Microbubble-Mediated Ultrasound Enhances the Lethal Effect of Gentamicin on Planktonic Escherichia coli

Previous research has found that low-intensity ultrasound enhanced the lethal effect of gentamicin on planktonic E. coli. We aimed to further investigate whether microbubble-mediated low-intensity ultrasound could further enhance the antimicrobial efficacy of gentamicin. The planktonic E. coli (ATCC 25922) was distributed to four different interventions: control (G(CON)), microbubble only (G(MB)), ultrasound only (G(US)), and microbubble-mediated ultrasound (G(MUS)). Ultrasound was applied with 100 mW/cm(2) (average intensity) and 46.5 KHz, which presented no bactericidal activity. After 12 h, plate counting was used to estimate the number of bacteria, and bacterial micromorphology was observed with transmission electron microscope. The results showed that the viable counts of E. coli in G(MUS) were decreased by 1.01 to 1.42 log(10) CFU/mL compared with G(US) (P < 0.01). The minimal inhibitory concentration (MIC) of gentamicin against E. coli was 1 μg/mL in the G(MUS) and G(US) groups, lower than that in the G(CON) and G(MB) groups (2 μg/mL). Transmission electron microscopy (TEM) images exhibited more destruction and higher thickness of bacterial cell membranes in the G(MUS) than those in other groups. The reason might be the increased permeability of cell membranes for gentamicin caused by acoustic cavitation.