Efflux inhibitors from Swazi medicinal plants

Efflux pumps are ubiquitous in all organisms and are one of the mechanisms by which cells, especially microorganisms, extrude unwanted substances [1]. Bacteria can utilise this mechanism to avert the inhibitory action of antibiotics and since efflux pumps are both highly efficient and non-specific in nature, they may confer multidrug resistance (MDR) to a wide variety of structurally diverse and unrelated substances [2]. The development of efflux inhibitors (EIs) is one of the methods being employed to counter multidrug resistance in bacteria. In this study, we have isolated some EIs from plants used in Swazi traditional medicine for the management of infectious diseases.

Hexane, chloroform and methanol extracts of the plants were subjected to bioassay-guided fractionation. Antibacterial activity was investigated using the broth microdilution method to determine minimum inhibitory concentrations (MICs) [3]. Thereafter, antibiotic potentiating activity was determined using sub-inhibitory concentrations of the test agent (crude extracts, fractions or isolated compounds) in combination with the relevant antibiotic, and tested against a strain possessing an efflux pump that extrudes the antibiotic. Two Staphylococcus aureus strains were used; SA-1199B, over-expressing the NorA MDR pump and XU212, over-expressing the TetK efflux pump. Chromatographic and spectroscopic (IR, UV, MS, NMR) techniques were used to isolate and characterize the compounds. Cardanols (Z)-3-(pentadec-8-en-1-yl) phenol; (Z)-3-(heptadec-8-en-1-yl) phenol and a biflavonoid (amentoflavone), isolated from Ozoroa sphaerocarpa R. Fern. & A. Fern (Anacardiaceae) potentiated the activity of tetracycline against the S. aureus XU212 strain, resulting in a 512-fold reduction in MIC (128 to 0.25 mg/L) and norfloxacin, against the SA-1199B (16-fold reduction; 32 to 2 mg/L), respectively. Some cinnamic acid derivatives and terpenoids isolated from Dioscorea sylvatica Eckl. var. sylvatica (Dioscoreaceae) and Dioscorea cotinifolia Kunth. (Dioscoreaceae), respectively, resulted in comparable potentiating activity.

Acknowledgements: Commonwealth Scholarship Commission, University of Swaziland.

Keywords: Efflux pump inhibitors, potentiating activity, MIC, antibacterial.

References:

[1] Piddock LJV. Multidrug-resistance efflux pumps – not just for resistance. Nat Rev Microbiol 2006; 4: 629 – 636

[2] Stavri M, Piddock LJ, Gibbons S. Bacterial efflux pump inhibitors from natural sources. J Antimicrob Chemother 2007; 59: 1247 – 1260

[3] Andrews JM. Determination of minimum inhibitory concentrations. J Antimicrob Chemother 2001; 48: 5 – 16