Background/Aims: Osteomyelitis is a debilitating infectious disease of the bone which is predominantly caused by Staphylococcus aureus (S. aureus). The purpose of this study was to investigate the role of the S. aureus virulence factors, i.e. protein A (SpA), Panton-Valentine leukocidin (PVL) and coagulase (Coa) on osteomyelitis. Methods: The effect of SpA, PVL and Coa on osteoblasts was studied through the following aspects including osteoblast proliferation, apoptosis, bone formation, bone mineralization and RANK-L expression. S. aureus overexpressing PVL, SpA or Coa was constructed and used to study the role of PVL, SpA and Coa, respectively. S. aureus silencing PVL, SpA or Coa was also constructed and used for reversing verification. Osteoblast proliferation was detected by MTT tetrazolium dye reduction assay. Apoptosis was determined by Annexin V-FITC staining. The levels of pro-caspase 3, cleaved-caspase 3, pro-caspase 9 and cleaved-caspase 9 were detected by western blot. Bone formation markers including collagen I, osteopontin and osteocalcin were detected by real time RT-PCR. Alkaline phosphatase activity was measured by adding p-nitrophenyl phosphate as a phosphatase substrate. Von kossa stain and alizarin red stain were applied for determining phosphate and calcium deposition, respectively. The RANK-L expression was tested by ELISA. Results: PVL, SpA and Coa inhibited osteoblast proliferation, induced osteoblast apotosis, prohibited bone formation and mineralization and upregulated RANK-L expression. Conclusions: PVL, SpA and Coa play a critical role on bone loss and bone destruction of osteomyelitis.

Community acquired methicillin resistant Staphylococcus aureus (CA-MRSA), and the USA300 strain of CA-MRSA in particular, are known for their rapid community transmission, and propensity to cause aggressive skin and soft tissue infections. To assess factors that contribute to these hallmark traits of CA-MRSA, we evaluated how growth of USA300 and production of secreted virulence factors was influenced on exposure to physiologic levels of unsaturated free fatty acids that would be encountered on the skin or anterior nares, which represent the first sites of contact with healthy human hosts. There was a sharp threshold between sub-inhibitory and inhibitory concentrations, such that 100 µM sapienic acid (C16∶1) and linoleic acid (C18∶1) were sufficient to prevent growth after 24 h incubation, while 25 µM allowed unrestricted growth, and 50 µM caused an approximate 10–12 h lag, followed by unimpeded exponential growth. Conversely, saturated palmitic or stearic acids did not affect growth at 100 µM. Although growth was not affected by 25 µM sapienic or linoleic acid, these and other unsaturated C16 and C18 fatty acids, but not their saturated counterparts, promoted robust production of secreted proteases comprising the Staphylococcal proteolytic cascade. This trait was also manifested to varying degrees in other CA-MRSA, and in genetically diverse methicillin susceptible S. aureus strains. Therefore, induction of the Staphylococcal proteolytic cascade by unsaturated fatty acids is another feature that should now be evaluated as a potential contributing factor in the aggressive nature of skin and soft tissue infections caused by USA300, and as a general virulence mechanism of S. aureus.