Evaluation of the efficiency of synthesized efflux pump inhibitors on Salmonella enterica ser. typhimurium cells

Mode of the Interaction of Efflux Inhibitor Phenylalanyl-arginyl-β-naphtylamide with Bacterial Cells

An increased efflux activity is one of the major reasons for bacterial antibiotic resistance. The usage of efflux pump inhibitors could be a promising approach to restoring the activity of inefficient antibiotics. The interaction of the RND family efflux pump inhibitor phenylalanyl-arginyl-β-naphthylamide (PAβN) with Salmonella enterica ser. Typhimurium cells was assayed using traditional microbiological techniques and a novel PAβN-selective electrode. Monitoring the PAβN concentration in the medium using the electrode enabled the real-time measurements of this compound's interaction with bacterial cells. We showed that S. Typhimurium cells accumulate a high amount of PAβN because of its high affinity to lipopolysaccharides (LPSs), the major constituent of the outer layer of the outer membrane, and does not affect the functioning of the plasma membrane. EDTA enhanced the binding of PAβN to S. Typhimurium cells and the purified E. coli LPSs, but the energization of the cells by glucose does not affect the cell-bound amount of this inhibitor. Polycationic antibiotic Polymyxin B released both the cells accumulated and the suspended LPS-bound PAβN.

Scalable and Chromatography-Free Synthesis of Efflux Pump Inhibitor Phenylalanine Arginine β-Naphthylamide for Its Validation in Wild-Type Bacterial Strains

Phenylalanine arginine β-naphthylamine, or PAβN, is a C-terminus capped dipeptide discovered in 1999 as an RND-type efflux pump inhibitor (EPI). Since then, PAβN has become a standard tool compound in EPI research and development. Despite this, PAβN lacks a detailed or efficient synthesis, and standard parameters for its use in wild-type bacterial strains are inconsistent or non-existent. Therefore, a scalable and chromatography-free synthesis of PAβN was developed using streamlined traditional solution-phase peptide coupling chemistry. With this procedure, gram scale quantities of PAβN were synthesized alongside analogues and stereoisomers to build a focused library to evaluate simple structure activity relationships. While most analogues were less active than the broadly utilized L,L-PAβN itself, we identified that its enantiomer, D,D-PAβN, also provided 8- to 16-fold potentiation of the antibiotic levofloxacin at 40 to 50 μg/mL concentrations of EPI in various wild-type Pseudomonas aeruginosa strains. Additionally, D,D-PAβN was shown to be significantly more hydrolytically stable than L,L-PAβN, indicating that it may be a useful, and now readily synthesized, tool compound facilitating future EPI research.

Ethidium Binding to Salmonella enterica ser. Typhimurium Cells and Salmon Sperm DNA

Bacterial resistance to antibiotics due to increased efficiency of the efflux is a serious problem in clinics of infectious diseases. Knowledge of the factors affecting the activity of efflux pumps would help to find the solution. For this, fast and trustful methods for efflux analysis are needed. Here, we analyzed how the assay conditions affect the accumulation of efflux indicators ethidium (Et⁺) and tetraphenylphosphonium in Salmonella enterica ser. Typhimurium cells. An inhibitor phenylalanyl-arginyl-β-naphtylamide was applied to evaluate the input of RND family pumps into the total efflux. In parallel to spectrofluorimetric analysis, we used an electrochemical assessment of Et⁺ concentration. The results of our experiments indicated that Et⁺ fluorescence increases immediately after the penetration of this indicator into the cells. However, when cells bind a high amount of Et⁺, the intensity of the fluorescence reaches the saturation level and stops reacting to the accumulated amount of this indicator. For this reason, electrochemical measurements provide more trustful information about the efficiency of efflux when cells accumulate high amounts of Et⁺. Measurements of Et⁺ interaction with the purified DNA demonstrated that the affinity of this lipophilic cation to DNA depends on the medium composition. The capacity of DNA to bind Et⁺ considerably decreases in the presence of Mg²⁺, Polymyxin B or when DNA is incubated in high ionic strength media.

Interplay Between the Phenotype and Genotype, and Efflux Pumps in Drug-Resistant Strains of Riemerella anatipestifer

The number of multidrug-resistant strains of Riemerella anatipestifer continues to increase, and new strategies for the treatment of associated infections are necessary. Recently, numerous studies have shown that efflux pumps (EPs) play key roles in universal bacterial mechanisms that contribute to antibiotic resistance. In addition, studies have shown that the effects of antibiotics that are subjected to efflux can be reinforced by their combined use with efflux pump inhibitors (EPIs). Unfortunately, the role of the efflux system in R. anatipestifer remains barely understood. In this study, we evaluated the role of EPs and resistance genes in the resistance generated by clinical strains of R. anatipestifer to antibiotics. A set of 10 R. anatipestifer strains were characterized by drug resistance, associated resistance genes, and antibiotic profiles in the presence and absence of EPIs. Efflux activity was studied on a real time basis through a fluorometric method. Quantification of the levels of mRNA transcription of efflux pump genes (EPGs) was determined by RT-qPCR. Several approaches (detection of resistance genes, drug susceptibility testing, and growth kinetics analysis) were used to assess the correlation between the effect of the EPIs and the resistance levels. Analysis of the R. anatipestifer growth inhibition tests showed that the antibiotic activity was enhanced by the synergy of EPIs. Among the various resistance genes that confer antibiotic resistance, different minimum inhibitory concentrations (MICs) were observed. The different levels of resistance were reduced by EPIs. Real time fluorometry showed that all the R. anatipestifer strains presented inherent efflux activity, conferring varying levels of inhibition in the presence of EPIs. Moreover, 15 EPGs were overexpressed in the presence of antibiotics. The addition of EPIs to antibiotics led to downregulation in the expression of some EPGs and a simultaneous increase in drug resistance and sensitivity. These results demonstrated the contribution of these EPs in the resistant phenotype of the clinical strains of R. anatipestifer that are under investigation, independently of the resistant genotype of the respective strains. Intrinsic efflux activity was possibly linked to the evolution of resistance in multidrug-resistant isolates of R. anatipestifer. Furthermore, the inhibition of EPs by EPIs could enhance the clinical effects of antibiotics.

Interaction of ethidium and tetraphenylphosphonium cations with Salmonella enterica cells

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. Simultaneously with monitoring of Et⁺ concentration in the cell incubation medium, electrochemical 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-β-naphtylamide (PAβN) 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°C was considerably faster than at 23°C, 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 400mmol/L Tris buffer was about twice higher compared to 100mmol/L one. The inhibitory action of TPP⁺ on Et⁺ efflux was evident only in 400mmol/L Tris although PAβN effectively increased Et⁺ fluorescence at both buffer concentrations.

CONCLUSIONS

Results of our experiments indicate that ionic strength of the incubation medium influence the selectivity, the medium temperature and the assay conditions impact the kinetics of efflux. The lower accumulated amount and the weaker fluorescence of Et⁺ registered in slightly acidic medium indicate that ΔΨ plays a role in the accumulation of this indicator cation. The bound amount of Et⁺ to the de-energized or permeabilized cells considerably varies depending on the conditions and methods of de-energization or permeabilization of cells. Tris/EDTA permeabilization of the cells does not inhibit the efflux.