Ionic Conductivity as a tool to study biocidal activity of sulfobetaine micelles against Saccharomyces cerevisiae model cells

Zwitterionic sulfobetaine surfactants are used in pharmaceutical or biomedical applications for the solubilization and delivery of hydrophobic molecules in aqueous medium or in biological environments. In a screening on the biocidal activity of synthetic surfactants on microbial cells, remarkable results have emerged with sulfobetaine amphiphiles. The interaction between eight zwitterionic sulfobetaine amphiphiles and Saccharomyces cerevisiae model cells was therefore analyzed. S. cerevisiae yeast cells were chosen, as they are a widely used unicellular eukaryotic model organism in cell biology. Conductivity measurements were used to investigate the interaction between surfactant solution and cells. Viable counts measurements were performed, and the mortality data correlated with the conductivity profiles very well, in terms of the inflection points (IPs) observed in the curves and in terms of supramolecular properties of the aggregates. A Fourier transform infrared (FTIR)-based bioassay was then performed to determine the metabolomic stress-response of the cells subjected to the action of zwitterionic surfactants. The surfactants showed nodal concentration (IPs) with all the techniques in their activities, corresponding to the critical micellar concentrations of the amphiphiles. This is due to the pseudocationic behavior of sulfobetaine micelles, because of their charge distribution and charge densities. This behavior permits the interaction of the micellar aggregates with the cells, and the structure of the surfactant monomers has impact on the mortality and the metabolomic response data observed. On the other hand, the concentrations that are necessary to provoke a biocidal activity do not promote these amphiphiles as potential antimicrobial agents. In fact, they are much higher than the ones of cationic surfactants.