Staphylokinase (SAK) is a plasminogen-activator protein produced by Staphylococcus aureus. SAK production was evaluated in vitro in S. aureus isolates from the bloodstream of patients with lethal (n = 56) and non-lethal (n = 57) bacteraemia and from anterior nares of healthy subjects (n = 48). Most isolates (93/161) produced SAK, and 68 % of SAK-producing isolates expressed both surface-bound and secreted types of SAK. SAK production was significantly less common among isolates from patients with lethal bacteraemia (39 %) than isolates from patients with non-lethal bacteraemia (68 %) or nasal carriage isolates (67 %) (P < 0.01). After adjusting for infection with methicillin-resistant S. aureus and APACHE II score, patients infected with SAK-deficient isolates were 4.3 times more likely to have lethal bacteraemia than patients whose infecting isolate produced high levels of SAK (> or =5 microg ml(-1)), suggesting that in vitro SAK production was inversely associated with clinical outcome among patients with S. aureus bacteraemia. The high frequency of SAK production in nasal isolates and in cases with uncomplicated bacteraemia suggests that SAK may be one of the adaptive mechanisms of S. aureus symbiosis with the host.
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