Low-frequency ultrasound increases outer membrane permeability of Pseudomonas aeruginosa

Abstract

Low-frequency ultrasound has been investigated asan adjuvant to antimicrobial therapy, targeted at bothplanktonic and biofilm (sessile) organisms. Our previ-ous work showed that ultrasound (US) effectively en-hances the bactericidal activity of certain antibioticsagainst planktonic cultures (Pitt et al., 1994; Rediskeet 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-positiveand gram-negative bacteria. Ultrasound was shown toincrease the transport of antibiotics through biofilms(Carmen et al., 2004a) which could account for some(or all) of the enhanced antibiotic activity against in-sonated biofilms; but such a mechanism could not ac-count for US-enhanced antibiotic activity in planktoniccultures which have no extensive exopolymer matrix toretard antibiotic transport.Because this ultrasonic enhancement of antibioticactivity operates on both planktonic and sessile bacte-ria, we posit that US does more than simply increasethe transport of antibiotic to the cells; ultrasound ispostulated to increase uptake of antibiotic into the cellsby rendering the cell membrane more permeable tothe antibiotic. To examine this postulate, we must firstreview how ultrasound interacts with cells.Bacterial cells are fairly transparent to ultrasound;that is, ultrasonic waves go right through cells with littleabsorption, scattering or other interaction. However,the pressure oscillations of ultrasound produce sizeoscillations in any gas bubbles in the liquid (Brennen,1995). These bubbles range in size from approxi-mately 1mm to 100mm in diameter (Brennen, 1995).The oscillations of bubbles, called cavitation, are gen-erally divided into “stable” and “collapse” types of cavi-tation. Stable cavitation is the low intensity oscillationof the bubbles without complete collapse of the bub-ble, while collapse cavitation occurs at higher intensitylevels and lower frequencies wherein these bubblescollapse and violently accelerate the fluid aroundthem. During bubble collapse, adiabatic heating of thegas produces very high temperature, produces freeradicals, generates very high liquid shear force, andgenerates a shock wave as the collapsing sphericalwall slams into itself (Brennen, 1995). With a sufficientnumber of collapse cavitation events, cell membranes