Abstract
Aim
To investigate the local intra-operative concentration of gentamicin needed to prevent biofilm formation in a porcine model of implant-associated osteomyelitis.
Method
In total 24 pigs were allocated to six groups. Group A (n=6) was inoculated with saline. Groups B (n=6), C (n=3), D (n=3), E (n=3) and F (n=4) were inoculated with 10 μL saline containing 104 CFU of Staphylococcus aureus, however, different minimal inhibitory concentrations (MIC) of gentamicin were added to the inoculum of Groups C(160xMIC), D(1600xMIC), E(16000xMIC) and F(160000xMIC). The inoculums were injected into a pre-drilled implant cavity proximally in the right tibial bone. Following inoculation, a steel implant (2 × 15 mm) was placed in the cavity. The pigs were euthanized after five days. The implants were sonicated and swabs were taken from the implant cavity for microbiological evaluation. The peri-implant tissue was analyzed by histopathology including estimation of neutrophil infiltration.
Results
The microbiological samples from Group A pigs were sterile. All implants and implant cavities of pigs inoculated with bacteria and bacteria + 160 or 1.600xMIC were positive for S. aureus. In each of the Groups E (16000xMIC) and F (160000xMIC) only one animal was found positive and 1/3 and 3/4 of the implants were sterile after sonication, respectively. All positive swabs were confirmed to be same spa-type as used for inoculation. By adding Groups C + D (<10000xMIC) and Groups E + F (>10000xMIC) a strong significant decrease (one-way ANOVA, P value = 0.001) of implant attached bacteria was only seen between the high MIC values and Group B (bacteria only). The histological examination demonstrated that 1600, 16000 and 160000 × MIC resulted in a peri-implant tissue reaction, including neutrophil estimation, comparable to saline inoculated animals. Patho-morphologically, it was not possible to distinguish between pigs inoculated with bacteria and bacteria + 160xMIC as both groups had a strong inflammatory response and an equal estimation of neutrophils.
Discussion
The antibiotic susceptibility for prevention of an in vivo biofilm infection is influenced by body fluids, host immune response, extracellular host proteins like fibrin, tissue necrosis and development of an anaerobic environment. With the present in-vivo setup, we have demonstrated that local intra-operative gentamicin might be given in concentrations of more than 10000 times the MIC value in order to prevent biofilm formation by planktonic bacteria. Our study supports that biofilm susceptibility testing performed in-vitro is yet still unreliable for prediction of prophylactic and therapeutic success.
