96 MOLECULES, VIRUSES, AND BACTERLV 



without an inducer would be able to grow at the same rate at a lower 

 lactose concentration because it would maintain maximum levels of 

 /?-galactosidase and galactoside permease no matter how low the con- 

 centration of lactose. These expectations were verified when Weiner 

 observed that in a chemostat with lactose as the controlling growth 

 factor, an inducible strain was replaced after about ten days, by a con- 

 stitutive strain ( one not requiring the inducer ) , 



The principles involved in the selection of such strains in the chem- 

 ostat may be explained more fully as follows. When a bacterium is 

 growing in the chemostat at a growth rate which is determined by 

 some low concentration of a required nutrilite, the rate-limiting step is 

 the capture of a nutrilite molecule by the bacterium, i.e., conversion of 

 the molecule to a chemical form which cannot escape from the cell. 

 Probably this capture is efiFected by a permease or by some enzyme 

 which attaches a charged or large group to the molecule. 



Two kinds of behavior can be expected, depending on whether the 

 enzyme involved in the capture step is inducible or constitutive. 



1. If the capturing enzyme is constitutive, i.e., if its level is inde- 

 pendent of the concentration of the substrate, then the growth rate 

 falls linearly with a decrease in the concentration of substrate at low 

 growth-rate-limiting concentrations. This appears to be the case when 

 tryptophan is used as the controlling growth factor for a tryptophan- 

 requiring strain ( Novick and Szilard, 1950b ) . When a chemostat is op- 

 erated for a long period, strains are selected which grow faster at the 

 low concentrations of tryptophan in the chemostat. This selection can 

 be understood from the curves in Figure 1, which give the growth rate 

 as a function of the concentration for the original strain in the chemo- 

 stat and for one of the strains that replaced it. In both cases the growth 

 rate falls linearly with the concentration of tryptophan, but the growth 

 rate of the later strain is higher at a given low concentration of sub- 

 strate. Thus selection occurs because initially, when the original strain 

 is predominant, the bacteria of what later becomes the predominant 

 strain have a higher growth rate, ( af ) than the washout rate ( «„ ) at the 

 concentration of tryptophan ( Ci ) established by the first strain. When 

 the faster strain becomes large in numbers, its growth rate must fall to a 

 value equal to the washout rate. The concentration of tryptophan then 

 falls to C2, where the original strain grows at a rate ( a^ ) much less than 

 the washout rate. As a result, the original strain is diluted out of the 

 population. 



2. If the amount of the capturing enzyme formed by the bacteria 

 depends on the substrate concentration, i.e., if it is an inducible enzyme, 

 different results are to be expected. At low concentrations of the sub- 

 strate the growth rate will fall off very rapidly with a decrease in the 



