Interaction of ethidium and tetraphenylphosphonium cations with Salmonella enterica cells

Abstract

Background and objective: One of the main causes of bacterial resistance to antimicrobials is multidrug resistance induced by the increased efficiency of the efflux pumps. In this study we analyzed how the conditions of assay affect the efflux of indicator substrates ethidium (Et+) and tetraphenylphosphonium (TPP+) in Salmonella enterica ser. Typhimurium cells. Impact of the outer membrane permeability barrier, composition and temperature of the medium on accumulation of the indicator compounds also was analyzed. Materials and methods: The fluorescence of Et+ and Nile Red was measured using 96-well plates and a plate reader. In parallel to traditional studies of fluorescence we applied a constructed selective electrode to follow the accumulation of Et+ in S. enterica cells. Simul- taneously with monitoring of Et+ concentration in the cell incubation medium, electrochem- ical measurements of TPP+ accumulation were performed. Furthermore, Et+ and TPP+ were used within the same sample as agents competing for the interaction with the efflux pumps. An inhibitor phenylalanyl-arginyl-b-naphtylamide (PAbN) was applied to evaluate the input of RND-family pumps in the total efflux of these indicator compounds. Results: S. enterica cells with the intact outer membrane (OM) bound very low amounts of Et+ or TPP+. Cells with the permeabilized OM accumulate considerably higher amounts of the indicator compounds at pH 8.0, but only Et+ was considerably accumulated at pH 6.5. At conditions of electrochemical monitoring accumulation of Et+ by the permeabilized cells at 37 8C was considerably faster than at 23 8C, but at the higher temperature most of the cell-accumulated Et+ was extruded back to the medium. The fluorescence of Et+ in suspension of cells incubated in 400 mmol/L Tris buffer was about twice higher compared to 100 mmol/L one. [...]