Hoechst likes to play hide and seek … use it with caution!

Critical discussion on drug efflux in Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) can withstand months of antibiotic treatment. An important goal of tuberculosis research is to shorten the treatment to reduce the burden on patients, increase adherence to the drug regimen and thereby slow down the spread of drug resistance. Inhibition of drug efflux pumps by small molecules has been advocated as a promising strategy to attack persistent Mtb and shorten therapy. Although mycobacterial drug efflux pumps have been broadly investigated, mechanistic studies are scarce. In this critical review, we shed light on drug efflux in its larger mechanistic context by considering the intricate interplay between membrane transporters annotated as drug efflux pumps, membrane energetics, efflux inhibitors and cell wall biosynthesis processes. We conclude that a great wealth of data on mycobacterial transporters is insufficient to distinguish by what mechanism they contribute to drug resistance. Recent studies suggest that some drug efflux pumps transport structural lipids of the mycobacterial cell wall and that the action of certain drug efflux inhibitors involves dissipation of the proton motive force, thereby draining the energy source of all active membrane transporters. We propose recommendations on the generation and interpretation of drug efflux data to reduce ambiguities and promote assigning novel roles to mycobacterial membrane transporters.

Reconstitution of Membrane Proteins in Liposomes

Membrane protein reconstitution in liposomes is an invaluable technique to study numerous properties of membrane proteins in vitro. Kinetics, substrate specificity, and protein-protein interaction of membrane proteins can be investigated once they are embedded in the liposome bilayer. Both protocols described here aim to investigate the efflux pump MexA-MexB-OprM from Pseudomonas aeruginosa, a proteins complex embedded in both membranes of this Gram-negative bacteria. This tri-partite system, which by-passes the periplams, is involved in antibiotic resistance. First, we describe a protocol to study MexB, the actual transporter, and second we propose an alternate protocol where the tripartite system is investigated as a whole.

In vitro transport activity of the fully assembled MexAB-OprM efflux pump from Pseudomonas aeruginosa

Antibiotic resistance is a major public health issue and many bacteria responsible for human infections have now developed a variety of antibiotic resistance mechanisms. For instance, Pseudomonas aeruginosa, a disease-causing Gram-negative bacteria, is now resistant to almost every class of antibiotics. Much of this resistance is attributable to multidrug efflux pumps, which are tripartite membrane protein complexes that span both membranes and actively expel antibiotics. Here we report an in vitro procedure to monitor transport by the tripartite MexAB-OprM pump. By combining proteoliposomes containing the MexAB and OprM portions of the complex, we are able to assay energy-dependent substrate translocation in a system that mimics the dual-membrane architecture of Gram-negative bacteria. This assay facilitates the study of pump transport dynamics and could be used to screen pump inhibitors with potential clinical use in restoring therapeutic activity of old antibiotics.