X. ULTRASONIC VIBRATIONS 331 



The general conclusion arrived at from an analysis of all the data 

 to date substantiates Harvey's observations, namely, that lethal 

 and sterilizing effects on small organisms occur only when there is 

 cavitation in the presence of dissolved gases and that cavitation does 

 not occur within the cells but rather causes external fluid movements 

 etc., which in turn tear the cells apart. 



Bacteria and other extremely small pathogenic organisms are af- 

 fected in many different ways. Williams and Gaines {15) reported 

 that Escherichia coli were killed and that the lethal effect was prob- 

 ably due to violent action set up within the cell. They used a low 

 frequency source of 8800 cycles per second, which has a wavelength 

 in water of about 17 cm. Since the wavelength in this case is very 

 large compared to the cell size, the lethal action of the sound was 

 probably due to some secondary effect of cavitation — such as local 

 fluid movements, high local temperatures, local potentials, or the 

 production of hydrogen peroxide. The latter effect has been shown 

 to exist by the oxidation of potassium iodide when irradiated in a 

 solution containing dissolved air. Use of another dissolved gas would 

 eliminate the latter factor but no data on such an effect with bacteria 

 have yet been reported. As far as the effect of fluid movements are 

 concerned, it is well known that a rapidly revolving (2000-3000 

 r.p.m.) spindle wdth vanes will break up a suspension of cells and 

 sand. Bacteria are destroyed this way in about five hours and while 

 it is possible that exposure to a cavitating ultrasonic field is a more 

 violent exhibition of this phenomenon, it does not explain w^hy some 

 bacteria remain completely refractory to ultrasonic treatment. 



Some investigators have reported increased agglutination and 

 diminution of virulence when certain bacteria are irradiated with ul- 

 trasound, while others have reported an opposite effect — for example, 

 that colon bacilli could not be influenced, no matter how long the 

 exposure or how high the intensity. Yeast cells have been found to 

 lose their power of reproduction and luminous bacteria their lumi- 

 nosity. All in all, the effect on bacteria, because of their small size, 

 seems to depend on the reaction of the particular organism to the 

 secondary effects of cavitation. While this represents a promising 

 field in biology, ultrasound does not seem to have the sterilizing ac- 

 tion popularly ascribed to it. 



Stumpf, Green, and Smith {31) have investigated ultrasonic dis- 

 integration as a method of extracting bacterial enzymes. They used 

 a quartz crystal generator {29) with a maximum power of 700 watts 



