In vitro anti-biofilm activity of a biphasic gentamicin-loaded calcium sulfate/hydroxyapatite bone graft substitute

Bone and implant-associated infections caused by microorganisms that grow in biofilms are difficult to treat because of persistence and recurrence of infection. Along with surgical debridement, the combination of systemic and local administration of antimicrobials represents the background for an efficient treatment strategy. Gentamicin is one of most used antibiotics for the local treatment of bone-related infections, alone or in combination, due to its bactericidal and broad-range activity. Gentamicin-loaded beads (GLBs), composed of calcium sulfate/hydroxyapatite, were assessed for their in vitro antimicrobial activity against planktonic and biofilm S. agalactiae, S. aureus, S. epidermidis, E. faecalis and E. coli, using standard methods and ultra-sensitive isothermal microcalorimetry. Gentamicin released from GLBs to clinically relevant concentrations (200–2500 μg/mL) within 1 h was able to kill planktonic S. agalactiae, S. epidermidis and E. coli at lower concentrations (MIC: ≤4 μg/mL). Moreover, 12 and 23 μg/mL of released gentamicin were able to prevent bacterial adhesion and suppress a 24 h-old biofilm of E. coli, respectively. Conversely, higher amounts of antibiotic, ranging from 171 to 1260 μg/mL, were needed to prevent and eradicate biofilms of gram-positive bacteria. Likewise, the emergence of resistance to GLBs in vitro and the bacterial attachment on the bone graft substitute, when the amount of gentamicin in the material is reduced, were also reported. This study provides further information regarding the in vitro anti-biofilm activity of the biphasic gentamicin-loaded bone graft substitute, suggesting the validity of this antibiotic-loaded material for the prophylaxis and treatment of bone and implant-associated infections.