G. S. STENT AND C. R. FUERST 443 



in which Ao is the specific radioactivity (in milUcuries per miUigram of phos- 

 phorus) of the growth medium and N the total number of phosphorus atoms 

 per phage particle. Hence, a plot of logio5 vs. (1 — e~^^), the fraction of all P^'^ 

 atoms decayed by the /'^ day, should be a straight line with slope proportional 

 to ^0, the relation actually observed experimentally. 



We have studied the inactivation by P"^ decay of five virulent coliphages 

 Tl, T2, T3, T5, T7, and of the temperate coliphage X. All these strains, except 

 the pair T3-T7, are serologically unrelated, differ in their chemical constitution, 

 morphology, genetic structure, and manner of interaction with bacterial host 

 cells. Radioactive stocks of each strain were grown by the procedure indicated 

 above in media ranging in specific radioactivity from 100 to 300 mc./mg. At 

 these specific activities, approximately 0.03 to 0.1 per cent of all phosphorus 

 atoms are present as the P^- isotope. The lysates, whose titer usually represented 

 at least a thousandfold increase over the inoculum, were stored at 4°C. in 

 casamino acid-glycerol medium and the number of infective centers assayed 

 from day to day. The results are presented in Fig. 1 in which the logarithm 

 of the fraction of the survivors in the different phage stocks is plotted against 

 (1 — e~^'). It is seen that in agreement with equation (2) a straight line survival 

 curve is obtained in every case. The specific death rates aN, having the di- 

 mension lethal atoms per phage and obtained by dividing the observed slopes 

 of the lines of Fig. 1 by -1.48 X 10-« Ao, are listed in Table I. Control ex- 

 periments, not shown in Fig. 1, indicated that non-radioactive stocks of all 

 six strains were stable in casamino acid-glycerol medium at 4°C. and that the 

 radioactive lysates had been diluted sufficiently far to avoid inactivation by 

 any external P^^. The six phages evidently fall into two classes of sensitivity 

 to P^- inactivation. One class, composed of T2 and T5, is characterized by 

 4.5 X 10"* lethal atoms per phage, the value already observed by Hershey 

 el al. for T2 and T4. The sensitivity of the other group, comprising Tl, T3, 

 T7, and X, corresponds to 1.5 X 10* lethal atoms per phage. Hence the strains 

 of the second group are only one-third as sensitive to inactivation by decay 

 of P^- as those of the first. 



Phosphorus Content and Efficiency of Killing. — 



The efficiency of killing per disintegration, a, may be calculated from the 

 specific death rate, aN, if the number of phosphorus atoms per infective unit is 

 known. The phosphorus content of each phage strain was, therefore, deter- 

 mined by means of the following procedure, the results of which are listed in 

 Table I. 



A stock of each phage was grown in H medium containing P^^ at a low but ac- 

 curately determined specific activity. The lysate was clarified and freed of bacterial 

 debris by two low speed centrifugations (10 minutes at 5,000 g) and the phage sedi- 



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