Mapping bacterial biofilms on recovered orthopaedic implants by a novel agar candle dip method

While the detrimental effects of periprosthetic joint infections (PJIs) are well known, the process of biofilm formation on orthopaedic hardware is unclear. Previous work has shown that encasement of explant hardware in agar can aid in identifying biofilms. This study tested the utility of agar ‘candle dip’ method in detecting and mapping the location of biofilm on infected orthopedic components. Explant components from 15 patients were rinsed, briefly submerged in agar to create a surface coating, and incubated. Larger components were coated by pipetting agar over them. After incubation, colony outgrowth on the component surface was documented (candle dip status). Data were compared with clinical laboratory results (clinical culture status) and the PJI diagnosis using Musculoskeletal Infection Society criteria (MSIS status). All six patients classified as MSIS and clinical culture positive were also positive with the candle dip technique. Of the nine candle dip negative cases, four were positive and five were negative for both MSIS and clinical culture status. Candle dip may be negative in few cases due to the residual antibiotic eluting from the spacers, limiting the growth of bacterial biofilms on the components. The candle dip method shows promise for biofilm mapping but requires additional testing to evaluate the clinical diagnostic potential.