Kinetics and morphology of polymicrobial biofilm formation on polypropylene mesh

We examined the ability of three clinical bacterial isolates to form mixed biofilms on surgical polypropylene mesh (PPM) in vitro. The three strains--Staphylococcus aureus, Enterococcus faecalis, and Enterobacter cloacae--were isolated from a patient with an infected PPM. Staphylococcus aureus and E. faecalis (alone and in combination) were inoculated into culture containing squares of PPM and allowed to attach and propagate into mature biofilms. Enterococcus faecalis initially attached to the mesh in greater numbers; however, 7 days postinoculation, there were more S. aureus cells attached, indicating that in vitro S. aureus is the out-competing species. All three isolates were then co-cultured to form mature biofilms on mesh, and the biofilms were examined by confocal microscopy using both Live/Dead staining and fluorescent in situ hybridization (FISH). Imaging revealed a dense biofilm structure with interstitial voids and channels; rods and cocci were interspersed throughout the biofilm, indicating bacterial coexistence in close proximity. FISH revealed staphylococci and enterococci adjacent to each other and also to the Enterobacter, distinguishable by its rod morphology. These studies show that different species can co-operatively form mature biofilms on mesh but that the relative abundance of a species within the biofilm may vary over time.