An in vitro analysis of various antibiotic cement combinations against Cutibacterium acnes

Background: Cutibacterium acnes is the most common cause of prosthetic shoulder infection. Treatment often requires the placement of an antibiotic-loaded cement spacer. The exact combination of antibiotic and cement that best targets this organism is not fully understood. In this study, we performed an in vitro investigation of various antibiotic combinations in Simplex and Palacos cement to determine their efficacy against three C acnes strains. We hypothesized that gentamicin alone would underperform other antibiotics including vancomycin and ertapenem against C acnes in an in vitro environment. Additionally, we hypothesized that Palacos cement would outperform Simplex cement. Methods: Strains were cultured and antibiotic cement beads were plated in six combinations. Zones of inhibition (mm) were measured, and beads were transferred to a new culture plate at 7-day intervals. This procedure was carried out for 81 days. Mixed model for longitudinal repeated measures for continuous data was used to determine the antibiotic combination that produced the most robust zone of inhibition over time. Results: For vancomycin, gentamicin, and vancomycin + gentamicin, these data demonstrated that for two strains, Palacos cement with vancomycin and gentamicin produced the most robust zone of inhibition over time (P < .0001, P < .0001, respectively). For one strain, Palacos cement combined with vancomycin alone produced the most robust response (P < .0001). The second highest response for this strain was seen in Palacos cement combined with vancomycin and gentamicin. Ertapenem induced the largest response among all antibiotics that we tested against all 3 strains, but its effect was short-lived (one week or less) compared to vancomycin or vancomycin with gentamicin (35-81 days). Gentamicin response in both cement combinations against C acnes was weak and lasted at most 14 days. Discussion: These data suggest that Palacos cement likely offers a better elution profile than Simplex in in vitro C acnes-related prosthetic shoulder infection strains. Vancomycin and gentamicin in combination demonstrated the most prolonged response for eradication of the 3 C acnes strains tested. Additionally, ertapenem may be a promising option as it showed a short-lived but robust clearing response. Gentamicin weakly inhibits C acnes growth calling into question its use in premade spacers.