Further Studies on the Effect of Supersonic Waves on Bacteria

Abstract

We have shown 1 that exposure to supersonic waves brings about killing and dissolution of certain bacteria. The question as to whether these effects are due to the mechanical waves motions in the medium or cavitation of the dissolved gases remained unanswered. The observations of the present study on the effect of supersonic waves on B. dysenteriae Shiga and B. coli suspended in gas-free solutions and in solutions saturated with air or hydrogen seem to indicate that the killing and dissolution of the bacteria is due to the cavitation of the dissolved gases.

The apparatus used for the generation of the supersonic waves at the rate of 1.5 × 10⁶ times per second was the same as described by Wu and Liu. 2 One cc. of the bacterial suspension to be exposed was placed in a pyrex tube (20 mm. in diameter) containing a glass cooling coil through which cold water circulated. The temperature of the bacterial suspension throughout the entire experiment was always below 20°C, thus eliminating the possibility of destruction of bacteria by heat. Twenty-four-hour cultures of B. dysenteriae Shiga and B. coli on agar slant were washed and suspended in saline. To make the suspension gas-free, it was placed in the test tube and subjected to suction with a vacuum pump until the mercury manometer showed a constant minimum reading of 18 to 20 mm. The rubber tubing connecting between the test tube and pump was clamped and disconnected from the pump. The test tube containing the gas-free suspension under vacuum was then exposed to supersonic waves. To obtain hydrogen saturated suspension, hydrogen gas was allowed to pass through a sterile glass tubing with a cotton plug into the gas-free suspension under vacuum until one atmospheric pressure was reached.