials and adjunctive therapies that are only recently being established for therapeutic use are discussed. EXPERT OPINION: The unfavorable safety profile of combination antimicrobial therapy in clinical trials has outweighed its benefits. Therefore, future investigation should focus on optimizing duration and de-escalation protocols. Antibody and bacteriophage lysin-based candidates have mostly been limited to safety trials, but progress with these agents is demonstrated through a lysin-based agent receiving a phase III trial. Antibiotics indicated for use in treating MRSA skin infections see continued investigation as treatments for MRSA bacteremia despite the difficulty of completing trials in this patient population. Promising agents include dalbavancin, ceftobiprole, ceftaroline, and exebacase.

Quantification of Staphylococcus aureus Biofilm Formation by Crystal Violet and Confocal Microscopy.

Most Staphylococcus aureus strains can grow as a multicellular biofilm, a phenotype of utmost importance to clinical infections such as endocarditis, osteomyelitis, and implanted medical device infection. As biofilms are inherently more tolerant to the host immune system and antibiotics, understanding the S. aureus genes and regulatory circuits that contribute to biofilm development is an active and on-going field of research. This chapter details a high-throughput and standardized way to grow S. aureus biofilms using a classical microtiter plate assay. Biofilms can be quantified using crystal violet or by confocal microscopy imaging and COMSTAT analysis.