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. 1998 Oct;64(10):3927-31.
doi: 10.1128/AEM.64.10.3927-3931.1998.

Bacterial stress responses to 1-megahertz pulsed ultrasound in the presence of microbubbles

A C Vollmer  1 , S Kwakye, M Halpern, E C Everbach
Affiliations

Bacterial stress responses to 1-megahertz pulsed ultrasound in the presence of microbubbles

A C Vollmer et al. Appl Environ Microbiol. 1998 Oct.

Abstract

Members of a panel of stress-responsive biosensors have been used to study the effect of megahertz frequency ultrasound on Escherichia 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.

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Figures

FIG. 1
FIG. 1
Kinetic graph showing induction of bioluminescence as a function of time for grpE′::lux promoter fusion bacteria after exposure to 5 min of 1-MHz pulsed ultrasound (ISPPA, 500 W cm−2). RLU are directly proportional to stress response. The sham-treated control was subjected to transport and all manipulations, as was the ultrasound-treated sample, without being subjected to the 1-MHz transducer. The untreated control was placed at 26°C and shaken but was not transported or placed in the apparatus.
FIG. 2
FIG. 2
Specific response ratio plotted versus time-averaged RMS cavitational signal (in millivolts) for all exposures in this study. Data shown are for individual experiments, not group means. Specific response ratio is defined as the ratio of the maximum RLU per viable cell for ultrasound-exposed bacteria to the maximum RLU per viable cell for sham-exposed controls. RMS signal from the 20-MHz passive cavitation detector is a measure of average acoustic emissions produced by collapsing bubbles occurring throughout the 5-min insonification.
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