HOST SPECIFICITY 133 



ported by Delbruck and Bailey (1946). When bacteria were 

 mixedly infected with the closely related phages T2 and T4, up 

 to 90 per cent of the bacterial population liberated both T2 and 

 T4 phage particles when plated on the mixed indicator bacteria 

 B/2 and B/4. However when plated on B/4, the indicator for 

 phage T2, only a small proportion seemed to yield T2. The 

 authors concluded that the liberated T2 particles had a low 

 efficiency of plating on B/4 as compared with that on mixed 

 indicator. 



This paradoxical result was explained by Novick and Szilard 

 (1951a) who found that bacteria mixedly infected with T2 and 

 T4 might liberate three kinds of phage particles, typical T2. 

 typical T4, and particles with the genotype of T2 but the host 

 range phenotype of T4. The latter particles adsorbed to host 

 strains B and B/2 but not to B/4. After one passage through 

 either B or B/2, the phage reverted to its T2 phenotype and 

 henceforth would multiply on B/4 but not on B/2. This ex- 

 plains why such particles produced plaques on mixed indicators 

 or on strain B, but not on either B/2 or B/4 by itself. In this 

 case the T4 phenotype was the result of the presence of particles 

 having the genotype of T4 in the mixedly infected bacteria. 

 Since the T4 host range characteristic is replaced by the T2 

 phenotype on subculture, these particles must be genotypically 

 T2. 



b. Host-Controlled Variation 



Another type of phenotypic host range modification was dis- 

 covered by Luria and Human (1952). This involves a phage 

 resistant mutant of E. coli called B/3,4,7 (2,6). This mutant may 

 be isolated from cultures of E. coli strain B by the selective action 

 of T3, T4, or T7 but not by T2 or T6. It is resistant to T3, T4, 

 and T7 because of failure of these phages to adsorb, but it's resist- 

 ance to T2 and T6 is of a different kind. When concentrated 

 T2 and T6 phage stocks are plated with this bacterial mutant, a 

 clear, sterile area results; but if the phage stocks are progres- 



