Group A Streptococcus (GAS) is a leading human pathogen infecting millions annually. Colonization is mediated through bacterial attachment to host epithelium. Internalization of epithelial cells may offer the bacteria protection from immune cells and antibiotics, and allow for penetration of epithelial barriers. We investigated the role of surface-anchored protein serum opacity factor (SOF) in the invasion of epithelial cells and virulence. Our data demonstrate that SOF plays a critical role in invasion of epithelial cells, mediated through its activity domain, independent of its fibronectin binding capacity. To this end, elimination of SOF in GAS produced smaller lesions and yielded significantly less bacteria per lesion than wild-type (WT) in a mouse model of necrotizing fasciitis. To cause invasive disease GAS must circumvent host immune defenses. Neutrophils circulate in the bloodstream until chemokine signals recruit them to sites of infections. GAS produce a protease, SpyCEP, which cleaves and inactivates one of the most potent neutrophil chemoattractants interleukin-8 (IL-8). We observe that this leads to decreased neutrophil migration compared to a GAS mutant lacking the SpyCEP gene. In addition, we demonstrate that SpyCEP inhibis the formation of neutrophil NETs and contributes to GAS-induced neutrophil apoptosis. The SpyCEP mutant was significantly attenuated for virulence in a mouse model of necrotizing fasciitis. Macrophages are necessary defenders against GAS infections. We have found that live GAS trigger significant accelerated caspase-dependent macrophage apoptosis upon phagocytosis. This apoptosis disrupts mitochondrial integrity and promotes membrane remodeling, leading to the release of cytochrome c into the cytosol. Apoptosis-induction is dependent upon the GAS pore-forming cytolysin streptolysin O (SLO), which is necessary and sufficient for this phenotype. Accelerated apoptosis hampers the ability of macrophages to kill the bacteria and release cytokines, blunting immune activation. An SLO mutant was attenuated in a mouse systemic infection model, with less bacteria surviving in the blood and reduced killing of mice. This work demonstrates the complexity of GAS interactions with the host during disease and the multitude of virulence mechanisms employed to escape immune defenses. A deeper understanding of GAS disease pathogenesis can lead to novel therapeutic strategies in the treatment of this disease

Group A Streptococcus is a leading human pathogen associated with a diverse array of mucosal and systemic infections. Cell wall anchored pili were recently described in several species of pathogenic streptococci, and in the case of GAS, these surface appendages were demonstrated to facilitate epithelial cell adherence. Here we use targeted mutagenesis to evaluate the contribution of pilus expression to virulence of the globally disseminated M1T1 GAS clone, the leading agent of both GAS pharyngitis and severe invasive infections. We confirm that pilus expression promotes GAS adherence to pharyngeal cells, keratinocytes, and skin. However, in contrast to findings reported for group B streptococcal and pneumococcal pili, we observe that pilus expression reduces GAS virulence in murine models of necrotizing fasciitis, pneumonia and sepsis, while decreasing GAS survival in human blood. Further analysis indicated the systemic virulence attenuation associated with pilus expression was not related to differences in phagocytic uptake, complement deposition or cathelicidin antimicrobial peptide sensitivity. Rather, GAS pili were found to induce neutrophil IL-8 production, promote neutrophil transcytosis of endothelial cells, and increase neutrophil release of DNA-based extracellular traps, ultimately promoting GAS entrapment and killing within these structures.