The pathogenesis of joint infections is not well understood. In particular, we do not know why these infections respond poorly to antibiotic treatment. Here we show that methicillin-resistant Staphylococcus aureus, a major cause of joint infections, forms exceptionally strong biofilmlike aggregates in human synovial fluid (SF), to an extent significantly exceeding biofilm formation observed in growth medium or serum. Screening a transposon bank identified bacterial fibronectin- and fibrinogen-binding proteins as important for the formation of macroscopic clumps in SF, suggesting an important role of fibrin-containing clots in the formation of bacterial aggregates during joint infection. Pretreatment of SF with plasmin led to a strongly reduced formation of aggregates and increased susceptibility to antibiotics. These results give important insight into the pathogenesis of staphylococcal joint infection and the mechanisms underlying resistance to treatment. Furthermore, they point toward a potential novel approach for treating joint infections.

Staphylococcus aureus is a highly versatile gram positive bacterium that is resident as an asymptomatic colonizer on the skin and in the nasopharynx of approximately 30% of individuals. Nasopharyngeal colonization is a risk for acquiring S. aureus infections, which can cause a range of clinical symptoms that are commonly associated with skin and soft-tissue infections. The emergence of S. aureus strains that are highly resistant to antimicrobials has recently become a major public health concern. In low-income countries the incidence of S. aureus disease is highest in neonates and children up to one year of age and mortality rates are estimated to be up to 50%. In the United States, S. aureus infection accounts for approximately 300,000 hospitalizations per year. A vaccine against multi-drug resistant S. aureus, therefore, is urgently needed. Two vaccine candidates have previously been evaluated in late-stage clinical trials but have not demonstrated efficacy. At present, one vaccine candidate and two monoclonal antibody are undergoing clinical evaluation in target groups at high risk for S. aureus infection. This review provides an overview of current vaccine development efforts and presents the major technical and regulatory challenges to developing a licensed S. aureus vaccine.

Staphylococcus aureus is a leading cause of prosthetic joint infections, which, as we recently showed, proceed with the involvement of biofilm-like clusters that cause recalcitrance to antibiotic treatment. Here we analyzed why these clusters grow extraordinarily large, reaching macroscopically visible extensions (>1 mm). We found that while specific S. aureus surface proteins are a prerequisite for agglomeration in synovial fluid, low activity of the Agr regulatory system and subsequent low production of the phenol-soluble modulin (PSM) surfactant peptides cause agglomerates to grow to exceptional dimensions. Our results indicate that PSMs function by disrupting interactions of biofilm matrix molecules, such as the polysaccharide intercellular adhesin (PIA), with the bacterial cell surface. Together, our findings support a two-step model of staphylococcal prosthetic joint infection: As we previously reported, interaction of S. aureus surface proteins with host matrix proteins such as fibrin initiates agglomeration; our present results show that, thereafter, the bacterial agglomerates grow to extremely large sizes owing to the lack of PSM expression under the specific conditions present in joints. Our findings provide a mechanistic explanation for the reported extreme resistance of joint infection to antibiotic treatment, lend support to the notions that Agr functionality and PSM production play a major role in defining different forms of S. aureus infection, and have important implications for antistaphylococcal therapeutic strategies.