Bacterial Stress Responses to 1-Megahertz Pulsed Ultrasound in the Presence of Microbubbles

Abstract

Members of a panel of stress-responsive biosensors have been used to study the effect of megahertz frequency ultrasound onEscherichia coli. Insonification causes acoustic cavitation, the collapse of oscillating microbubbles in solution, which can damage bacterial cells. A focused 1-MHz ultrasound transducer, capable of generating a spatial peak pulse average intensity of 500 W/cm2, was used to treat liquid bacterial cultures. Stress-responsive promoters fused to luxCDABE allowed the continuous measurement of light produced as a result of protein damage, DNA damage, oxidative stress, and membrane perturbation. A promoter responsive to ammonia limitation was not transcriptionally activated under test conditions. In contrast to bacteria in exponentially growing cultures, those in stationary-phase cultures were more resistant to the effects of ultrasound treatment. Quantification of the degree of acoustic cavitation due to symmetric bubble collapse was measured by a 20-MHz passive transducer, the output of which appears to be only partially correlated with cellular damage and survival. The methods and results summarized here provide the basis for further investigation into applications, including the purification of water samples.

Sonobactericide: An Emerging Treatment Strategy for Bacterial Infections

“…It is likely that bacterial cells in different metabolic states, stages of cell division or growth phases will respond differently to sonobactericide. This hypothesis is supported by the observation that planktonic bacteria in stationary growth phase are more resistant to ultrasound alone or combined with cavitation nuclei (Vollmer et al 1998).…”

Ultrasound Increases the Rate of Bacterial Cell Growth

“…Therefore high intensity ultrasound can kill cells in addition to partially removing them from surfaces. Because cavitation is usually more intense at low frequencies, low frequency ultrasound is commonly used to perturb or disrupt cell membranes and lyse cells (22,23,25,27,(29)(30)(31).…”

Book Review

“…28 After exclusions, the literature search on ultrasound's potential for damage to DNA resulted in 19 new papers being added to the database. [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47] Eighteen were level II evidence in which ultrasound was applied to in vitro collections of cells or DNA. The exception was a level III-2 paper that compared workers who had undergone long-term occupational ultrasound exposure to unexposed controls.…”

“…40,42 There is also some evidence that rapidly dividing cells are more easily damaged by cavitation than stable phase cells. 38,46 The only in vitro study examined here that did not demonstrate any evidence of DNA damage from ultrasound used a procedure in which the target cells were suspended in phosphate-buffered solution to which methylcellulose had been added with the specific intention of preventing cavitation. 47 However, it would appear to be difficult to extrapolate the results of these studies to make realistic estimates about the mutagenic risks of ultrasound in vivo.…”