Energy-dependent stimulatory and inhibitory effects of extracorporeal shock waves on bacteria and on gentamicin activity

Background: Local infection is considered a contraindication for extracorporeal shock wave (ESW) application, although the antibacterial effects of ESW have been clearly demonstrated in vitro. This study aimed to assess the effects of ESW on bacteria under growth-promoting conditions and to evaluate interactions with the activity of gentamicin. Material/Methods: Standardized suspensions of Staphylococcus aureus were exposed to 4000 shock wave impulses of various energy flux densities (EFD) both at 37°C in growth medium and at 20°C in saline. Bacterial viability of treatment groups and controls were quantified. Furthermore, the MICs of gentamicin against ESW-treated and untreated suspensions of S. aureus were compared. Finally, suspensions of S. aureus containing graded concentrations of gentamicin were exposed to ESW and bacterial growth was assessed. Results: Antibacterial effects of ESW (0.59-0.96 mJ/mm²) were confirmed with bacteria suspended in normal saline (20°C, p<0.05). However, bacteria suspended in growth medium at 37°C demonstrated significantly increased proliferation (p=0.009) after treatment with shock waves of lower EFD (0.59 mJ/mm²). At higher EFD a significant reduction of bacteria was observed (p=0.009). The MIC of gentamicin against S. aureus was not altered by ESW application. Furthermore, the combination of gentamicin and ESW did not alter gentamicin activity (p>0.05). Nevertheless, a growthpromoting effect of ESW at 0.59 mJ/mm² was demonstrated despite simultaneous administration of gentamicin. Conclusions: This is the first study reporting energy-dependent stimulation of bacterial growth by ESW. Also important is that ESW did not alter the activity of gentamicin.