Unbalanced Growth and Death 79 



of 15 T - in the uninfected state. 5 The properties of the uninfected cell 

 have proven to be so unusual that we have been unable until just 

 recently to return to the original problem of the behavior of infected 

 cells. 



Attempts to obtain an experimentally useful set of relations among 

 turbidity, cell number, and the thymine content of the medium led at 

 first to confusing results. The turbidity of a culture increased signifi- 

 cantly in the absence of thymine, in the presence of a carbon source 

 such as glucose, plus the usual salts, including NH 4 + . In cultures 

 from which exogenous thymine had been exhausted, viable cell number 

 varied widely. It was then observed that cells which utilized glucose, 

 nitrogen, and phosphorus in the absence of thymine soon lost the power 

 to multiply, i.e., they died, at the rate of 90% per division time. In 

 fact all of these constituents were essential for death to occur. Mere 

 metabolism, in the sense of some partial metabolic event such as 

 glycolysis, was not the key to the killing process. The bacteria had 

 to grow to die. Such death was irreversible since the power to multiply 

 was tested by plating the cells on a medium containing thymine. 



The increase in turbidity of 15 T - in the absence of exogenous thymine 

 was due to growth of the cells. There was an increase of the mass 

 of protein and nucleic acid in the cells and not merely a swelling due 

 to water uptake. The cells increased considerably in length and 

 breadth; the protein and ribose nucleic acid of the cells doubled at 

 least, as did the rate of respiration on glucose; the latter phenomenon 

 suggests an increase in enzyme content as well. However, the DNA 

 content of the bacteria barely increased, according to the usual colori- 

 metric 'procedures. It appears, then, that the cytoplasmic constituents 

 of the cells increase, but nuclear synthesis is prevented by the lack 

 of a substance found uniquely in a nuclear constituent, namely, the 

 thymine present in DNA. It is suggested that death is caused by this 

 unbalanced growth, that the cytoplasmic growth has created a struc- 

 tural framework within the cell in which nuclear division has become 

 impossible. We have noted that addition of thymine to the dead cells 

 permits DNA synthesis; however, viable count remains unchanged, 

 signifying that matters have proceeded past the point at which the 

 formation of DNA can effect division. 



The net synthesis of nucleic acid constituents in 15 T - in the absence 

 of exogenous thymine has been studied by means of uniformly labeled 

 C 14 -glucose. 5 Thus in a two-hour period RNA synthesis in the absence 

 of thymine equals the amount of RNA originally present in the bac- 

 teria, as determined by the radioactivity found in the specific RNA 



