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

Table 2

Reports about effect of antibiotics combined with LFU on biofilm.

Authors (year)	Type of research	Pathogens	Frequency, density, and time of LFU	Combined antibiotics	Results
Qian et al. (1996) [26]	In vitro study on biofilm	P. aeruginosa	500 kHz; Average insonation intensity of 10 mW/cm^2; CW, 2 h	Gentamicin	LFU enhanced bacteria bactericidal effect of gentamicin on 24 h old biofilm of P. aeruginosa. CLSM results showed that LFU does not disrupt biofilm or disperse the bacteria.
Qian et al. (1997) [27]	In vitro study on biofilm	P. aeruginosa	70, 500 kHz; 2.25, 10 MHz; 10 mW/cm^2; CW	Gentamicin	A significantly greater fraction of the bacteria was killed by gentamicin when they were subjected to ultrasound. Ultrasound by itself did not have any deleterious effect on the biofilm viability. LFU is significantly more effective than higher frequency ultrasound in reducing bacterial viability within the biofilm.
Johnson et al. (1998) [29]	In vitro study on biofilm	E. coli	70, 500 kHz; 20, 100 mW/cm^2; CW, 2, 4, 6 h	Gentamicin	The combination of 70 kHz ultrasound with the antibiotic more significantly reduced bacterial viability than antibiotic alone, resulting in about 97% killing in 2 h. The combination of 500 kHz ultrasound and antibiotic produced only a slight, insignificant reduction in the killing caused by antibiotic alone.
Rediske et al. (1999) [30]	In vivo study of biofilm-infected rabbit model	E. coli	28.48 kHz; 100–300 mW/cm^2; CW, 24 h	Gentamicin	Exposure to ultrasound only caused no significant difference in bacterial viability. But in the presence of antibiotic, bacterial viability was significantly reduced due to 300 mW/cm^2 ultrasound () and insignificant reduction due to 100 mW/cm^2 ultrasound.
Qian et al. (1999) [28]	In vitro study on biofilm	P. aeruginosa	44 kHz–10 MHz; 1 and 10 mW/cm^2; duty cycle of 1 : 10; 2 h	Gentamicin	The enhanced bactericidal effect of antibiotic due to sonication showed a monotonic decrease as the frequency increased from 44 kHz to 10 MHz, indicating that the lower frequencies are more effective in enhancing the antibiotic action. A power density of 10 mW/cm^2 is more effective in enhancing the bactericidal effect of the antibiotic than the 1 mW/cm^2 intensity.
Rediske et al. (2000) [31]	In vivo study of biofilm-infected rabbit model	E. coli	28.48 kHz,; 1 : 3 duty cycle with 300 mW/cm^2; 1 : 6 duty cycle with 600 mW/cm^2; 24 h	Gentamicin	The average bacterial viability was reduced from 2.94 to 0.99  CFU/cm^2 by 300 mW/cm^2 pulsed ultrasound and from 2.93 to 1.69  CFU/cm^2 by 600 mW/cm^2 pulsed ultrasound. No discoloration or damage of the skin was apparent.
Carmen et al. (2004) [34]	In vivo study of biofilm-infected rabbit model	S. epidermidis	28.48 kHz; 1 : 3 duty cycle; 500 mW/cm^2; 24, 48 h	Vancomycin	Application of LFU enhanced the activity of vancomycin against implanted S. epidermidis biofilms. 48 h of insonation significantly reduced the count of viable bacteria in the biofilm.
Carmen et al. (2004) [32]	In vitro study on biofilm	P. aeruginosa; E. coli	70-kHz; 1.9 and 2.9 W/cm^2 for E. coli; 1.5 and 2.5 W/cm^2 for P. aeruginosa; CW, 15, 30, 45 min	Gentamicin	Ultrasonication significantly increased transport of gentamicin across biofilms that normally blocked or slowed gentamicin transport when not exposed to ultrasound.
Carmen et al. (2005) [33]	In vivo study of biofilm-infected rabbit model	P. aeruginosa; E. coli	28.5 kHz; 1 : 3 duty cycle; 500 mW/cm^2; 24, 48 h	Gentamicin	The number of viable bacteria in E. coli biofilm was reduced to 2.29 ± 0.40  CFU/cm^2 after 72 h of treatment with gentamicin alone and to 0.011 ± 1.02  CFU/cm^2 after treatment with both gentamicin (for 72 h) and ultrasound (for 48 h) compared with that after treatment with antibiotic alone. But 24 or 48 h of ultrasound combined with gentamicin failed to significantly enhance the bactericidal effect of P. aeruginosa in the biofilm.
Seth et al. (2013) [35]	In vivo study of biofilm-infected rabbit model	P. aeruginosa	3 min every other day or every day as instructions of MIST Therapy System	Ciprofloxacin (topical)	LFU has a significant impact on biofilm-infected wounds, including a decrease in viable bacteria and an overall improvement in wound healing and host inflammatory dynamics.
Li et al. (2015) [36]	In vivo study of biofilm-infected mouse model	S. epidermidis; S. aureus	200 mW/cm^2; 1 : 1 duty cycle; 20 min, 3 times a day	HBD-3	Biofilm densities, the percentage of live cells, and the viable counts from the biofilm on the titanium surface in mice were significantly decreased in the group of the HBD-3 combined with ultrasound targeted microbubble destruction.
Liu et al. (2016) [37]	In vitro study on biofilm	Pan-resistant; A. baumannii	40 kHz; 1 : 9 duty cycle; 600 mW/cm^2; 30 min	Colistin; Vancomycin	Reductions > 2 log CFU/mL were observed for colistin plus vancomycin with LFU than without LFU after 12 h of incubation. Bacterial counts declined continuously for 24 h, with a reduction of 3.77 log CFU/mL from with LFU to without LFU.

CW: continuous wave ultrasound.