Cross-resistance and collateral susceptibility to antifolic antimalarial compounds.

Series of strains of Streptococcus faecium ATCC 8043, Lactobacillus casei ATCC 7469, and Pediococcus cerevisiae ATCC 8081 with increasing resistance to the active antifolate antimalarial drugs chlorguanide triazine (CGT), pyrimethamine (PM), and trimethoprim (TMP) were isolated. These mutant strains, stable for at least 3 to 5 years, were examined for cross-resistance and collateral susceptibility to the above compounds and to methotrexate (MTX). Generally, they exhibited cross-resistance to all four compounds, but resistance of a strain to one compound did not predict accurately its resistance to another drug. Unexpectedly, L. casei resistant to CGT exhibited collateral susceptibility to MTX, TMP, and PM varying from 5- to 20-fold. P. cerevisiae developed resistance to CGT readily but maintained its susceptibility to PM and TMP after prolonged exposure to these compounds. Resistance to these antimalarial antifolates was accompanied by only low-grade cross-resistance to MTX, a representative antileukemic antifolate agent.