J. Gen. Appl. Microbiol., 52, 295–301 (2006)

an adjuvant to antimicrobial therapy, targeted at both planktonic and biofilm (sessile) organisms. Our previous work showed that ultrasound (US) effectively enhances the bactericidal activity of certain antibiotics against planktonic cultures (Pitt et al., 1994; Rediske et al., 1999) and in vitro biofilms (Johnson et al., 1998; Qian et al., 1999) and in vivo biofilms (Carmen et al., 2004b, 2005; Rediske et al., 2000) of gram-positive and gram-negative bacteria. Ultrasound was shown to increase the transport of antibiotics through biofilms (Carmen et al., 2004a) which could account for some (or all) of the enhanced antibiotic activity against insonated biofilms; but such a mechanism could not account for US-enhanced antibiotic activity in planktonic cultures which have no extensive exopolymer matrix to retard antibiotic transport. Because this ultrasonic enhancement of antibiotic activity operates on both planktonic and sessile bacteria, we posit that US does more than simply increase the transport of antibiotic to the cells; ultrasound is postulated to increase uptake of antibiotic into the cells by rendering the cell membrane more permeable to the antibiotic. To examine this postulate, we must first review how ultrasound interacts with cells. Bacterial cells are fairly transparent to ultrasound; that is, ultrasonic waves go right through cells with little absorption, scattering or other interaction. However, the pressure oscillations of ultrasound produce size oscillations in any gas bubbles in the liquid (Brennen, 1995). These bubbles range in size from approximately 1 mm to 100 mm in diameter (Brennen, 1995). The oscillations of bubbles, called cavitation, are generally divided into “stable” and “collapse” types of cavitation. Stable cavitation is the low intensity oscillation of the bubbles without complete collapse of the bubble, while collapse cavitation occurs at higher intensity levels and lower frequencies wherein these bubbles collapse and violently accelerate the fluid around them. During bubble collapse, adiabatic heating of the gas produces very high temperature, produces free radicals, generates very high liquid shear force, and generates a shock wave as the collapsing spherical wall slams into itself (Brennen, 1995). With a sufficient number of collapse cavitation events, cell membranes Low-frequency ultrasound increases outer membrane permeability of Pseudomonas aeruginosa