By Peter Weiss
Bubbles can be microbe killers. Scientists have long known that ultrasound in liquids causes gas bubbles to form and then often collapse violently. When those bubbles implode in cleaning solutions, they break up dirt and destroy some microbes. Doctors have eyed high-frequency sound as a quick, low-heat way to sterilize medical instruments, but no ultrasonic device yet has killed germs efficiently enough.
A study unveiled this week at the First Pan-American/Iberian Meeting on Acoustics in Cancun, Mexico, suggests that an effect known from submarine research may make ultrasound sterilization possible. In a liquid exposed to ultrasound, moderately increasing the pressure dramatically boosts microbe destruction, according to Kenneth A. Cunefare of the Georgia Institute of Technology in Atlanta and his colleagues.
In related microbe-blasting research presented at the Cancun meeting, Mexican scientists described work using powerful electric discharges in water to produce shock waves. Achim M. Loske of the Universidad Nacional Autónoma de México in Querétaro and his colleagues found that the pressure, bursting bubbles, and light from the discharges combine to slay bacteria in complex ways.
Loske’s group also found that ultrasound-induced bubble formation, or cavitation, and bursting might not affect different bacteria in the same way.
Boosting the number of cavitation bubbles increased the kill of one pathogenic bacterium, Escherichia coli, but not of a Listeria strain.
In a liquid, bubbles form when falling pressure permits dissolved gases to pop out of solution. A churning submarine propeller or the low-pressure phase of a sound wave can create such cavitation. When the pressure jumps back up, the bubbles violently collapse (SN: 8/24/02, p. 125: Violent chemistry saps sonobubble energy).
In previous work, scientists studying damage to submarine propellers from such implosions found that bubbles formed at moderate pressure, at shallow depths, do the most harm. Initially, as depth and pressure increase, the bubbles implode with much more force. At greater depths, however, the pressure becomes too high for bubbles to form at all.
In the Georgia team’s experiments, the researchers tested the impact of the disinfectant isopropyl alcohol on bacteria and bacterial spores when combined with ultrasound. In tests of a 66 percent alcohol solution on spores of harmless Bacillus subtilis and Bacillus stearothermophilus, ultrasound had a negligible effect at atmospheric pressure, even after 20 minutes, says Stephen D. Carter, a Snellville, Ga., dentist who worked with Cunefare on the study.
However, doubling the pressure caused 90 percent of the spores to die in just 1 minute. For the effect to be considered sterilization, however, the concentration of surviving microbes must drop to 1 in a million or less, he notes.
Acoustics specialists have failed for decades to achieve ultrasonic sterilization, comments Lawrence A. Crum of the University of Washington in Seattle. That’s because creating an implosion near every microbe is a tall order, he says.
The Georgia team plans to circulate the fluid to meet that challenge. If the technique works, Carter predicts that it could achieve sterilization in 10 minutes–a third of the time required by an automated system that washes instruments with peracetic acid, the leading low-temperature sterilization method now in use.
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