APPENDIX 509 



periment 65/85 or 76 per cent of the possible mixedly infected 

 bacteria actually liberated both kinds of phage. This must be a 

 minimum estimate since not all mottled plaques can be recog- 

 nized. Mutual exclusion, if it occurs in this case, must be very 

 weak. Furthermore the burst size of the mixedly infected bac- 

 teria in this instance was the same as that of bacteria multiply in- 

 fected with T2r+ or T2r alone so that there is no depressor effect. 

 These observations have been confirmed by means of the single- 

 cell burst technique (Delbriick and Bailey, 1946) (pp. 485 ff.). 



Recombination of Genetic Characters in Mixedly hifected Bacteria 



The discovery of these genetic markers has made it possible to 

 analyze the yields from bacteria infected with phage particles 

 differing in 2 genetic characters (Hershey, 1 946b ; Hershey and 

 Rotman, 1949), for instance, T2/z+r and T2/zr+. T2h+ and T2A 

 are readily distinguished from each other by use of a mixture of B 

 and B/2 for plating. When plated on this mixture, T2A+ will 

 give turbid plaques since it will lyse B but not B/2, while T2^ will 

 give clear plaques since it lyses both strains. A culture of B is 

 mixedly infected with equal numbers of T2A+r and T2/?r.+ Af- 

 ter adsorption the mixture is diluted into anti-T2 serum to neu- 

 tralize free phage and further diluted into F.G.T. and S.G.T. 

 An aliquot of F.G.T. is plated on B before the end of the latent 

 period and a sample of S.G.T. on B + B/2 after all bursts have 

 occurred. About 65 per cent of the plaques from F.G.T. are 

 then found to be mottled, indicating that at least this proportion of 

 the infected bacteria are liberating both ; + and r forms. On the 

 mixed indicator plate from S.G.T., r+ is readily distinguished 

 from r by plaque size and h from /i+ by turbidity. Actually this 

 plate reveals 4 kinds of plaques; T2A+r and T2/?r+, the parent 

 types, and T2A+r"'" and T2hr, 2 new types resulting from recom- 

 bination of the genetic characters of the parents. This method 

 is ideally suited for study of exchanges of genetic characteristics 

 between phages during intracellular growth in mixedly infected 

 bacteria. 



