We
previously demonstrated that mutation of the staphylococcal accessory regulator
(sarA) reduces the capacity of Staphylococcus
aureus to cause hematogenous osteomyelitis and that this is due to the
increased production of extracellular proteases. To determine whether this is
also true in acute, post-traumatic osteomyelitis, we used a murine model in
which bacteria were deposited directly into a bone defect. These experiments
were done with the S. aureus clinical
isolate LAC, its isogenic sarA mutant,
and an isogenic sarA mutant unable to
produce extracellular proteases. The results confirmed that mutation of sarA limits the bone destruction that
characterizes the acute phase of infection and that this is due to the
increased production of extracellular proteases. They also established that
these phenotypes can be correlated with cytoxicity in both established cell
lines and primary bone marrow derived osteoblasts and osteoclasts. Subsequent
studies confirmed that, while multiple proteases appear to be involved,
aureolysin plays the most important role in defining these phenotypes. This is
consistent with the observation that eliminating the production of aureolysin
most closely restored the proteome of a LAC sarA mutant to that of its isogenic
parent strain. These results confirm that sarA
controls the production and/or accumulation of virulence factors that limit
both osteoblast and osteoclast function and thereby limit bone repair and
induce reactive bone formation during the acute phase of S. aureus post-traumatic osteomyelitis. Furthermore, as with
hematogenous osteomyelitis, there is an important role for S. aureus secreted proteases, most notably aureolysin, in the sarA mutant phenotype. To the extent
that these changes compromise the local blood supply and consequently limit the
efficacy of systemic antibiotic therapy, this suggests that inhibitors of sarA expression and/or function could be
used to therapeutic advantage in an orthopaedic context.