ent-16-Kauren-19-oic acid is a bacteriolytic diterpenoid present in the resin of the medicinal plant Pseudognaphalium vira vira. The compound and its methyl ester showed strong activity against gram-positive bacteria, whereas the derivative 3beta-hydroxy-ent-kauren-19-oic acid was inactive against all assayed bacteria at the maximal concentration used (250 microg/mL). The bacteriolytic effect of ent-16-kauren-19-oic acid (5 microg/mL) was confirmed with cultures of Bacillus cereus and Staphylococcus aureus, whereas the methyl derivative (12 microg/mL) showed only a bacteriostatic effect. Both compounds stimulated oxygen consumption and proton conduction of whole cells, as reflected by an abrupt increase in the extracellular pH. These results indicate that ent-16-kauren-19-oic acid acts as a respiratory chain uncoupler, and that this function is strongly affected by minor structural substitutions, suggesting a tight activity-structure relationship. The ultimate effect of the uncoupling mechanism demonstrated by ent-16-kauren-19-oic acid is bacterial lysis. The disruption of the bacterial membrane integrity caused by the diterpenoid compound was determined using SYTOX Green stain and visualized by fluorescence microscopy.