Development of Resistance to 4-Amino-N10-Methyl Pteroylglutamic Acid (Amethopterin) by Leuconostoc citrovorum.

Abstract
In further studies on the development of resistance to amethopterin in the leukemic process a resistance was induced in a biological system which specifically required the citrovorum factor (CF). By serial transfer in media containing amethopterin and CF it has been possible to obtain an amethopterin-fast strain of L. citrovorum. A 2000 fold increase in resistance to amethopterin was induced in L. citrovorum without any other change in the physiology or morphology of the micro-organism.