Assessment of the activity of RND-type multidrug efflux pumps in Pseudomonas aeruginosa using tetraphenylphosphonium ions

Multidrug resistance (MDR) pumps are one of the major causes of antibiotic resistance in Pseudomonas aeruginosa. Thus, fast and reliable methods are needed to assay the efficiency of MDR pumps in these bacteria. In this study, it was demonstrated that a membrane voltage (Δψ) indicator tetraphenylphosphonium (TPP+) in combination with the efflux pump inhibitor phenylalanyl-arginyl-β-naphthylamide can be used to monitor the activity of resistance–nodulation–cell division (RND)-type efflux pumps in P. aeruginosa. By controlling the outer membrane permeability and Δψ, electrochemical measurements of RND pump activity in real time were performed. It was demonstrated that the composition of the medium, the presence of nutrients and the level of aeration affect the efficiency of the TPP+-extruding activity of P. aeruginosa, urging the standardisation of experimental conditions to obtain quantitative and comparative results.