Staphylococcus aureus Resists Human Defensins by Production of Staphylokinase, a Novel Bacterial Evasion Mechanism

Abstract

α-Defensins are peptides secreted by polymorphonuclear cells and provide antimicrobial protection mediated by disruption of the integrity of bacterial cell walls. Staphylokinase is an exoprotein produced by Staphylococcus aureus, which activates host plasminogen. In this study, we analyzed the impact of interaction between α-defensins and staphylokinase on staphylococcal growth. We observed that staphylokinase induced extracellular release of α-defensins from polymorphonuclear cells. Moreover, a direct binding between α-defensins and staphylokinase was shown to result in a complex formation. The biological consequence of this interaction was an almost complete inhibition of the bactericidal effect of α-defensins. Notably, staphylokinase with blocked plasminogen binding site still retained its ability to neutralize the bactericidal effect of α-defensins. In contrast, a single mutation of a staphylokinase molecule at position 74, substituting lysine for alanine, resulted in a 50% reduction of its α-defensin-neutralizing properties. The bactericidal properties of α-defensins were tested in 19 S. aureus strains in vitro and in a murine model of S. aureus arthritis. Staphylococcal strains producing staphylokinase were protected against the bactericidal effect of α-defensins. When staphylokinase was added to staphylokinase-negative S. aureus cultures, it almost totally abrogated the effect of α-defensins. Finally, human neutrophil peptide 2 injected intra-articularly along with bacteria alleviated joint destruction. In this study, we report a new property of staphylokinase, its ability to induce secretion of defensins, to complex bind them and to neutralize their bactericidal effect. Staphylokinase production may therefore be responsible in vivo for defensin resistance during S. aureus infections.

Growth-phase dependence of susceptibility to antimicrobial peptides in Staphylococcus aureus

“…To date, in S. aureus, three types of mechanism that affect susceptibility to antimicrobial peptides have been identified. These are (1) trapping defensins by binding to staphylokinase (Braff et al, 2007;Jin et al, 2004), (2) digestion of peptides by proteinase (Sieprawska-Lupa et al, 2004), and (3) changing the bacterial cell surface charge via dlt and mprF (Li et al, 2007a;Peschel et al, 1999Peschel et al, , 2001. mprF was first identified as fmtC, which was shown to affect the meticillin-resistance level in MRSA (Komatsuzawa et al, 2001).…”

Innate barriers against skin infection and associated disorders

“…It is now appreciated that SK is independently able to directly bind α-defensins produced by human neutrophils, inhibiting their bactericidal activity [36]. Testing of a panel of SA strains found that those producing SK were resistant to α-defensins, and that addition of purified SK to SK-negative SA cultures rescued them from α-defensin killing [36].…”

“…It is now appreciated that SK is independently able to directly bind α-defensins produced by human neutrophils, inhibiting their bactericidal activity [36]. Testing of a panel of SA strains found that those producing SK were resistant to α-defensins, and that addition of purified SK to SK-negative SA cultures rescued them from α-defensin killing [36]. Interestingly, SA upregulates cathelicidin expression during infection, and the binding of cathelicidin to SK augments the ability of this virulence factor to activate plasminogen, promote fibrinolysis, and allow bacterial dissemination [37].…”

Innate defences against methicillin‐resistant Staphylococcus aureus (MRSA) infection

“…Both mechanisms increased susceptibility to the peptides [124,127]. Staphylokinase inhibits the antibacterial activity of the α-defensins by reducing the binding affinity of these peptides [128]. Therefore, it is possible that up-or down-regulation of these factors in S. aureus may influence the susceptibilities.…”