SULPHONAMIDE RESISTANCE IN PnEUMOCOCCI 189 



giving different internal concentrations of SA within the cell, 

 are formally very much like those of an enzyme which mutates 

 so that it responds differently to the same concentration 

 of drug. In both cases a single entity is inferred which can 

 exist in several states having different affinities for the 

 sulphonamide. 



Sulphonamide Inhibition of Cell Growth 



There is no great difficulty in assessing accurately the level 

 of SA which just permits or slightly inhibits growth of a 

 pneumococcal strain. In such an experiment one is testing 



/^q = /^q/inl ©[sulfanilamide 



Fig. 2. 



12345678 

 Hours 

 Inhibition of pneumococcal growth by sulphanilamide 

 in excess. 



for indefinitely continued division of virtually every cell 

 within the culture. Such a threshold concentration of drug, 

 however, does not at first sight seem to have much effect 

 upon cell metabolism. Even when so gross a measure of 

 growth as the total turbidity of the culture is followed, the 

 effect of SA seems to be slight. Such turbidity curves as those 



