Gene deletions differentially affect surface-associated biofilms and staphylococcal abscess communities.

Staphylococcus aureus is a major human pathogen that causes persistent infections characterised by the formation of aggregates such as surface-attached biofilm and staphylococcal abscess communities (SACs). Both consist of dense bacterial populations associated with self-produced matrices that impair immune cell and antibiotic access. Surface-attached biofilms have mostly protein- or polysaccharide-rich matrices, whereas SACs are spherical structures within tissue encased in a fibrin pseudocapsule and microcolony-associated meshwork. It remains unclear whether SAC are simply biofilms within tissue or if they display distinct features with unique genetic regulation. Here, we investigated genetic determinants of SAC and biofilm formation using four S. aureus deletion mutants targeting staphylokinase (Δsak), coagulase (Δcoa), the alternative sigma factor SigB (ΔrpoF), and sortase A (ΔsrtA). SACs were grown in collagen gels, whilst biofilms were grown on titanium discs, with bacterial viability, fibrin deposition, and biofilm biomass assessed by microscopy, image analysis, and crystal violet staining. Although deletion of coa or sak did not significantly impact either SAC or biofilm formation, biofilm surface coverage was markedly increased in ΔsrtA and biomass decreased in ΔrpoF; however, these mutations had no effect on SAC. These findings suggest that certain mutations can have different effects in the two experimental systems, and that the tested gene deletions were more important for surface-associated biofilm development than SAC formation.