S. E. LURIA 



2. A plaque of r type can be distinguished without difficulty from a "mottled 

 plaque" stemming from a bacterium infected with a mixture of r and wild-type, 

 even when r is in moderate excess. Thus, it is possible to distinguish a plaque 

 originating from an r particle from one resulting from a mutation that occurs on 

 the plate during the development of a wild type plaque, unless the r mutation 

 occurred in the first bacterium infected after plating and produced within that 

 bacterium a large majority of r mutants — an improbable occurrence, as the 

 results of the present work will show. 



3. As for the w mutants, mixed plaques of w and wild type closely resemble the 

 plaques of wild type, so that the chances of mistaking a mixed plaque for a pure 

 w one are rather small. 



Several single burst experiments of T2L on B, with single mfection in Difco 

 nutrient broth, gave average yields per bacterium around 60-100; the burst size 

 distributions are shown in Table 1. The reasons for the lower average yield of 

 T2L in Difco nutrient broth, as compared with the yields obtained several years 

 ago in the same system (Delbriick and Luria, 1942), are unknown; they may 

 have to do with changes in medium composition, in the bacterial host, or in the 

 virus itself. 



Mass phage lysates often contain more phage particles than plaque-forming 

 units; most of the particles can be caused to form plaques by treatment with 

 distilled water or Zn++ before plating (Bertani, unpub.). Electron micrographic 

 counts give, for carefully assayed lysates of T2L, ratios "particles/plaques" 

 between 1.0 and 2.0 (Luria, Williams and Backus, 1951 and unpub.). The 

 failure of some phage particles to form plaques is apparently a peculiarity of 

 phage in mass lysates, probably due to a combination between phage and 

 inhibitors of bacterial origin. Repeated attempts to reveal, by various treat- 

 ments, any increase in the plaque count of dilute lysates similar to those used in 

 the experiments here reported, constantly failed. It is likely that plaque counts 

 reveal nearly 100 per cent of the phage particles plated. 



In our experiments, we plated the full content of tubes in which one or several 

 bacteria had lysed. The loss of phage remaining in each tube after plating is of 

 the order of five per cent or less. Thus, we feel that we recovered and examined 

 practically the totahty of the active phage produced by the bacteria. 



Preliminary experiments indicated that mutants were present in abdut one 

 burst out of 200. This made it possible to examine on each plate the phage yields 

 from several infected bacteria, and yet to have almost never more than one 

 mutant clone per plate. As many as 20 bursts per plate were examined in some 

 experiments, particularly when only the r mutants were scored, since these are 

 more easily recognized than the w mutants. 



Independently isolated r mutants are generally found to be nonallelic, yielding 

 wild-type recombinants in mixed infection (Hershey and Rotman, 1948). To 

 test for allelism among r mutants isolated in our experiments, stocks were pre- 

 pared from individual mutant plaques and used, separately or m mixtures, to 

 infect bacteria. The yields were examined for wild-type plaques. Similar tests 

 with w mutants are technically more difficult and were therefore not attempted. 



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