Il8 BENZER ET AL. 



Strain in the presence of an excess of the phage. The sensitive bacteria 

 are thereby destroyed, the resistant ones survive and form colonies. 



Since the resistant mutants so obtained always fail to adsorb the 

 phage to which they have become resistant, it is reasonable to assume 

 (but difficult to prove) that the resistance is due solely to the failure of 

 the adsorption mechanism. One may imagine that the mutation has 

 led to the formation of altered surface elements unable to adsorb the 

 phage. This point of view will be elaborated in more detail in the next 

 section. 



In one instance, we have suggestive evidence that it is really only 

 the adsorption mechanism whose failure makes the bacterium resistant, 

 and not a failure of the ability of the bacterium to synthesize the phage 

 in question. This evidence is derived from the following case: When 

 bacterium B is mixedly infected with T2 and T4, there results particles 

 like the parental types, then some genetic hybrids as will be described 

 later (32), and in addition, particles of a peculiar type, which will be 

 designated as T2(4). Such particles have the following properties 

 (Szilard and Novick, personal communication; Delbriick, unpublished) : 



1 ) They are adsorbed by B/2 as if they were T4 particles. 



2) They multiply vsdthin B/2, but their offspring consists entirely 

 of T2 particles. 



The T2(4) particles are interpreted as having the genotype of T2, 

 but phenotypically they have some of the T4 properties, and it is these 

 T4 properties which enable the particles to adsorb to and enter B/2 

 bacteria. 



This case is important in that it shows that B/2 which cannot ad- 

 sorb T2 can nevertheless reproduce T2 when it is infected with a suit- 

 ably equipped T2 particle. The adsorptive abilities of the bacterium, 

 therefore, do not necessarily constitute a complete display of the syn- 

 thetic or reproductive abilities of a bacterium. 



17. Receptor Spots. — The close resemblance between neutraliza- 

 tion of an antigen by an antibody and the adsorption of a phage on to a 

 sensitive bacterium led to early attempts to isolate substances from 

 bacteria, which would represent the "receptor spots" of the bacterial 

 surface responsible for the specific attachment of the infecting phage 

 particle to the sensitive cell. In vitro the reaction between receptor sub- 

 stance and phage should lead to an inactivation of the phage, which 

 could serve as a test in the course of purification procedures. Indeed it 

 is possible to obtain extracts from bacteria, which inactivate specifically 

 phages infective for the extracted bacterial strain (Beumer, 1947). 



