348 HAUIATION HIOLOGY 



Thus ultraviolet irradiation separates the early phases of infection from 

 the later ones; it provides evidence that bacterial killinji; occurs througli 

 the disrupting; action of the infectin}^ pha}>;e and does not re(iuire its rei)ro- 

 (hiction. With other ultraviolet-inactivated phages (e.g., Tl and T7j 

 bacterial infection is followed by an increase of material that reacts cyto- 

 ehemically like deoxyribonucleic acid, but no active phage can be 

 recovered from the bacteria. 



For X-ray inactivation, it is necessary to distinguish between direct and 

 indirect effects (see Sect. 2-1). Bacteriophage particles inactivated by 

 the direct effects of X rays (Watson, 1950) are normally adsorbed by host 

 bacteria, but their bacteria-killing ability is often lost. The fraction of 

 "killing" particles diminishes logarithmically with the X-ray dose, with a 

 slope approximately one-third the slope of the inactivation curve. The 

 killing particles affect the bacteria in the same way as does ultraviolet- 

 inactivated phage. All adsorbable phage particles, whether killers or not , 

 retain both the "lysis-inhibiting" property and the ability to produce 

 "lysis from without." These effects, then, require only the changes 

 brought about by phage adsorption, without further intromission of the 

 virus particle into the economy of the host cell. Thus the comparison 

 between active phage particles and particles inactivated by ultraviolet 

 and by X rays permits the distinguishing, in the early preproductive 

 phases of host-virus interaction, of two stages — one of "adsorption" and 

 one of "invasion." The latter involves the disruption of that part of the 

 bacterial machinery that impresses on the newly synthesized material the 

 specificity of bacterial protoplasm. Interference phenomena (see Sect. 

 4-1) reciuire particles capable of invasion; they are produced only by 

 particles that can kill bacteria. 



Phage particles inactivated by the indirect effect of X rays (exposure in 

 the absence of protective substances; Watson, 1952) exhibit a greatly 

 reduced rate of adsorption onto the host cells, which hinders the analysis 

 of those phage properties that manifest themselves in later stages of the 

 host-virus interaction. This suppression of adsorption results only from 

 exposure to the short-lived toxic agent present during actual irradiation in 

 water. Phage particles inactivated by introduction into a freshly irradi- 

 ated medium give a completely different picture; they are readily adsorbed 

 and retain their killing ability, with all the properties that attend this. 

 No interpretation in chemical terms of the effects of indirect irradiation 

 on various phage properties is available; differential effects of chemical 

 poisons on different parts of virus particles are clearly to be expected. 



(lenerally, inactive virus particles that are still adsorbable by the bac- 

 terial cells are not physically disintegrated and can still be recognized, e.g., 

 in electron micrographs. Very large doses of ultraviolet radiation disrupt 

 the complex morphological structure of some bacteriophages. Upon dis- 

 ruption, some of the large coli bacteriophages release part of their nucleic 



