190 ROLLIN D. HOTCHKISS AND AuDREY H. EvANS 



shown in Fig. 2 seem confusing when it is reahzed that tur- 

 bidity increases almost as much as in the control when 10 to 

 20 times the inhibitory concentration of SA is present. 



The explanation lies in the limited number of divisions 

 observed in the experiment. As indicated in Fig. 3, different 

 growth media can supply different samples and quantities of 

 the eventual products of PAB metabolism. In addition, the 

 cells themselves can accumulate and later use substantial 

 amounts of the catalytically effective product(s) of PAB, folic 



Environment 



Metabolism 



Measurement 



cSi?i;?s4Tctori-'-Cf^""i'>^p-=--o- 



COOH 



Sulfonamide 

 inhibition 



streptococcal assay 



TOLIC ACID -' 



cell substance 

 (nucleic acid; protein) 



Fig. 3. Determination of inhibitory effects of sulphonamides. 



medium supplies 

 non- competitive products 

 of folic acid system 



- pneumococcal 

 endogenous assay 



-->- turbidity 



acid(s). Therefore, when placed under conditions fully 

 inhibitory to folic acid synthesis the cells nevertheless are 

 able to complete a limited number (usually three to five) ter- 

 minal divisions. When using such measures of growth as 

 turbidity, one may purposely start with an initial culture 

 heavy enough to measure. An increase of 10- to 30-fold may 

 be possible in sulphonamide so that the measuring instru- 

 ment may only reveal the late stages of inhibition or none at 

 all, as in Fig. 2. 



It is clear, therefore, that the true ability to survive in 

 sulphonamide is only assessed when indefinite propagation is 

 demanded, as when single organisms are required to produce 

 colonies. A second difficulty is the PAB-sparing effect of end- 



