Sunday, June 12, 2016

Why prosthesis exchange is important: biofilms protect bacteria from antibiotics

Antibiotic-tolerant Staphylococcus aureus Biofilm Persists on Arthroplasty Materials.

These authors cultured methicillin-sensitive Staphylococcus aureus biofilm on total knee arthroplasty materials and exposed these biofilms to increasing concentrations of cefazolin (control, 0.5, 1.0, 10.0, 100.0 μg/mL) to determine if the biofilm could be treated with antibiotics.

Quantitative confocal microscopy and quantitative culture were used to measure viable biofilm cell density.

At the highest concentration tested (100 µg/mL), residual viable biofilm was present on all three materials, and there were no differences in percent biofilm survival among cobalt-chromium (18.5% ± 15.1%), polymethylmethacrylate (22.8% ± 20.2%), and polyethylene (14.7% ± 10.4%). They found that tolerance was a phenotypic phenomenon, because increasing cefazolin exposure did not result in changes in minimum inhibitory concentration as compared with controls.

They concluded that antibiotics are inadequate at complete removal of the biofilm from the surface of arthroplasty materials.

Comment: Biofilms provide a protective niche for bacteria. The biofilms impede antibiotic access by providing a barrier to diffusion. They also allow bacteria to persist at a lower metabolic rate, again making them less susceptible to the effects of antibiotics. 

These results help explain the ineffectiveness of washout and partial prosthesis exchange in managing  colonized implants.