Inhibition of efflux pumps in meticillin-resistant Staphylococcus aureus and Enterococcus faecalis resistant strains by triterpenoids from Momordica balsamina

Six cucurbitane-type triterpenoids (1–6) isolated from the aerial parts of Momordica balsamina were evaluated for their ability to inhibit the activity of bacterial efflux pumps of meticillin-resistant Staphylococcus aureus (MRSA) COLOXA, Enterococcus faecalis ATCC 29212, Salmonella enterica subsp. I serovar Typhimurium 5408 and S. Typhimurium 5408CIP strains. The latter strain overproduces the AcrB transporter of the AcrAB-TolC efflux pump six-fold compared with its parent. Compounds 4–6 were also tested for similar activity against Escherichia coli AG100 wild-type strain and E. coli AG100TET8 that overproduces the AcrAB-TolC efflux pump. Evaluation of efflux activity was performed using a semi-automated method that measures accumulation of the universal efflux pump substrate ethidium bromide (EtBr). Some of the compounds significantly inhibited efflux of EtBr by MRSA COLOXA and E. faecalis ATCC 29212. A correlation between activity and the topological polar surface area of the compounds was found for MRSA COLOXA.