Abstract
Aim
Staphylococcus aureus (SA) chronic bone and joint infections (BJI) are characterized by a progressive destruction of bone tissue associated to SA persistence which results in a large number of relapses (10–20%). The main factors proposed for these failures are: i) a weak diffusion of antibiotics in bone tissue, ii) formation of biofilm, iii) the bacterial internalization by the cells responsible for bone mineralization, namely the osteoblasts (OB).
Our in vitro and in vivo work aimed at providing new information on the impact of SA, more specifically of internalized SA, on bone homeostasis.
Method
Effect of SA infection (8325–4/FnBP+; DU5883/FnBP-) on the viability, differentiation and mineralization of an OB cell line was measured in vitro by MTT and Phosphatase Alcaline (PAL) activity assays and quantification of calcium deposits using Alizarin red, respectively. A gentamicin protection assay (GPA) confirmed that the effects observed are due solely to the internalized SA. In vivo, X-ray microtomography (μCT) and 3D reconstruction was used to evaluate the impact of SA infection on bone formation and bone resorption in a mouse model of femur infection.
Results
In vitro, the infection of pre-OB decreases their capacity of differentiation into mature OB displaying a PAL activity. This effect depends on both the multiplicity of infection and invasion capacities of the strains used (8325–4 (invasion competent) vs DU5883 (invasion incompetent)). The infection delays mineralization after 5 days (p <0.0001), likely due to a cytotoxic effect. Indeed, after bacterial clearance at J21, this delay is made up (no difference between infected and uninfected cells). These results are consistent with the preliminary in vivo observations (μCT) showing a significant decrease in the thickness of trabecular of infected femurs with 8325–4 compared to DU5883 and non-infected femurs (p< 0, 0041).
Conclusions
These results suggest that the internalization of SA leads to an imbalance of bone remodeling, in particular by a cytotoxic effect on the pre-OB and a slowed-down formation of bone tissue by OB, leading to a significant bone loss. The ongoing study of the cellular and bacterial mechanisms involved in this internalization should allow a better management of chronic BJI.
