No abstract

Staphylococcus aureus is a prominent human pathogen in bone and soft tissue infections. Pathophysiology involves abscess formation, which consists of central staphylococcal abscess communities (SACs), surrounded by a fibrin pseudocapsule and infiltrating immune cells. Protection against ingress of immune cells such as neutrophils, or tolerance to antibiotics, remains largely unknown for SACs and is limited by the lack of availability of in vitro models. We describe a 3D in vitro model of SACs grown in a human plasma-supplemented collagen gel. The in vitro SACs reached their maximum size by 24 hours and elaborated a fibrin pseudocapsule as confirmed by electron and immunofluorescence microscopy. The in vitro SACs tolerated 100x the minimal inhibitory concentration (MIC) of gentamicin alone and in combination with rifampicin, whilst planktonic controls and mechanically dispersed SACs were efficiently killed. To simulate a host response, SACs were exposed to differentiated PLB-985 neutrophil-like (dPLB) cells and to primary human neutrophils at an early stage of SAC formation, or after maturation at 24 hours. Both cell types were unable to clear mature in vitro SACs, but dPLB cells prevented SAC growth upon early exposure before pseudocapsule maturation. Neutrophil exposure after plasmin pre-treatment of the SACs resulted in a significant decrease in number of bacteria within the SACs. The in vitro SAC model mimics key in vivo features, offers a new tool to study host-pathogen interactions and drug efficacy assessment, and has revealed the functionality of the S. aureus pseudocapsule in preventing the bacteria against host phagocytic responses and antibiotics.