Galbanic acid, a sesquiterpene coumarin from Ferula szowitsiana roots, was investigated for its
potentiating effect on the antimicrobial activity of antibiotics as well as ethidium bromide, in 6 multidrug
resistance (MDR) clinical isolates of Staphylococcus aureus. Galbanic acid had inhibitory effect on none
of the isolated bacteria tested (up to 800 μg /ml). The MIC range of ciprofloxacin, tetracycline and
ethidium bromide, against all tested S. aureus were 10-80, 10-80 and 4-16 μg/ml, respectively. These were
reduced to 2.5-5, 2.5-5 and 0.5-2 μg/ml in the presence of galbanic acid (300 μg /ml) or verapamil (100
μg /ml). The rate of ethidium bromide (2 μg /ml) accumulation in clinical isolates was enhanced with
galbanic acid (300 μg /ml). There is also a decrease in loss of ethidium bromide from bacteria in the
presence of galbanic acid. Similar results were obtained when verapamil (100 μg /ml) was used as an
efflux pump inhibitor. Galbanic acid, like verapamil, a typical inhibitor of efflux pump, reduced the MIC
of ethidium bromide and tested antibiotics. Since efflux is the only known reported mechanism for
ethidium bromide resistance, the reduction in ethidium bromide MIC and enhanced accumulation as well
as decreased efflux of ethidium bromide in the presence of galbanic acid, can be attributed to this efflux
inhibitory properties.
