Ctrl-atl-delete: Re-thinking the mechanisms of biofilm formation by staphylococci

Abstract

Biofilm formation by staphylococci on implanted medical devices represents a major virulence mechanism and a serious threat to hospital patients. Staphylococcus aureus strains including, methicillin-resistant S. aureus (MRSA), produce biofilms dependent on the major autolysin Atl. Antibodies to the amidase (AM) and glucosaminidase (GL) domains of Atl inhibited in vitro Atl-dependent biofilm formation and Atl was required for the normal expression of cytolytic toxins. However, an atl mutant was not significantly attenuated in a mouse model of catheter-related infections. Consistent with this finding, Atl and a number of previously characterised biofilm factors were subsequently shown to be redundant for biofilm formation on surfaces coated with human plasma. In contrast, coagulase activity, and specifically coagulase-mediated conversion of fibrinogen to fibrin, was critical for biofilm formation under these physiologically relevant conditions. These findings identify a novel third mechanism of S. aureus biofilm development, the other two being mediated by the extracellular polysaccharide poly-N-acetylglucosamine (PNAG) or a combination of cell surface proteins (including Atl) and extracellular DNA (eDNA). A correlation between upregulated autolytic activity and high-level homogeneous methicillin resistance or growth in sub-MIC oxacillin was demonstrated in MRSA. This observation appears to be related to altered cell wall architecture and, when combined with previous observations showing the impact of high level antibiotic resistance on biofilm and virulence, further shows the pleiotrophic effects of antibiotic resistance on the physiology of S. aureus. Extension of the studies on PNAG-independent mechanisms of biofilm formation revealed that the Staphylococcus epidermidis Atl homologue protein, AtlE, and the accumulation associated protein, Aap, contribute to S. epidermidis PNAG-independent biofilm formation in a strain-dependent manner. The role for autolytic activity in biofilm formation differed between strains and proteolytic cleavage of surface proteins was identified as having a significant influence on PNAG-independent biofilm formation.