INACTIVATION OF VIRUSES 361 



iiig of virus particles by receptor substance, will be discussed in detail in 

 other chapters and are therefore not treated here. 



II. Physical Agents 



0. Maal0e 



A. Mechanical Treatments 



1. Sonic Irradiation 



In virology, the use of sonic oscillators is primarily of theoretical interest. 

 Even prolonged treatment with the high energy waves emitted by modern 

 machines usually does not sterilize virus suspensions, and it has not yet been 

 possible to produce vaccines usmg "sonic" inactivation alone. 



Two types of generators, magneto-striction and piezoelectric oscillators, 

 are being used. The physical principles involved have been reviewed by Gregg 

 (1944) and by PoUard (1953). The forces which seem to be responsible for 

 inactivation are those created by the rapid shifts between high and very low 

 pressures that occur rhythmically as the sound wave travels through a liquid. 

 If the energy applied is great enough, submicroscopic bubbles of dissolved 

 gas develop as the pressure drops and rapidly coUapse as the pressure again 

 goes up; this phenomenon is known as cavitation, and very great shearing 

 forces are generated ui this way. Difficulties in interpreting correctly what 

 happens in the minute regions within which these forces operate are due 

 mainly to the inevitable changes in temperature accompanying the trans- 

 mission of the sonic wave. WTien cavitation occurs, the local temperature may 

 rise to over 200°C. for very short times (Harvey et at., 1944). Nevertheless, 

 heat inactivation seems not to play an important role; thus Oster (1947) has 

 shown that the properties of sonically disrupted tobacco mosaic virus (TMV) 

 are quite different from those of heat-denatured virus. Nor are the pressures 

 generated by sound waves sufficient to cause inactivation (Pollard, 1953). 



Elimination of the dissolved gases, as well as increase of pressure, is known 

 to comiteract inactivation (Stanley, 1934a), and lowering the surface tension 

 is reported to have increased the effect on bacteria (Harare, 1949). These 

 observations indicate that inactivation by sonic treatment is due mainly to 

 the process of cavitation. In this connection it may be mentioned that there 

 seems to exist an upper Hmit for the biological efficiency of the applied acous- 

 tic energy: Horton and Horwood (1951), working with Escherichia coli, found 

 that maximum efficiency was reached when the energy was about 18 acous- 

 tical watts cm.^2_ The explanation offered is that, at high intensities of cavi- 

 tation, the gas bubbles prevent the effective transmission of the sound wave. 



It was early demonstrated that TMV can be inactivated by sonic treatment 

 (Takahashi and Christensen, 1934; Stanley, 1934a), Kausche et al. (1941) 

 demonstrated that the characteristic rods of TMV are actually broken by the 

 treatment. 



