Development of Resistance to 4-Amino-N10-Methyl Pteroylglutamic Acid (Amethopterin) by Streptococcus faecalls

(1) By growing Streptococcus jaecalis (ATCC No. 8043) in a medium containing successively higher concentrations of amethopterin, more than a thousand-fold increase in resistance has been achieved. (2) This amethopterin-fast variant has a slightly lower requirement for folic acid in the absence of amethopterin than the parent sensitive strain. In the absence of folic acid, however, amethopterin and certain other 4-amino derivatives of PGA can replace this folic acid requirement in the resistant strain.