Drug efflux protein complexes confer multidrug resistance on bacteria by transporting a wide spectrum of structurally diverse antibiotics. Moreover, organisms can only acquire resistance in the presence of an active efflux pump. The substrate range of drug efflux pumps is not limited to antibiotics, but it also includes toxins, dyes, detergents, lipids and molecules involved in quorum sensing; hence efflux pumps are also associated with virulence and biofilm formation. Inhibitors of efflux pumps (EPI) are therefore attractive compounds to reverse multidrug resistance, reduce biofilm formation, attenuate the pathogen and to prevent the development of resistance in clinically relevant bacterial pathogens.
We have recently produced the first structure of an active, intact, multiprotein drug efflux complex from a Gram-negative bacterium (Du et al, 2014). Our group is now using in silico screening to translate this structural information into the development of efflux pump inhibitors. Compounds identified are tested for (a) their antibacterial activity, (b) their ability to restore sensitivity to antibiotics and (c) the inhibition of substrate efflux. Hit compounds are further characterised to ensure their effects are target-specific by measuring their effect on cells which do not contain efflux pumps and by ensuring that they do not permeablise the inner- or outer membranes.
Using this approach we have been able to identify several phytochemicals isolated from plants used in traditional medicine as EPIs. A range of derivatives were synthesized from the original compounds and structure activity relationship optimisation was performed on all the derivatives.
In conclusion, we have developed new chemicals which could be lead compounds for an antimicrobial drug discovery program aimed at stemming the tide of untreatable, multidrug resistant infections.