Studies on the nature of resistance of streptococcus faecalis to folic acid antagonists

Abstract

Mutants of Streptococcus faecalis were selected at different levels of amethopterin (“methotrexate”) in a medium containing pteroylglutamic acid(PGA). An increased capacity to reduce pteroylglutamic acid to the tetrahydro-level was a consistent characteristic of each of the resistant strains. The amount of aminopterin bound by the sensitive and resistant variants did not differ significantly. One highly resistant strain, which had been maintained for 2 years as a stock culture, took up much greater amounts of PGA and aminopterin than did the sensitive parent strain or the newly-selected resistant strains. This example of the concomitant occurrence of increased uptake and decreased effectiveness of a drug demonstrates the need for methods to study specific and non-specific binding in relation to drug resistance. An eighty-fold higher concentration of amethopterin relative to 4-amino-4-deoxy- 10N-methylpteroic acid, was required to obtain comparable degrees of inhibition of the growth of this amethopterin-resistant strain. Similarly, an eighty-fold higher concentration of amethopterin was required to inhibit the conversion of PGA to folinic acid by the intact cells; however, the soluble enzyme in cell-free filtrates was equally sensitive to inhibition by the two antagonists. Additional studies are needed to explain the mechanisms by which the access of the inhibitors to the sensitive enzyme system is limited in the drug-resistant cells.