476 THE BIOCHEMISTRY OF B VITAMINS 



hibitor can usually be increased gradually until the organism becomes 

 resistant to relatively high concentrations of the inhibitory compound. 

 The resistance acquired by the organism in the early stages of the de- 

 velopment of resistant strains is usually lost after the organism is cul- 

 tured in the absence of the inhibitor. The initial resistance may be highly 

 specific for the inhibitory analogue, or the organism may be sensitive to 

 analogues of similar structure. However, the resistance gained after 

 prolonged culturing in the presence of the inhibitor usually is relatively 

 permanent, and may be specific not for the individual inhibitor but for 

 the majority, if not all, of the inhibitory analogues of the vitamin, the 

 functioning of which is prevented by the inhibitor against which the 

 resistance is developed. 



The development of resistant strains of an organism would be expected 

 to result either from a selection of a naturally occurring mutant strain, 

 or from an induced mutation, or a combination of both. Obviously, in 

 the presence of the drug, the environment favors the selection of natural 

 resistant strains, and no convincing data indicating that an inhibitor 

 specifically induces the mutation to a strain which is resistant have been 

 presented. While the process of development of resistance is gradual in 

 most instances, suggesting that the final mutant strain which possesses 

 the resistance is not present in the initial population, the possibility 

 exists that continual selection of progeny more and more resistant to the 

 inhibitor allows the isolation of a mutant strain which occurs normally, 

 but only so infrequently that an ordinary culture would have little chance 

 initially of containing a single such resistant cell. 



Many types of biochemical differences between normal strains of an 

 organism sensitive to an inhibitory analogue of a metabolite and strains 

 resistant to the inhibitor could account for the development of resistant 

 strains. Biochemical differences between resistant and parent strains 

 include: (1) increase in the biosynthesis and concentration of the metab- 

 olite in the cells; (2) increase in the effective concentration of the in- 

 hibited enzyme; (3) increase in the synthesis of other factors limiting 

 the utilization of the product of the inhibited enzyme system, i.e., factors 

 which exert a "sparing action" on the product; (4) more extensive de- 

 velopment of other mechanisms by which the product of the inhibited 

 enzyme system is synthesized; (5) presence of an enzyme of slightly 

 different structure with normal affinity for the metabolite, but with less 

 or no affinity for the analogue, i.e., different cells of the same organism 

 may not necessarily produce structurally identical molecules of an en- 

 zyme catalyzing the same reaction in all the cells, and such variations 

 could conceivably occur within a single cell; (6) presence of an enzyme 

 system which destroys the inhibitor; (7) decrease in cell permeability 

 specifically to the inhibitor. 



