SULPHONAMIDE RESISTANCE IN PnEUMOCOCCI 191 



products of PAB metabolism such as the purines, pyrimidines 

 and amino acids, normal constituents of the complex media 

 used for pneumococci. Not only the number of terminal 

 divisions but the limiting steady growth of pneumococci in 

 sulphonamide is modified by such metabolites. Accordingly, 

 the inhibitory levels of SA or the SA/PAB ratios for limiting 

 growth can be made to vary in absolute value when such 

 factors in the media are varied. 



A strong indication that only a single site of action is 

 involved is found when one relates these indices for mutant 

 strains and wild types to each other, as the medium is altered 

 deliberately in this fashion. It was found that the half 

 inhibitory SA concentrations and SA/PAB ratios could be 

 altered 5- to 10-fold in magnitude through changing the 

 medium, and yet the relation between the indices for mutant/ 

 wild type remained constant. It appears reasonable therefore 

 that the actual basis for the sulphonamide resistance in the 

 mutants, at least for the two or three so far tested, is an 

 altered affinity of sites on some single enzyme (or possibly 

 a single permeability-determining concentrating system) for 

 SA relative to PAB. If this proves to be true, it may be hoped 

 that we are now in possession of a system in which a series of 

 interrelated alterations within a DNA particle exerts genetic 

 control over corresponding properties at a site within a pro- 

 tein molecule, altering its affinities for a known metabolite 

 and drug. 



The further definition of the phenotypic modifications 

 existing in these resistant mutants seems to be possible when 

 folic acid synthesis is measured, somewhat along the lines of 

 the method of Nimmo-Smith, Lascelles and Woods (1948). 

 The sulphonamide inhibition of this function is, in contrast 

 with growth, not greatly modified by constituents of the 

 medium, and we have little doubt that more fundamental 

 SA affinities will soon be established for each strain. The 

 details of these studies and of a considerably simplified auto- 

 genous pneumococcal assay for folic acid w^ill be published 

 elsewhere. 



