AbstractPromysalin was previously described as a narrow spectrum molecule with a unique species-specific activity against Pseudomonas aeruginosa. Here we demonstrate that promysalin is active against Gram-positive and Gram-negative bacteria using a microdilution assay. Promysalin acts on Gram-positive bacteria with a mechanism of action involving cell membrane damage with leakage of intracellular components. The evaluation of MICs and MBCs on 11 promysalin analogs, synthesized utilizing diverted total synthesis, allowed the identification of the structural moieties potentially involved in cell membrane interaction and damage. The mechanism of action of promysalin against Gram-negative bacteria is still not clarified, even if a synergistic effect with the bisguanidine chlorhexidine on cell membrane disruption has been observed.