A Review of the Combination Therapy of Low Frequency Ultrasound with Antibiotics

Table 1

Reports about effect of antibiotics combined with LFU on planktonic bacteria.

Authors (year)	Type of research	Pathogens	Frequency, density, and time of LFU	Combined antibiotics	Results
Pitt et al. (1994) [14]	In vitro study on planktonic cultures	P. aeruginosa; E. coli; S. epidermidis; S. aureus	67 kHz; 0.3 W/cm^2; CW, 1, 2, 3, 6, 9, 12, 14, 18, 24 h	Gentamicin	A synergistic effect was observed and bacterial viability of P. aeruginosa and E. coli was reduced by several orders of magnitude when gentamicin and ultrasound were combined, while ultrasound alone did not kill bacteria. The synergistic effect was not observed with S. epidermidis and S. aureus.
Williams and Pitt (1997) [15]	In vitro study on planktonic cultures	E. coli	70 kHz; 0.01–4.5 W/cm^2; CW, 1, 2, 3 h	Gentamicin	Combined with ultrasonication greatly enhanced the activity of gentamicin. The greatest bactericidal effect (approximately 5-log reduction in viable population) was realized at 4.5 W/cm^2 and decreased with reductions in power density. At 10 mW/cm^2, no significant acoustic enhanced bactericidal effect was noted.
Rapoport et al. (1997) [19]	In vitro study on planktonic cultures	P. aeruginosa	80 kHz; 0.68, 3.2 W/cm^2; CW, 2, 4, 6 h	Erythromycin	The efficiency of erythromycin in killing planktonic P. aeruginosa increased more than an order of magnitude upon the simultaneous application of ultrasound. Ultrasound alone does not kill the cells but rather sensitizes the cells to antibiotic action.
Rediske et al. (1998) [22]	In vitro study on planktonic cultures	E. aerogenes; S. marcescens; Salmonella derby; Streptococcus mitis; S. epidermidis	70 kHz; 3 W/cm^2; CW, 1, 2, 3, 6 h	Gentamicin Streptomycin Kanamycin Tetracycline Ampicillin	Simultaneous application of ultrasound and antibiotic significantly increased the effectiveness of the selected antibiotics. Bacterial viability was reduced by several orders of magnitude when harmless levels of ultrasound were combined with the selected antibiotics, especially the aminoglycosides.
Rediske et al. (1999) [18]	In vitro study on planktonic cultures	P. aeruginosa	70 kHz; 2.2 W/cm^2; CW, 1, 3, 6, 12 h	Erythromycin	Ultrasound in combination with erythromycin reduced the viability of P. aeruginosa by 1-2 orders of magnitude compared with antibiotic alone, even at concentrations below MIC.
Rapoport et al (1999) [17]	In vitro study on planktonic cultures	P. aeruginosa; E. coli	80 kHz; 0.8–2.4 W/cm^2; CW, 2 h	Gentamicin	The penetration of spin-labeled gentamicin was not affected by insonation below the cavitation threshold. It implies that synergistic effect between hydrophilic antibiotics and LFU in killing Gram-negative bacteria did not result from the enhanced antibiotic penetration through bacterial cell walls.
Runyan et al. (2006) [20]	In vitro study on planktonic cultures	P. aeruginosa	70 kHz; 0.5–4.7 W/cm^2; CW, 3 min	Nitrocefin	The rate of nitrocefin hydrolysis is increased by ultrasound in an intensity-dependent manner, which reflects the rate of entry of the antibiotic.
Ayan et al. (2008) [23]	In vitro study on planktonic cultures	S. aureus	1.5 MHz; 0.03–0.161 W/cm^2; CW, 20 min	Penicillin; Oxacillin; Teicoplanin; Vancomycin; Erythromycin; Clindamycin; Levofloxacin; Ciprofloxacin	The samples treated with LFU showed a significantly lower number of bacteria colonies compared to the antibiotic alone. Partial destruction or disintegration of the cell walls was detected in some bacteria using the electron microscopy.
Conner-Kerr et al. (2010) [24]	In vitro study on planktonic cultures	MRSA	35 kHz; 2.0 W/cm^2; CW, 0.5, 1, 3 min	Oxacillin	LFU reduces CFU of bacteria, punctures, and fractures cell walls and alters colonial characteristics of MRSA, including resistance to the methicillin.
Liu et al. (2011) [21]	In vitro study on planktonic cultures	E. coli	40 kHz; 1 W/cm^2; CW, 15, 30, 45 min	Levofloxacin; Ciprofloxacin	Addition of LFU to levofloxacin and ciprofloxacin exposure enhances the effectiveness of the antibiotics in killing E. coli. LFU can activate fluoroquinolones to produce reactive oxygen species, which are mainly determined as superoxide radical anion and hydroxyl radical.
Zhu et al. (2014) [16]	In vitro study on planktonic cultures	E. coli	46.5 kHz; 100 mW/cm^2; 1 : 3 duty cycle	Gentamicin	Microbubble-mediated LFU could further enhance the antimicrobial efficacy of gentamicin compared with LFU alone. Transmission electron microscopy images showed more destruction and higher thickness of bacterial cell membranes in the microbubble-mediated LFU than those in other groups.

CW: continuous wave ultrasound.