Trans-envelope multidrug efflux pumps of Gram-negative bacteria and their synergism with the outer membrane barrier

Abstract

Antibiotic resistance is a serious threat to public health. Significant efforts are currently directed toward containment of the spread of resistance, finding new therapeutic options concerning resistant human and animal pathogens, and addressing the gaps in the fundamental understanding of mechanisms of resistance. Experimental data and kinetic modeling revealed a major factor in resistance, the synergy between active efflux and the low permeability barrier of the outer membrane, which dramatically reduces the intracellular accumulation of many antibiotics. The structural and mechanistic particularities of trans-envelope efflux pumps amplify the effectiveness of cell envelopes as permeability barriers. An important feature of this synergism is that efflux pumps and the outer membrane barriers are mechanistically independent and select antibiotics based on different physicochemical properties. The synergism amplifies even weak polyspecificity of multidrug efflux pumps and creates a major hurdle in the discovery and development of new therapeutics against Gram-negative pathogens.

Keywords: Active drug efflux; Hyperporination; Kinetic model; Permeability barrier; Substrate specificity.

Fig. 1

Schematic representation of drug fluxes across the two-membrane cell envelopes of Gram-negative bacteria and the mathematical model that describes this permeability barrier. Modified from [50].

Fig. 2

Substrate specificities of MexEF-OprN and MexHI-OpmD from P. aeruginosa. When overproduced, and in the context of the outer membrane permeability barrier, the two pumps have overlapping substrate specificities and provide the same levels of resistance to structurally diverse fluoroquinolones. Their actual substrate specificities are distinct, with MexHI-OpmD selective for phenazines [59]. EtBr, ethidium bromide; PMS, phenazine methosulfate; PCA, phenazine-1-carboxylic acid.