HOST SPECIFICITY 125 



manner. Its resistance to infection is not due solely to failure of 

 attachment. A different mutant of the same strain of E. coli, 

 called B/1,5, is resistant to phage Tl because of failure of adsorp- 

 tion (Garen and Puck, 1951). There are numerous other refer- 

 ences in the literature to phage-resistant variants that adsorb the 

 phage to which they are resistant. In such cases the mechanism 

 of resistance is not known. 



Some bacteria adsorb a phage, are killed by it, but fail to 

 liberate viable phage progeny. In such cases the phage simu- 

 lates an antibiotic and the bacterium cannot be considered a host 

 for the phage. Although the individual bacterial cell is killed, 

 the ultimate result may be protection of the bacterial culture 

 because the adsorbed phage particle is destroyed. In some cases 

 a bacterium may either respond in this way to phage attack or 

 may respond normally with phage multiplication depending on 

 environmental conditions such as nutrients or temperature. 

 Literature references to the antibiotic action of phages have been 

 collected by Fredericq (1952b), and "abortive infection" is dis- 

 cussed in Chapter XV. 



5. Acquisition of Phage Resistance by Mutation 



Bacteria may become resistant to phage attack either by 

 mutation or by becoming lysogenic. Since the mechanisms of 

 these processes are distinct, they will be discussed separately. 



A mutation is a permanent, heritable, modification in some 

 property of a cell. Mutations occur spontaneously during cell 

 reproduction. Since mutation is typically a rare event, m.utants 

 are usually detected by the use of appropriate selective agents in 

 the environment. For instance the presence of phage-resistant 

 mutants in a population of phage-susceptible cells may be de- 

 tected by plating the population with an excess of phage. The 

 phage-susceptible bacteria are destroyed and the phage-resistant 

 mutant cells produce colonies which may be readily subcultured 

 and free of phage. 



That the mutation occurs spontaneously prior to the addition 

 of the selecting agent has been demonstrated by the fluctuation 



