Detection and measurement of antagonism to folic acid.

Methods of testing competitive antagonism in bacterial systems are considered and evaluated with particular reference to antifolic acids. Four antifolic acids-pteridine O/129, methotrexate, CB2295 and CB2335-were examined for competitiveness and potency in three test systems-Leuc. citrovorum and folinic acid, Str. faecalis and folinic acid, and Str. faecalis and pteroylglutamic acid. For comparison, phenol and formaldehyde were studied in the Leuc. citrovorum and folinic acid system. Interpretation of results was based on families of log. concentration/growth curves obtained in each system in the presence and absence of each antagonist. These curves were usually simple sigmoids. From these families of curves, pA values were derived and reciprocal growth/reciprocal concentration curves were drawn. Pteridine O/129 was fully competitive in all systems, while methotrexate was fully competitive in the Str. faecalis and folinic acid system, but did not give simple sigmoids in the other two systems. CB2295 showed some competitive features in all and CB2335 in the two systems in which it was active. It is concluded that the methods adopted, based on those used in vertebrate pharmacology, provide a stringent test of competitiveness in bacterial systems.