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The antifolate compound methotrexate (MTX) is toxic to the gram-positive bacterium Streptococcus pneumoniae. Interaction of MTX with this bacterium resulted in an increase in the electric transmembrane potential (delta psi) and enhanced the delta psi-dependent uptake of isoleucine and MTX. In contrast, delta psi-independent uptake of glutamine was not changed. Folate, a nontoxic analog of MTX, did not exhibit these membrane effects, nor did it prevent the effect of MTX, suggesting that the NH2 in position 4 of the pteridine ring of the MTX molecule is involved in the MTX response. A strain bearing the nonsense mutation amiA9, selected for MTX resistance, did not exhibit increased membrane potential after MTX pretreatment. This suggests that MTX interacts with a specific membrane component in S. pneumoniae. A resulting change in ion permeability could lead to changes in the magnitude of the delta psi. The MTX-sensitive component is altered or absent in mutant amiA9.

Alteration of Streptococcus pneumoniae membrane properties by the folate analog methotrexate