Effect of low frequency ultrasound on immune escape of Staphylococcus epidermidis biofilms

OBJECTIVE To explore the effect of low frequency ultrasound on Staphylococcus epidermidis biofilms (BF) and observe change of neutrophil activity against bactericidal effect of BF.METHODS In vitro S.epidermidis BF models were established and exposed to low frequency ultrasound at 500 mW/cm² or 800 mW/cm² of sound intensity, the counts of viable bacteria and total biomass of BF were detected by respectively using the plate counting and crystal violet semi-quantitative test, the BF were incubated in vitro with neutrophils isolated from rabbits for 1 hour. The phagocytosis, chemotaxis and respiratory burst of neutrophils against post-treatment BF were detected by using gram staining, transwell migration test and flow cytometry, respectively.RESULTS There was no significant difference in the total counts of viable S. epidermidis BF before and after the ultrasound intervention. The crystal violet semi-quantitative test indicated that the structure of BF was loosened after the low frequency ultrasound intervention, and the total biomass of BF was significantly reduced, as compared with the control group (P<0.05); the total biomass of BF was reduced more significantly by using the sound intensity of 800 mW/cm² than by using the sound intensity of 500 mW/cm² (1.06±0.09 vs. 1.34±0.08,P<0.05). The phagocytosis and chemotaxis of neutrophils against BF were enhanced more significantly in the intervention group than in the control group by using the 500 mW/cm² and 800 mW/cm² of sound intensities (P<0.05); the mean fluorescence intensity of respiratory burst of neutrophils was(568±53) in the control group, (637±74) in the intervention group by using the 500 mW/cm² of sound intensity and (718±59) in the intervention group by using 800 mW/cm² of sound intensity, indicating that the ultrasound intervention effect was enhanced significantly with the increase of the sound intensity (P<0.05).CONCLUSION After the low frequency ultrasound intervention, the structure of S.epidermidis BF is loosened, the total biomass is reduced, and the phagocytosis, chemotaxis and respiratory burst of neutrophils against the post-treatment BF are enhanced.