Date Published: July 21, 2016
Publisher: Public Library of Science
Author(s): Marta Zapotoczna, Eoghan O’Neill, James P. O’Gara, William E. Goldman.
From the earliest identification of poly-N-acetylglucosamine (PNAG)/polysaccharide intercellular adhesin (PIA) as a first known mediator of Staphylococcus epidermidis biofilm formation (reviewed in ), interest in this important virulence determinant has led to the discovery of multiple biofilm mechanisms in S. epidermidis and S. aureus. The LPXTG-cell wall-anchored biofilm-associated protein (BAP) in bovine mastitis S. aureus isolates , the accumulation-associated protein (Aap) in S. epidermidis , and the fibronectin binding proteins (FnBPs) in human methicillin-resistant S. aureus (MRSA) isolates  were among the first PIA/PNAG-independent biofilm mechanisms to be described. Other protein adhesins include the cell wall-anchored clumping factor A (ClfA), cell wall-anchored clumping factor B (ClfB), S. aureus surface protein G (SasG), S. aureus surface protein C (SasC), Staphylococcus aureus protein A (Spa), and S. epidermidis surface protein C (SesC), as well as the cell surface extracellular matrix binding protein (Embp) and extracellular adherence protein (Eap) (reviewed in ). Release of extracellular DNA following lysis mediated by the major autolysin contributes to biofilm development in both species [6,7]. Lysis-dependent release of cytoplasmic proteins has also been implicated in the biofilm phenotype . Protein adhesins and extracellular DNA (eDNA) are in turn susceptible to protease and nuclease degradation, which can modulate biofilm development, architecture, and release . The small-peptide toxins, termed the phenol-soluble modulins (PSMs), have surfactant qualities that regulate biofilm maturation and dissemination . PSMs can also aggregate into amyloid structures that enhance biofilm formation , building on previously described roles for extracellular amyloid fibres in biofilm formation in other bacteria . Surface charge influenced by wall teichoic acid composition also impacts staphylococcal cell interactions with surfaces and the initiation of biofilm formation . Clearly staphylococci possess an array of biofilm mechanisms (Fig 1), and significant progress has been made over the last number of years in our understanding of the complexity of the various stages involved in staphylococcal biofilm attachment, formation, regulation, and disassembly. Application of this knowledge base and future studies will investigate how interactions between different adhesins influence the biofilm phenotype and the pathogenesis of biofilm-associated infections. Such interactions remain largely unexplored, but studies in a number of bacteria have shown interactions between eDNA and other matrix components such as polysaccharide and amyloid [13–15]. In S. aureus, interactions between extracellular DNA, amyloid fibres , and beta toxin  or between PIA/PNAG and teichoic acids  have also been reported.
The ability of S. aureus to survive in human blood is facilitated by production of coagulase (Coa), which is up-regulated in vivo by the two-component system SaeRS. In the clinical laboratory, Coa or staphylocoagulase production is routinely used to differentiate between S. aureus isolates and the coagulase-negative staphylococci. Whereas the contribution of surface proteins, secreted and lysis-derived proteins, polysaccharide, and eDNA adhesins, is influenced by strain background, the production of Coa, which we recently reported plays a critical role in biofilm formation under physiologically relevant conditions, is universal for all S. aureus strains. Upon maturation, like other biofilm types, the fibrin-shielded biofilms exhibit increased resistance to antimicrobial drugs.
Given that S. aureus is highly unlikely to have retained the capacity to express multiple biofilm phenotypes when just one would suffice, it seems reasonable to suggest that these environmentally regulated biofilm mechanisms are niche-specific and may play overlapping roles in both colonisation and biofilm formation. On the skin where NaCl concentrations are relatively high and water availability is low, production of PIA/PNAG may serve primarily to trap water with its role in intercellular adherence a secondary function. Similarly, up-regulation of FnBP expression in host niches where the pH is more acidic (e.g., urinary tract, vagina, mouth, and skin) appear to favour a biofilm mechanism that also promotes bacterial adherence to extracellular matrix proteins such as fibronectin, Fg, and elastin. Indeed, this general hypothesis may also extend to all surface proteins as well as the autolysin-mediated release of cytoplasmic proteins  and extracellular DNA with adherence properties . Physiological levels of Zn2+, which can be elevated at infection sites, play an important role in promoting Aap/SasG-dependent intercellular adhesion, perhaps in part by altering the cell surface via interactions with negatively charged teichoic acids . On the other hand, the regulation of proteinaceous biofilms by bacterial and host proteases may reflect both a bacterial dispersal mechanism and a host response to infections involving protein adhesin-mediated biofilms.
As noted above, comparative studies revealed that the antibiotics daptomycin, tigecycline, and rifampicin were capable of an almost complete inactivation of 24-hour fibrin-mediated biofilms, whereas FnBP-mediated biofilms were significantly more resistant . A recent study using an antibiotic lock model of infection showed that very high doses of these antibiotics retained significant activity against mature three- and five-day biofilms, which is more likely to reflect the “real-life” clinical scenario in which treatment is started following diagnosis of a DRI . Early diagnosis and intervention against biofilm-associated infections may therefore be of significant therapeutic importance using existing antimicrobial drugs, although the need to administer antimicrobials at many thousand times the minimum inhibitory concentration of the organism to achieve adequate biofilm inactivation remains the major challenge.