Persistence of bacteria on antibiotic loaded acrylic depots. A reason for caution.

Bacterial growth on the surface of antibiotic loaded acrylic cement was examined in an in vitro model. Tobramycin or vancomycin impregnated discs were incubated in broth containing either Staphylococcus epidermidis or Staphylococcus aureus organisms. At 24, 48, and 96 hours, the broth and the surface of the acrylic discs were examined for viable organisms. In the broth, only 6% (8 of 136) of samples contained viable organisms at 24 hours, and all samples were sterile by 96 hours. This is in contrast to the surface of the discs, which revealed the presence of viable organisms at all study time periods. Growth was noted in 99% and 30% of the Staphylococcus epidermidis and Staphylococcus aureus discs, respectively, at 24 hours. Viable organisms were found on both types of discs through 96 hours (20% of Staphylococcus epidermidis and 15% of Staphylococcus aureus). The surface of bone cement is therefore a suitable substrate for bacterial growth, even in the presence of antibiotics. In clinical practice, antibiotic loaded acrylic cement should therefore be used with caution and clear indications.