Early fibrin biofilm development in cardiovascular infections

The single most common microbe causing cardiovascular infections is <i>Staphylococcus aureus</i> (<i>S. aureus</i>). <i>S. aureus</i> produces coagulase that converts fibrinogen to fibrin, which is incorporated into biofilms. This process aids in adherence to intravascular structures, defense against the host immune system, and resistance to antimicrobial treatment. Despite its significance, fibrin formation in <i>S. aureus</i> biofilms remains poorly understood. Therefore, this study aimed to elucidate the early development of cardiovascular biofilms. Clinically isolated coagulase-positive <i>S. aureus</i> and coagulase-negative <i>Staphylococcus lugdunensis</i> (<i>S. lugdunensis</i>) from patients with cardiovascular infections, and a coagulase mutant <i>S. aureus</i> Δcoa, were grown in tryptic soy broth (TSB), Iscove's Modified Dulbecco's Medium (IMDM), and pooled human plasma, with or without porcine heart valves. Bacterial growth, metabolic activity, and bacterial fibrinogen utilization were measured over 24 h at 37 °C. Time-lapse confocal microscopy was used to visualize and track biofilm development. <i>S. aureus</i> exhibited more growth in TSB and human plasma than <i>S. lugdunensis</i> and <i>S. aureus</i> Δcoa, but showed similar growth in IMDM after 24 h. Peak metabolic activity for all isolates was highest in TSB and lowest in human plasma. The presence of porcine valves caused strain-dependent alterations in time to peak metabolic activity. Confocal imaging revealed fibrin-based biofilm development exclusively in the coagulase-producing <i>S. aureus</i> strains. Between 2 and 6 h of biofilm development, 74.9 % (p = 0.034) of the fibrinogen from the medium was converted to fibrin. Variations in fibrin network porosity and density were observed among different coagulase-producing <i>S. aureus</i> strains. Fibrin formation is mediated by <i>S. aureus</i> coagulase and first strands occurred within 3 h for clinical strains after exposure to human plasma. This study stresses the importance of experimental design given the bacterial changes due to different media and substrates and provides insights into the early pathogenesis of <i>S. aureus</i> cardiovascular biofilms.