Bacteria butchered by bubbles

An acoustic phenomenon previously studied for its effects on submarines could be the basis for an improved disinfection technique that kills microorganisms on medical instruments.

An acoustic phenomenon previously studied for its effects on submarines could be the basis for an improved disinfection technique able to rapidly kill microorganisms on medical instruments without high temperatures or harsh chemicals.

Preliminary research by scientists at the Georgia Institute of Technology and Georgia State University showed the technique killed more than 90 percent of bacteria in a test vial that also contained a mild solution of isopropyl alcohol.

‘Complex and extremely expensive endoscopes and related surgical equipment are very vulnerable to heat, and they are challenging to clean,’ explained Dr. Stephen Carter, an Atlanta-area dentist who is working with Georgia Tech Professor Kenneth Cunefare to develop the technique. ‘We believe that our methods will sterilise in shorter periods of time, which would be a substantial advantage for expensive medical equipment.’

The patented technique uses a form of cavitation, a phenomenon in which acoustic energy applied to a liquid induces the creation of voids — or bubbles — that release energy when they collapse. By pressurising their test chamber while inducing cavitation, Cunefare and Carter create a form of transient cavitation that is said to cause violent collapse of the bubbles.

The enhanced cavitation takes advantage of the ‘anomalous depth effect,’ in which the impact of bubble collapse increases dramatically when subjected to roughly twice the normal atmospheric pressure. Scientists have studied the phenomenon for years because it can damage submarines’ propellers when operating at certain depths.

When applied to a solution of 66 percent isopropyl alcohol containing two forms of ‘marker’ bacterial spores – Bacillus stearothermophilus and Bacillus subtilis -the enhanced cavitation reduced the bacterial count by more than 90 percent, Cunefare said. Research indicates that both the alcohol solution and increased pressure are necessary for killing the spores with cavitation.

Tests showed little effect on the spores from the transient cavitation in plain water, or from cavitation of the alcohol solution at standard atmospheric pressure.

As part of their evaluation, researchers subjected a test vial of fluid containing bacterial spores in the alcohol solution to short bursts of cavitation over a period of 10-15 minutes. When the power was applied, the test vial appeared to be filled with foam that subsided when the power was switched off, Cunefare said. The cavitation was active for up to 60 seconds of the test period.

Because acoustic disinfection could be carried out more quickly than existing heat and chemical techniques, Carter believes it could offer significant cost advantages by reducing the amount of time that expensive equipment is out of service. And it would also have the potential for minimising the risk of cross transmission of infection caused by contaminated instruments, he added.

The idea for using transient cavitation to disinfect instruments originated with Carter, who had been interested in a new approach for sterilising the growing number of instruments that are vulnerable to damage from traditional heat disinfection. He reasoned that rapid decompression might be able to kill microbes by breaking their cell walls, and obtained a patent for the idea in 1994.

Subsequent testing, however, showed that even ‘explosive decompression’ failed to kill the hardiest of bacterial spores, so Carter sought to enhance the technique by combining pressure with powerful cycles of ultrasonic energy. Though he obtained a patent for that approach in 1997, it still was unable to kill the toughest of microbes. Undaunted, he approached Georgia Tech for help.

A professor in the School of Mechanical Engineering and a specialist in acoustics, Cunefare determined that Carter was on the right track, but needed to increase the amount of ultrasonic energy and change the pressure to optimise the effects of cavitation. Working together, Carter and Cunefare selected the right combination of energy, pressure and alcohol content to dramatically reduce the number of bacterial spores.

The mechanism by which the cavitation, pressure and alcohol combine to kill bacteria remains under study.

‘We don’t know exactly how the cells die, but we know the end phenomenon,’ said Donald Ahearn, professor emeritus of biology at Georgia State University. ‘Increased pressure and disinfectant molecules are somehow enhanced by the cavitation process, but the physiology of the death has yet to be determined.’

Ultrasound has been used elsewhere to make skin permeable enough to admit drug compounds. Cunefare suspects that the cavitation may induce a similar effect, making the bacterial cell walls permeable enough to admit the alcohol molecules.

Though the researchers studied only the technique’s effect on bacteria, Ahearn — who did the biological assays for the study — expects it would also work against viral organisms.

The researchers are now seeking support from the National Institutes of Health to optimise the technique, scale it up to a practical size, ensure that it would adequately kill the microorganisms — and assess the potential for damaging medical instruments.

‘We are seeking funding to further develop our understanding of the parameters that affect the transient cavitation,’ Cunefare explained. ‘We want to optimise the effects, and explore other additives that might enhance it.’

The researchers will also have to improve techniques for coupling power into the fluid in order to treat larger volumes of liquid. Since the amount of energy that can be induced into a liquid depends on the surface area, there may be limits to the volume that can be treated by inducing energy from the boundaries. They also need new power electronics and transducers that can operate continuously.

Beyond sterilisation of medical instruments, Cunefare also sees potential applications in the continuous treatment of water and wastewater, and potentially in low-temperature pasteurisation of food products such as milk or orange juice.