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McDonagh P1, Widaa A2,3, Claro T2, O’Brien FJ3, Kerrigan S2

Author affilitations
1RCSI medical student
2Molecular and Cellular Therapeutics, RCSI
3Department of Anatomy, RCSI



Background
Staphylococcus aureus is a commensal gram-positive organism of the skin and mucous membranes but may become pathogenic in the right circumstance. As a pathogen, S. aureus represents the most common cause of osteomyelitis and localised bone destruction. S. aureus binds to the osteoblast, is internalised and then inhibits osteoblast proliferation, leading to weakening of the bone.1 Previous reports demonstrate that internalisation is mediated by S. aureus cell wall protein FnbpA binding to the osteoblast.2 This paper investigates if S. aureus FnbpA plays a role in inhibiting osteoblast proliferation.

Methods
S. aureus strain SH1000 and a mutant lacking expression of FnbpA were grown to the exponential phase of growth for four hours in brain heart infusion broth. Bacteria fixed in formaldehyde were allowed to adhere to cell culture plates. Mouse osteoblast cells (2×105 cells/well) were added at 0 hour. Following a proliferation time of 24 and 48 hours, osteoblasts were removed and counted using a haemocytometer. The experiment was repeated three times with three wells allocated to each strain and the control. Statistical analysis was carried out using paired student t-tests.

Results
In the absence of S. aureus, osteoblasts exponentially proliferated over 24 hours and 48 hours. Addition of S. aureus strain SH1000 ablated proliferation at both 24 hours and 48 hours (0% growth at both time points, p<0.01, n=3). Furthermore, deletion of FnbpA from S. aureus SH1000 failed to recover proliferation after 24 hours or 48 hours (0% growth, p<0.05, n=3).

Conclusion
When S. aureus enters bone it binds viable osteoblasts, is internalised and then inhibits proliferation. Binding and internalisation is mediated by S. aureus protein FnbpA. Deletion of FnbpA from parent strain S. aureus SH1000 failed to recover the proliferation, which suggests that another protein expressed on S. aureus is responsible for preventing osteoblast proliferation. We therefore propose that S. aureus binding to osteoblasts is a multifactorial event involving several protein-protein interactions. Understanding the molecular mechanisms of osteomyelitis will aid in the development of novel treatments for this disease.

This work was kindly supported by a grant from the Royal College of Surgeons in Ireland Charitable Infirmary Charitable Trust.

References

  1. Tucker KA, Reilly SS, Leslie CS, Hudson MC. Intracellular Staphylococcus aureus induces apoptosis in mouse osteoblasts. FEMS Microbiology Lett 2000; 186: 151-6.
  2. Sinha B, François PP, Nüße O, Foti M, Hartford OM, Vaudaux P et al. Fibronectin-binding protein acts as Staphylococcus aureus invasion via fibronectin bridging to integrin 5β1. Cellular Microbiology 1999; 1 (2): 101-17.

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