Background: Methicillin-resistant Staphylococcus aureus (MRSA) generates NO through bacterial NO synthase (bNOS). Results: Loss of bNOS increases MRSA sensitivity to host neutrophils, cathelicidin antimicrobial peptides, and cell envelopeactive antibiotics. Conclusion: bNOS influences MRSA disease pathology. Significance: Future development of bNOS-specific inhibitors could provide dual activities to reduce MRSA pathology and increase antibiotic effectiveness.

c Streptococcal collagen-like protein 1 (Scl-1) is one of the most highly expressed proteins in the invasive M1T1 serotype group A Streptococcus (GAS), a globally disseminated clone associated with higher risk of severe invasive infections. Previous studies using recombinant Scl-1 protein suggested a role in cell attachment and binding and inhibition of serum proteins. Here, we studied the contribution of Scl-1 to the virulence of the M1T1 clone in the physiological context of the live bacterium by generating an isogenic strain lacking the scl-1 gene. Upon subcutaneous infection in mice, wild-type bacteria induced larger lesions than the ⌬scl mutant. However, loss of Scl-1 did not alter bacterial adherence to or invasion of skin keratinocytes. We found instead that Scl-1 plays a critical role in GAS resistance to human and murine phagocytic cells, allowing the bacteria to persist at the site of infection. Phenotypic analyses demonstrated that Scl-1 mediates bacterial survival in neutrophil extracellular traps (NETs) and protects GAS from antimicrobial peptides found within the NETs. Additionally, Scl-1 interferes with myeloperoxidase (MPO) release, a prerequisite for NET production, thereby suppressing NET formation. We conclude that Scl-1 is a virulence determinant in the M1T1 GAS clone, allowing GAS to subvert innate immune functions that are critical in clearing bacterial infections.

The up‐regulation of the gene encoding Streptococcal collagen‐like protein (scl‐1) in invasive Group A Streptococcus has been hypothesized to promote bacterial resistance to host defenses. Our previous data demonstrated that when incubated with phagocytic cells, an scl‐1 mutant strain exhibits significantly decreased survival compared to wild‐type and plasmid‐complemented strains. In our current study, we have collected results that support two possible molecular mechanisms by which Scl‐1 contributes to bacterial evasion of phagocytic killing. First, the surface protein Scl‐1 may serve as a physical barrier to mediate resistance to antimicrobial molecules contained in phagocytic cell granules. Scl‐1 directly contributes to bacterial resistance to the antimicrobial peptide human cathelicidin LL‐37, granule proteases and reactive oxygen species, though Scl‐1 does not affect peroxide production by phagocytic cells per se. Secondly, Scl‐1 contributes to bacterial survival by suppression of degranulation and release of antimicrobial molecules, and consequently the formation of neutrophil extracellular traps (NETs). Thus, the results show that Scl‐1 is a multi‐faceted virulence factor that mediates bacterial evasion to phagocytic killing by both suppressing the host response and by mediating direct resistance to antimicrobial molecules.