166 BACTERIOPHAGES 



phage and their genetic exclusion, led to the recognition that in- 

 fected cells rapidly develop refractoriness to penetration by phage 

 in a reaction completed after about two minutes (Chapter 

 XVII). Visconti (1953) showed that a reaction of similar swift- 

 ness produces almost complete resistance to lysis from without by 

 superinfecting phage. The resistance to lysis cannot reflect 

 merely the failure of normal penetration, because cells infected 

 at multiplicities sufficient to cause lysis in the presence of cyanide 

 do not lyse if the cyanide is added a few minutes later. Evi- 

 dently the properties of the cell surface as a whole are very quickly 

 altered by metabolic processes occurring after infection. This 

 alteration must account for faulty penetration, genetic exclusion, 

 and breakdown of superinfecting phage, as well as resistance to 

 lysis by numerous agencies. 



Puck and Lee (1954, 1955) described important experiments 

 that clarified considerably the nature of these processes. They 

 measured leakage of cell constituents, labeled with radioactive 

 phosphorus or sulfur, into the culture fluid during the first few 

 minutes following infection. Most of the experiments were per- 

 formed with T2, although a slight leakage occurred also with 

 several other phages tested. Like lysis from without, leakage is 

 prevented by magnesium ions and, unexpectedly, also by po- 

 tassium ions. 



The chief difliculty in these experiments is to discriminate be- 

 tween leakage accompanying normal infection, and frank lysis 

 from without. The difficulty is increased because in most of the 

 experiments of Puck and Lee infection was allowed to occur at 

 low temperatures, which causes abortive infection (Chapter XV). 



In one experim.ent (Figure 3 of Puck and Lee, 1955) infection 

 at moderate multiplicities liberated considerable amounts of 

 acid-soluble phosphorus but practically no ribonucleic acid or 

 galactosidase. Since all these substances are released during 

 lysis from without, it seems clear that the leakage of acid-soluble 

 phosphorus is something different. In other experiments galac- 

 tosidase was released at moderate multiplicities and the authors 



