Multidrug-resistance among bacteria has become a global issue and bacterial resistance
occurs as a result of mutations in bacterial genes or the acquisition of resistance
determinants borne on plasmids, bacteriophages, transposons and/or by the action of
multidrug-efflux pumps [1]. These pumps work to remove a variation of structurally
unrelated antibiotics from the microorganism resulting in reduced susceptibility of
the antibiotic [2]. This study has evaluated natural products for their in vitro antibiotic potentiation against S. aureus strains possessing distinct efflux-related multidrug-resistance pumps. Quercus robur, Fraxinus excelsior, Fagus sylvatica, Ulmus minor, Robinia pseudoacacia,
Baptisia tinctoria and Frangula alnus were extracted. Hexane, chloroform, methanol and aqueous extracts and fractions of each of the samples
were investigated for their antibiotic-potentiation activity against effluxing strains
of S. aureus using a modulation assay. These strains included SA1199B (fluoroquinolone-resistant;
NorA), RN4220 (macrolide-resistant; MsrA) and XU212 (tetracycline-resistant; TetK).
Extracts and fractions were produced using a Soxhlet aparatus, ultrasound-assisted.
The results showed that the chloroform extract of B. tinctoria and R. pseudoacacia had the highest antibiotic-potentiation activity against all S. aureus effluxing strains. They also enhanced the activity of norfloxacin against SA1199B
with a 256-fold reduction in norfloxacin minimum inhibitory concentration. The chloroform
extract of B. tinctoria enhanced the activity of erythromycin against RN4220 with a 128-fold reduction. The
potentiation activity of these extracts indicates that efflux inhibitor natural products
are present and studies are underway to isolate and characterise these compounds.
Keywords: Quercus robur, Fraxinus excelsior, Fagus sylvatica, Ulmus minor, Robinia pseudoacacia, Baptisia tinctoria, Frangula alnus.
References:
[1] Piddock LJV. Clinically relevant chromosomally encoded multidrug resistance efflux pumps in bacteria.
Clin Microbiol Rev 2006; 19: 382 – 402
[2] Smith ECJ, Williamson EM, Wareham N, Kaatz GW, Gibbons S. Antibacterials and modulators of bacterial resistance from the immature cones of Chamaecyparis lawsoniana. Phytochemistry 2007; 68: 210 – 21
