PmrAB, a Two-Component Regulatory System of Pseudomonas aeruginosa That Modulates Resistance to Cationic Antimicrobial Peptides and Addition of Aminoarabinose to Lipid A

Abstract

Spontaneous polymyxin-resistant mutants of Pseudomonas aeruginosa were isolated. The mutations responsible for this phenotype were mapped to a two-component signal transduction system similar to PmrAB of Salmonella enterica serovar Typhimurium. Lipid A of these mutants contained aminoarabinose, an inducible modification that is associated with polymyxin resistance. Thus, P. aeruginosa possesses a mechanism that induces resistance to cationic antimicrobial peptides in response to environmental conditions. Cationic antimicrobial peptides (CAPs) are a widely conserved host defense mechanism of plants and animals. Their antimicrobial effects can be attributed to their amphipathic, detergent-like nature, which enables individual CAP molecules to interact with both anionic and hydrophobic components of the bacterial envelope. CAPs bind to lipopolysaccharide (LPS), a major component of the gram-negative cell surface, through interactions with phosphates and fatty acids of LPS core and lipid A moieties (31). These molecules cross the outer membrane and periplasm, disrupt the membrane potential of the inner membrane, and thereby cause cell death (25). In vertebrates, the CAPs that pathogens encounter at epithelial surfaces are a major component of innate immunity, an ancient system of host defense that is stimulated via receptors that recognize pathogen-associated molecular patterns (29).Pseudomonas aeruginosa is an opportunistic pathogen of humans that causes infections in those with host defense defects such as epidermal injury, immunodeficiency, and impaired epithelial clearance mechanisms. In the human host, P. aeruginosa is exposed to endogenous CAPs such as ␤-defensins (37) and cathelicidins (5) at epithelial surfaces. It may also encounter exogenous CAPs in this setting, when agents such as the polymyxins, acylated cyclic CAPs synthesized by the grampositive soil bacterium Bacillus polymyxa, are used as antibiotics. Since the discovery and initial clinical use of the polymyxins more than 50 years ago, both clinical (12, 23, 26) and experimental (7, 16, 32) P. aeruginosa polymyxin resistance has been reported. P. aeruginosa possesses proteases that can degrade some CAPs (35); in addition, physiological (or "adaptive") polymyxin resistance may occur in response to membrane stresses such as divalent cation limitation (7,13,27,30) and polymyxin exposure (9, 16, 36), the latter being associated with the modulation of lipid A fatty acid composition (9). The P. aeruginosa PhoPQ two-component system contributes to the induction of these resistance phenotypes; however, its role appears to be complex (13,27), and the potential roles of other regulatory systems related to PmrAB, a response regulatorsensor kinase pair that regulates polymyxin resistance in Salmonella enterica serovar Typhimurium (18, 34), have not been defined.Isolation of polymyxin-resistant mutants of P. aeruginosa. Conditions that physiologically induce polymyxin resistance have not been fully defined for P. aeruginosa and could involve multiple regulatory...

The pmrF polymyxin-resistance operon of Yersinia pseudotuberculosis is upregulated by the PhoP-PhoQ two-component system but not by PmrA-PmrB, and is not required for virulence

“…As in Ps. aeruginosa (Moskowitz et al, 2004), the pmrD gene (which flanks the right extremity of the Salmonella pmrF operon and encodes a pmrA activator) was not found in the immediate vicinity of the Y. pseudotuberculosis pmrF operon, and subsequent whole-genome in silico analysis revealed that the gene was missing in the micro-organism studied here.…”

The sensor kinase PhoQ mediates virulence in Pseudomonas aeruginosa

“…Mg 2+ ; McPhee et al, 2006). Through regulation of the arnBCADTEF-pmrE LPS modification operon, they are critically important in controlling intrinsic and mutational resistance to cationic antimicrobial peptides (Ernst et al, 1999;Lewenza et al, 2005;Macfarlane et al, 2000;McPhee et al, 2006;Moskowitz et al, 2004). However, mutants in the PhoP and PmrA response regulators, and the PmrB sensor kinase, were not found here to be attenuated for virulence in the rat model of chronic lung infection and were fully capable of competing with the wild-type parental strain.…”

Capsule polysaccharide is a bacterial decoy for antimicrobial peptides

“…1). For P. aeruginosa and Salmonalla typhimurium, there are studies showing increases of the IC 50 of polymyxin B of 6 and 10-fold respectively (Gunn & Miller, 1996;Moskowitz et al, 2004). However, it is worth noting that the Pseudomonas and Salmonella strains used in those studies are not wild-type ) bound to CPSs (1 mg ml "1 ) from K. pneumoniae (CPS Kpn); S. pneumoniae (CPS Spn) and P. aeruginosa (CPS Pa).…”