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.

FIG. 1.

Role of the pmrAB locus in P. aeruginosa polymyxin resistance. (A) Resistance of strains with an aacC1 (Gm^r) cassette insertion. Triangles, PAKpmrB6 pmrA::aacC1; circles, PAKpmrB6; squares, PAKpmrB6 phoP::aacC1. (B) Resistance of strains with an episomal copy of the indicated pmrAB allele. Diamonds, PAKpmrA::aacC1(pMMB67HE::pmrAB6); circles, PAKpmrB6; inverted triangles, PAKpmrA::aacC1(pMMB67HE::pmrAB⁺).

FIG. 2.

Association of pmrB mutations with constitutive addition of aminoarabinose to P. aeruginosa lipid A. (A) MALDI-TOF negative-ion mode analysis of lipid A purified from P. aeruginosa strain PAK. In the negative-ion mode, observed molecular species lack at least one proton: [M—H]⁻. The difference between [M—H]⁻ at m/z 1,447 and m/z 1,617 (Δm/z = 170) in the mass spectrum indicates the loss of 3-hydroxydecanoate from position 3 of the reducing diglucosamine (right-hand ring in panel C). (B) Mass spectrum for lipid A from strain PAKpmrB6. The difference between [M—H]⁻ at m/z 1,617, 1,748, and 1,879 (Δm/z = 131) indicates the addition of aminoarabinose to the 1 and 4′ phosphates of lipid A. (C) Structure of P. aeruginosa lipid A without and with aminoarabinose. The X⁻ symbol at the right side of the lower structure represents either H (corresponding to m/z 1,748) or aminoarabinose (corresponding to m/z 1,879). The fatty acids depicted are 3-hydroxydecanoate (3-OH C_10:0), laurate (C_12:0), 2-hydroxylaurate (2-OH C_12:0), and 3-hydroxylaurate (3-OH C_12:0).