Study by NMR of the mode of action of monensin on Streptococcus faecalis de-energized and energized cells.

Streptococcus faecalis was used as a bacterial model for studying the mode of action of monensin by NMR investigations. Experiments were carried out in two states, characterized by several complementary methods: (i) the resting (de-energized) cell which was considered as an inert biological membrane, on which cationic transport induced by the ionophore alone can be investigated; (ii) the active (energized) cell where the ionophore-sensitive response of the living organism, particularly the cation pumps and the glycolysis, is probed. Studies of resting cells were performed, with changing external ionic concentrations, in the presence of monensin, which is preferentially a sodium carrier. Internal and external Na+ and H+ were followed by corresponding 23Na and 31P (inorganic phosphate) NMR resonances, K+ fluxes were measured by atomic absorption. It was shown that the induced cationic movements were linked to the existing ionic gradients for K+ and Na+. 31P and 13C NMR spectra for the intermediary metabolites detected in active cells showed that glycolysis is dramatically modified in the presence of monensin.