Interaction between Staphylococcus Agr virulence and neutrophils regulates pathogen expansion in the skin.

Staphylococcus aureus commonly infects the skin, but the host-pathogen interactions controlling bacterial growth remain unclear. S. aureus virulence is regulated by the Agr quorum-sensing system that controls factors including phenol-soluble modulins (PSMs), a group of cytotoxic peptides. We found a differential requirement for Agr and PSMα for pathogen growth in the skin. In neutrophil-deficient mice, S. aureus growth on the epidermis was unaffected, but the pathogen penetrated the dermis through mechanisms that require PSMα. In the dermis, pathogen expansion required Agr in wild-type mice, but not in neutrophil-deficient mice. Agr limited oxidative and non-oxidative killing in neutrophils by inhibiting pathogen late endosome localization and promoting phagosome escape. Unlike Agr, the SaeR/S virulence program was dispensable for growth in the epidermis and promoted dermal pathogen expansion independently of neutrophils. Thus, S. aureus growth and invasion are differentially regulated with Agr limiting intracellular killing within neutrophils to promote pathogen expansion in the dermis and subcutaneous tissue.

Aspartic Acid Residue 51 of SaeR Is Essential for Staphylococcus aureus Virulence.

Staphylococcus aureus is a common Gram-positive bacteria that is a major cause of human morbidity and mortality. The SaeR/S two-component sensory system of S. aureus is important for virulence gene transcription and pathogenesis. However, the influence of SaeR phosphorylation on virulence gene transcription is not clear. To determine the importance of potential SaeR phosphorylation sites for S. aureus virulence, we generated genomic alanine substitutions at conserved aspartic acid residues in the receiver domain of the SaeR response regulator in clinically significant S. aureus pulsed-field gel electrophoresis (PFGE) type USA300. Transcriptional analysis demonstrated a dramatic reduction in the transcript abundance of various toxins, adhesins, and immunomodulatory proteins for SaeR with an aspartic acid to alanine substitution at residue 51. These findings corresponded to a significant decrease in cytotoxicity against human erythrocytes and polymorphonuclear leukocytes, the ability to block human myeloperoxidase activity, and pathogenesis during murine soft-tissue infection. Analysis of SaeR sequences from over 8,000 draft S. aureus genomes revealed that aspartic acid residue 51 is 100% conserved. Collectively, these results demonstrate that aspartic acid residue 51 of SaeR is essential for S. aureus virulence and underscore a conserved target for novel antimicrobial strategies that treat infection caused by this pathogen.

Staphylococcus aureus Inhibits Neutrophil-derived IL-8 to Promote Cell Death.

While Staphylococcus aureus accelerates human neutrophil cell death, the underlying host- and pathogen-derived mechanisms remain incompletely defined. Previous studies demonstrated that the S. aureus SaeR/S sensory system is essential for pathogen survival following neutrophil phagocytosis. Herein, we demonstrate that the SaeR/S system promoted accelerated cell death, suppressed phosphorylation of nuclear factor-κB, and reduced interleukin-8 (IL-8) production in human neutrophils. Treatment of neutrophils with recombinant IL-8 significantly reduced bacterial burden and apoptosis. Our findings demonstrate a mechanism by which S. aureus suppresses the early neutrophil-derived IL-8 response to disrupt cell fate and promote disease.

The SaeR/S two-component system induces interferon-gamma production in neutrophils during invasive Staphylococcus aureus infection.

Invasive Staphylococcus aureus (S. aureus) disease is associated with neutrophil activity and pro-inflammatory cytokine expression, including interferon-gamma (IFNγ). Using a mouse model of S. aureus peritonitis, we identify neutrophils as the predominant source of IFNγ and link this induction with the SaeR/S two-component gene regulatory system. Relative to wild-type (BALB/c) mice, IFNγ-deficient mice demonstrated increased bacterial clearance and reduced cellular cytotoxicity following intraperitoneal challenge with S. aureus. Interestingly, bacterial burden and cytotoxicity were similar in BALB/c and IFNγ-deficient mice when infected with an isogenic saeR/S mutant strain. These findings suggest saeR/S-mediated neutrophil-derived IFNγ diminishes innate antibacterial mechanisms against S. aureus.