Adaptive Resistance to the “Last Hope” Antibiotics Polymyxin B and Colistin in Pseudomonas aeruginosa Is Mediated by the Novel Two-Component Regulatory System ParR-ParS

Abstract

As multidrug resistance increases alarmingly, polymyxin B and colistin are increasingly being used in the clinic to treat serious Pseudomonas aeruginosa infections. In this opportunistic pathogen, subinhibitory levels of polymyxins and certain antimicrobial peptides induce resistance toward higher, otherwise lethal, levels of these antimicrobial agents. It is known that the modification of lipid A of lipopolysaccharide (LPS) is a key component of this adaptive peptide resistance, but to date, the regulatory mechanism underlying peptide regulation in P. aeruginosa has remained elusive. The PhoP-PhoQ and PmrA-PmrB two-component systems, which control this modification under low-Mg 2؉ conditions, do not appear to play a major role in peptidemediated adaptive resistance, unlike in Salmonella where PhoQ is a peptide sensor. Here we describe the identification and characterization of a novel P. aeruginosa two-component regulator affecting polymyxinadaptive resistance, ParR-ParS (PA1799-PA1798). This system was required for activation of the arnBCADTEF LPS modification operon in the presence of subinhibitory concentrations of polymyxin, colistin, or the bovine peptide indolicidin, leading to increased resistance to various polycationic antibiotics, including aminoglycosides. This study highlights the complexity of the regulatory network controlling resistance to cationic antibiotics and host peptides in P. aeruginosa, which has major relevance in the development and deployment of cationic antimicrobials.

Irreproducible and Uninterpretable Polymyxin B MICs for Enterobacter cloacae and Enterobacter aerogenes

“…Unacceptable errors have been identified with the disk diffusion and Etest methods due to the poor diffusion of these compounds in agar (5)(6)(7)(8). Dilution methods for susceptibility testing are preferred, but problems with antibiotic adsorption to labware and adaptive resistance confound this methodology (9,18). Our results with E. cloacae and E. aerogenes again highlight the difficulties associated with susceptibility testing with these agents.…”

“…Several two-component regulatory systems that control expression of the arn operon have been identified. Under the pressure of low-Mg conditions (19) or exposure to polymyxins (12,18,20,21), PhoPQ has been demonstrated to regulate expression of the arn operon. Similarly, PmrAB can also affect expression of the arn operon under low-cation conditions (22) or following exposure to polymyxins (12,18,22,23).…”

“…Under the pressure of low-Mg conditions (19) or exposure to polymyxins (12,18,20,21), PhoPQ has been demonstrated to regulate expression of the arn operon. Similarly, PmrAB can also affect expression of the arn operon under low-cation conditions (22) or following exposure to polymyxins (12,18,22,23). Polymyxin resistance has been associated with altered expression of these regulators (12,24), including upregulation of pmrA (18), pmrB (12,23), and phoQ (12) and the disabling of phoQ (21).…”

Adaptive and Mutational Resistance: Role of Porins and Efflux Pumps in Drug Resistance

“…However, two-component regulators can also exert an influence on the transcription of efflux pumps encoded by genes located in separate regions of the chromosome as part of a more general response. Recently, the ParRS system of P. aeruginosa was identified as an important regulator of resistance to several antibiotics in response to the presence of certain antimicrobial peptides, including the clinically relevant polymyxins (61,169). The transcriptional responses mediated by ParRS include the upregulation of the arn operon, involved in peptide resistance, as well as the enhanced expression of the MexXY pump and the downregulation of the OprD porin, which determine susceptibility to aminoglycosides and imipenem, respectively.…”

Cation Concentration Variability of Four Distinct Mueller-Hinton Agar Brands Influences Polymyxin B Susceptibility Results

“…These systems respond to environmental variations such as an elevated calcium or a low magnesium concentration, distinct pHs, and the presence of iron in the medium. The environmental signals are detected by the sensor kinase gene that modifies the expression of the second gene, a response regulator, which in turn promotes modifications in the Gram-negative lipopolysaccha- ride, leading to polymyxin resistance (1,4,12,21,24). Previous studies have also reported increased polymyxin B and colistin MICs against P. aeruginosa and A. baumannii associated with high calcium or magnesium concentration in culture media (9,11,26).…”