Bone Infection and MRSA Vaccine

Although improvements in surgical technique and aggressive antibiotic prophylaxis have decreased the infection rate following orthopaedic implant surgery to 1-5%, osteomyelitis (OM) remains a serious problem and appears to be on the rise from minimally invasive surgery. The significance of this resurgence, 80% of which is due to Staphylococcus aureus, is amplified by the fact that ˜50% of clinical isolates are methicillin resistant S. aureus (MRSA). Current estimates of revision surgery for MRSA OM suggest that reinfection rates are over 50%, and most surgeons will not even attempt this procedure on septic total joint replacements (TJR).

 

Pictured Right: X-ray of a patient with an external fixation device that was surgically implanted to hold the fractured bones together so that they can heal. Evidence of bone infection is apparent from the radiolucency around the top pin (unpublished).

 

This indicates that there is a major need for alternative interventional strategies, particularly for immune compromised elderly who are the primary recipients of TJR. To this end, we have developed a novel murine model of implant-associated OM in which a stainless steel pin is coated with S. aureus and implanted transcortically through the tibial metaphysis. This leads to a highly reproducible OM with Gram+ biofilm, osteolysis, sequestrum/involucrum formation, and closely resembles clinical OM.

 

Pictured Left: Scanning electron micrograph illustrated the cytotoxic effect of anti-glucosaminidase antibodies, which cause some Staphylococcus aureus bacterium to explode (unpublished).

 

Furthermore, we have developed in vivo bioluminescence imaging to quantify the planktonic growth phase of the bacteria; real time quantitative-PCR (RTQ-PCR) to determine nuc gene copy number in infected bone tissue to quantify the total bacteria load; and micro-CT to quantify osteolysis. In preliminary studies we have characterized the host immune response during this infection, and cloned the glucosaminidase (Gmd) subunit of autolysin (Alt) as an immuno-dominant antigen. Moreover, we demonstrated that immunization with an active Gmd vaccine protects mice from S. aureus OM. Using this vaccine we generated 36 anti-Gmd monoclonal antibodies (mAb), and are currently characterizing their binding characteristics, cytolytic activity, and ability to protect mice in our transtibial model of osteomyelitis.  This indicates that there is a major need for alternative interventional strategies, particularly for immune compromised elderly who are the primary recipients of TJR. To this end, we have developed a novel murine model of implant-associated OM in which a stainless steel pin is coated with S. aureus and implanted transcortically through the tibial metaphysis. This leads to a highly reproducible OM with Gram+ biofilm, osteolysis, sequestrum/involucrum formation, and closely resembles clinical OM.

 

Pictured Bottom Left: Gram stained section of a mouse tibia that contained an infected implant as described in Li et al., 2008.

 

Pictured Bottom Right: Hematoxylin and eosin stained section of a mouse tibia that contained an infected implant as described in Li et al., 2008.