The effect of frequency and power density on the ultrasonically-enhanced killing of biofilm-sequestered Escherichia coli

Infection on implanted medical devices is a critical concern because the bacteria are recalcitrant to antibiotic therapy; currently the only way to eliminate the infection is to remove the device. We have found that low-frequency ultrasound renders bacteria more susceptible to antibiotics. The effect of low-intensity ultrasound on the enhancement of antibiotic action against biofilm bacteria was measured by subjecting thick E. coli biofilms for 2 h at 37°C to one of four conditions: (1) incubation in nutrient broth; (2) incubation in nutrient broth with antibiotic; (3) ultrasonication in nutrient broth without antibiotic; and (4) ultrasonication in nutrient broth with antibiotic. Two frequencies (70 and 500 kHz) and several ultrasonic intensities were examined, ranging from 2 to 200 mW/cm². It was determined that low-intensity ultrasound significantly enhanced killing of biofilm E. coli by gentamicin. This enhancement increased with increasing ultrasonic intensity and decreased with increasing frequency. A mathematical model of ultrasonically-enhanced transport in cylindrical pores and channels shows that gentamicin transport increases with ultrasonic intensity and decreases with increasing frequency. However, the magnitude of increased transport is so small that it is difficult to attribute enhanced killing of bacteria to enhanced antibiotic transport through the pores and channels of the biofilm; therefore, other mechanisms must play a role. The use of low-intensity ultrasound in conjunction with antibiotic treatment may prove to be a viable clinical method of eliminating biofilm infections from the surfaces of implanted medical devices.