Efficacy of Topical Vancomycin- and Gentamicin-Loaded Calcium Sulfate Beads or Systemic Antibiotics in Eradicating Polymicrobial Biofilms Isolated from Diabetic Foot Infections within an In Vitro Wound Model

Abstract

Diabetic foot ulcers are notoriously difficult to heal, with ulcers often becoming chronic, in many cases leading to amputation despite weeks or months of antibiotic therapy in addition to debridement and offloading. Alternative wound biofilm management options, such as topical rather than systemic delivery of antimicrobials, have been investigated by clinicians in order to improve treatment outcomes. Here, we collected blood and tissue from six subjects with diabetic foot infections, measured the concentrations of antibiotics in the samples after treatment, and compared the microbiota within the tissue before treatment and after 7 days of antibiotic therapy. We used an in vitro model of polymicrobial biofilm infection inoculated with isolates from the tissue we collected to simulate different methods of antibiotic administration by simulated systemic therapy or topical release from calcium sulfate beads. We saw no difference in biofilm bioburden in the models after simulated systemic therapy (representative of antibiotics used in the clinic), but we did see reductions in bioburden of between 5 and 8 logs in five of the six biofilms that we tested with topical release of antibiotics via calcium sulfate beads. Yeast is insensitive to antibiotics and was a component of the sixth biofilm. These data support further studies of the topical release of antibiotics from calcium sulfate beads in diabetic foot infections to combat the aggregate issues of infectious organisms taking the biofilm mode of growth, compromised immune involvement, and poor systemic delivery of antibiotics via the bloodstream to the site of infection in patients with diabetes.

Keywords: biofilm; clinical; diabetes; diabetic foot; infection; polymicrobial; postantibiotic effect.

FIG 1

Microbiota from tissue samples of DFIs for six subjects. (a) Total counts of aerobic and facultative anaerobic microorganisms isolated from debrided tissue from patients pre- and posttreatment (systemic) for a diabetic foot infection. (b) Average combined total counts ± the standard deviations (SD) for all patients pre- and posttreatment. (c to h) Heat maps showing the change in microbiota groups isolated from debrided tissue under aerobic or anaerobic growth conditions from each patient upon presentation and after 7 days of systemic treatment with antibiotics for a diabetic foot infection. The limit of detection for each species was 1.7 log₁₀(CFU/ml) in this study. ND1 and ND2 refer to two strains that we were unable to identify.

FIG 2

Mean total bacterial counts ± the SD within the wound model after exposure to topical therapy, simulated systemic therapy, or no antibiotics. “Control” denotes models to which no antibiotics were added. (a to f) The model designations (labeled in lowercase letters) correspond to patients DFG, DFN, DFR, DFB, DFK, and DFM discussed in the text. *, P < 0.05; **, P < 0.01; ***, P < 0.001; ns, not significant.