Neutrophils play a key role in the human immune response to Staphylococcus aureus infections. These professional phagocytes rapidly migrate to the site of infection to engulf bacteria and destroy them via specialized intracellular killing mechanisms. Here we describe a robust and relatively high-throughput flow cytometry assay to quantify phagocytosis of S. aureus by human neutrophils. We show that effective phagocytic uptake of S. aureus is greatly enhanced by opsonization, i.e. the tagging of microbial surfaces with plasma-derived host proteins like antibodies and complement. Our rapid assay to monitor phagocytosis can be used to study neutrophil deficiencies and bacterial evasion, but also provides a powerful tool to assess the opsonic capacity of antibodies, either in the context of natural immune responses or immune therapies.

Streptococcus agalactiae, also known as group B Streptococcus (GBS), is the major cause of neonatal sepsis in humans. A critical step to infection is adhesion of bacteria to epithelial surfaces. GBS adhesins have been identified to bind extracellular matrix components and cellular receptors. However, several putative adhesins have no host binding partner characterised. We report here that surfaceexpressed b protein of GBS binds to human CEACAM1 and CEACAM5 receptors. A crystal structure of the complex showed that an IgSF domain in b represents a novel Ig-fold subtype called IgI3, in which unique features allow binding to CEACAM1. Bioinformatic assessment revealed that this newly identified IgI3 fold is not exclusively present in GBS but is predicted to be present in adhesins from other clinically important human pathogens. In agreement with this prediction, we found that CEACAM1 binds to an IgI3 domain found in an adhesin from a different streptococcal species. Overall, our results indicate that the IgI3 fold could provide a broadly applied mechanism for bacteria to target CEACAMs.