RpiRc regulates RsbU to modulate eDNA-dependent biofilm formation andin vivovirulence ofStaphylococcus aureusin a mouse model of catheter infection
Authors: Fischer Adrien , Girard Myriam , Laumay Floriane , Woischnig Anne-Kathrin , Khanna Nina , François Patrice , Schrenzel Jacques 
Published: September 2019

Staphylococcus aureusis a major human pathogen. Despite high incidence and morbidity, molecular mechanisms occurring during infection remain largely unknown. Under defined conditions, biofilm formation contributes to the severity ofS. aureusrelated infections. Extracellular DNA (eDNA), a component of biofilm matrix released from apoptotic bacteria, is involved in biofilm structure and stability. In many bacterial biofilms, eDNA originates from cell lysis although eDNA can also be actively secreted or exported by bacterial membrane vesicles. By screening the Nebraska transposon library, we identifiedrpiRcas a biofilm regulator involved in eDNA regulation. RpiRc is a transcription factor from the pentose phosphate pathway (PPP) whose product is a polysaccharide intercellular adhesin (PIA) precursor. However,rpiRcmutant strain showed neither susceptibility to DispersinB® (a commercially available enzyme disrupting PIA biofilms) nor alteration oficatranscription (the operon regulating PIA production). Decreased biofilm formation was linked to Sln, an extracellular compound degrading eDNA in an autolysis independent pathway. Biofilm susceptibility to antibiotics in wt and mutant strains was tested using a similar protocol as the Calgary biofilm device. Involvement of RpiRc inS. aureusvirulence was assessedex vivoby internalization experiments into HEK293 cells andin vivoin a mouse model of subcutaneous catheter infection. While minimum inhibitory concentrations (MICs) of planktonic cells were not affected in the mutant strain, we observed increased biofilm susceptibility to almost all tested antibiotics, regardless of their mode of action. More importantly, therpiRcmutant showed reduced virulence in bothex vivoandin vivoexperiments related to decreasedfnbpA-Btranscription and eDNA production. RpiRc is an important regulator involved in eDNA degradation inside the matrix of mature PIA independent biofilms. These results illustrate that RpiRc contributes to increased antibiotic tolerance in mature bacterial biofilm and also toS. aureuscell adhesion and virulence during subcutaneous infection.

Author summary

Biofilm formation contributes to the severity ofStaphylococcus aureusrelated infections. Biofilm matrix is mainly composed by polysaccharide intercellular adhesion (PIA), proteins and extracellular DNA (eDNA). By screening a mutant library ofS. aureus, RpiRc was identified as a new regulator of eDNA dependent biofilm formation. How RpiRc regulates biofilm and its role in S. aureus virulence was studied in four differentS. aureusstrains. Deletion of RpiRc resulted in a pronounced decreased eDNA dependent biofilm formation, but not PIA dependent biofilm formation. Decreased biofilm formation was not related to increased autolysis, but was linked to extracellular compounds found in the supernatant of mutant biofilms. Sln was identified as one of this compound. RpiRc deletion also decreased biofilm recalcitrance (resistance) to selected antibiotics. Involvement of RpiRc inS. aureuspathogenesis was investigatedex vivoby internalization into HEK293 cells andin vivoin a mouse model of catheter infection. RpiRc deletion resulted in decreased virulence related to decreased expression of surface proteins like the fibronectin binding proteins A and B (FnbpA-B). These results illustrate that RpiRc contributes to increased antibiotic tolerance in mature bacterial biofilm and also toS. aureuscell adhesion and virulence during subcutaneous infection.